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ScienceWeek
SCIENCE-WEEK
A Weekly Email Digest of the News of Science
A journal devoted to the improvement of communication
between the scientific disciplines, and between scientists,
science educators, and science policy-makers.
April 27, 2001 -- Vol. 5 Number 17
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Geological time inspires awe, and there are no
certainties about the future, but perhaps it is
almost certain that someday they will collect
our skulls and call us Early Man.
-- Anonymous
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Section 1
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Contents of this Issue (Full reports in Section 2):
1. PLANETARY SCIENCE:
ON THE SOLAR WIND AND ATMOSPHERE EROSION
The planets in the Solar System are exposed to a variable but
persistent bombardment by the solar wind, with the intensity of
the flux dependent on how close a planet is to the Sun. The
Earthlike planets, Mercury, Venus, Earth, and Mars, are thus the
planets most affected by the solar wind. The power per unit area
of the solar wind is 6 orders of magnitude less than that of the
solar electromagnetic radiation, but the solar wind is much more
effective in removing planetary atmospheres. Thus, an important
question is why there is so little loss from Earth Assuming no
chemical differences between the Earthlike planets during
formation, an important requirement for an Earthlike planet to
retain its atmosphere and hydrosphere is thus to have a
protection mechanism against the eroding solar wind. A strong
intrinsic magnetic dynamo, like that of Earth, appears to help.
(Rickard Lundin: Science 9 Mar 01 291:1909)
2. PLANETARY SCIENCE:
ON AMINO ACIDS IN THE ORGUEIL AND IVUNA METEORITES
Researchers report that amino acid analyses of pristine interior
pieces of the Orgueil and Ivuna meteorites reveal beta-alanine,
glycine, and gamma-amino-n-butyric acid as the most abundant
amino acids in these two meteorites, with concentrations ranging
from approximately 600 to 2000 parts per billion. Other alpha-
amino acids such as alanine, alpha-amino-n-butyric acid, alpha-
aminoisobutyric acid, and isovaline are present only in trace
amounts (less than 200 parts per billion). Carbon isotope
measurements of beta-alanine and glycine in the Orgueil meteorite
suggest that these amino acids are extraterrestrial in origin. In
contrast to the CM-type Murchison and Murray meteorites, the
amino acid composition of the CI-type Orgueil and Ivuna
meteorites is strikingly distinct, and the authors suggest that
the Orgueil and Ivuna meteorites derive from a different type of
parent body, possibly an extinct comet.
(P. Ehrenfreund et al:
Proc. Natl. Acad. Sci. US 27 Feb 01 98:2138)
3. HISTORY OF PHYSICS: ON X-RAYS AND X-RAY DIFFRACTION
In 1888, Wilhelm Roentgen became head of the physics department
in the University of Wurzburg, and like many other scientists of
his day, he studied the visible light emitted in a closed glass
tube (Crookes tube) under reduced pressure when high voltage is
applied. On November 8, 1895, Roentgen wrapped his glass tube in
black paper to prevent escape of the light, and darkened the
room. He used fluorescent screens in his experiments, and to his
great surprise, when voltage was applied across the glass tube,
the fluorescent screen showed flashes of light. Roentgen
attempted to block the fluorescent screen with his hand, and he
then saw the shadow of his hand upon the screen. He soon
discovered that the radiation had great penetrating power, was
absorbed by lead, and could blacken a photographic plate. On the
28th of December 1895, Roentgen published his first paper,
announcing the discovery of x-rays, in which he included a
radiograph of the hand of his wife. The impact of his paper was
enormous. (Ron Jenkins: J. Chem. Educ. 5 May 01 78:601)
4. EVOLUTIONARY BIOLOGY:
ON THE EVOLUTION OF SIZE IN LIVING SYSTEMS
There is a long history of support for the general notion of
overall evolutionary trends toward increases in size, complexity,
and diversity. However, there are two fundamentally distinct
mechanisms that have been proposed to explain these trends. One
proposed mechanism is a random and passive tendency to evolve
away from the initial minima of organismal size, complexity, and
diversity through an overall increase in variance ("there is no
where to go but up"). The second proposed mechanism is a non-
random, active or "driven" process that biases evolution towards
increased size or complexity. What must be noted is that there
are relationships between size and complexity and between
complexity and diversity that are intuitive apparent. Increases
in organismal size through increases in cell number create the
potential for increases in diversity of cell type, and as a
result, increases in anatomical complexity. Increases in
morphological complexity then may lead to expansions into
previously unoccupied "ecospace" and an accompanying expansion of
species diversity. (Sean B. Carroll: Nature 22 Feb 01 409:1102)
5. CHEMICAL ECOLOGY:
DEFENSIVE FUNCTION OF HERBIVORE-INDUCED PLANT VOLATILE EMISSIONS
In many land-based ecosystems, the major autotrophs are plants,
and the major heterotrophs feeding on plants are insects. One of
the more intriguing ecological relationships in nature is that of
the "trophic triangle", an often remarkable instance of co-
evolution among different species. One example is the insect-
plant-insect trophic triangle, in which a species of plant, prey
to a predatory insect species that destructively feeds on it,
evolves a defense mechanism that sends out a signal that attracts
another species of insect that feeds on the first species of
insect. Such co-evolutionary triangles, delicate minuets of prey
and predation, have become an important focus of experimental
ecology. Researchers now report a study in which they quantified
volatile emissions from plants growing in natural populations
during attack by 3 species of leaf-feeding herbivores. Three
compounds increased egg predation rates by a generalist predator,
and the complete blend decreased lepidopteran egg-deposition
rates. As a consequence, a plant could reduce the number of
herbivores by more than 90 percent by releasing volatiles.
(A. Kessler and I.T. Baldwin: Science 16 Mar 01 291:2141)
6. MEDICAL BIOLOGY: ON CUTANEOUS SQUAMOUS CELL CARCINOMA
Non-melanoma skin cancer is the most common cancer in the US,
with over 1.3 million cases expected to occur in the year 2001.
Approximately 80 percent of non-melanoma skin cancers are basal-
cell carcinomas, and 20 percent are squamous-cell carcinomas.
Squamous-cell carcinoma is the second most common cancer among
Whites. Unlike almost all basal-cell carcinomas, cutaneous
squamous-cell carcinomas are associated with a substantial risk
of metastasis. Exposure to ultraviolet radiation is the most
common cause of squamous-cell carcinoma of the skin. Ultraviolet
B radiation (wavelength 290 to 320 nanometers) from sunlight is
principally responsible, with ultraviolet A radiation (320 to 400
nanometers) adding to the risk. Ultraviolet radiation produces
mutations in DNA, usually the formation of thymidine dimers in
the p53 tumor-suppressor gene. Failure to repair these mutations
may result in tumor formation.
(M. Alam and D. Ratner: New England J. Med. 29 Mar 01 344:975)
7. IN FOCUS: ON THE DEVELOPMENT OF GENE CLONING
8. SCIENCEWEEK NOTES:
EARTH SCIENCE: THE MASSES OF EARTH'S COMPONENTS
9. FROM THE SCIENCEWEEK ARCHIVE:
ASTROPHYSICS: ON THE COMPLEXITY OF THE DEATH OF STARS
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Section 2
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1. PLANETARY SCIENCE:
ON THE SOLAR WIND AND ATMOSPHERE EROSION
The surface of the Sun and the regions immediately exterior
to it constitute a domain in which various forces are played out
on an immense scale. The bright surface layer of the Sun is
called the "photosphere", a region a few hundred kilometers thick
at a temperature that ranges from 5770 degrees kelvin at its
innermost part to 4400 degrees kelvin at its outermost part, the
Sun's temperature minimum. Above this is the "chromosphere",
approximately 9000 kilometers thick, where the temperature ranges
from the minimum at the photosphere-chromosphere interface to
approximately 20,000 degrees kelvin. And above the chromosphere
is the "corona", the Sun's faint outer atmosphere, a low-density
hot gas where the temperature is 2 million degrees kelvin or
more.
The term "solar wind" refers to the flux of particles,
primarily protons and electrons together with helium nuclei
(alpha particles) and nuclei of heavier elements in smaller
numbers. This flux is accelerated by the high temperatures of the
solar corona to velocities large enough to permit escape from the
Sun's gravitational field. The solar wind is responsible for both
deflecting the tail of the Earth's "magnetosphere" (see below)
and the tails of comets away from the Sun. At a distance of 1
astronomical unit (AU) (i.e., the mean distance between the Sun
and Earth), during a relatively quiet period, the solar wind
contains approximately 1 to 10 protons per cubic centimeter
moving outward from the Sun at velocities of 350 to 700
kilometers per second. This creates a positive ion flux of 10^(8)
to 10^(9) ions per square centimeter per second, each ion with an
energy of at least 15 electron volts. The solar wind is
essentially a highly tenuous ionized gas (a "plasma") that
carries mass and angular momentum away from the Sun, the wind
propelled in an outgoing spiral by the rotation of the Sun, the
charged particles of the wind carrying the Sun's magnetic field.
During solar flares, the proton velocity, flux, plasma
temperature, and associated turbulence increase substantially.
The term "magnetosphere" refers to a region surrounding a
planet in which charged particles are controlled by the magnetic
field of the planet rather than by the magnetic field of the Sun.
These charged particles derive mainly from the solar wind. The
magnetosphere of Earth extends to 60,000 kilometers on the
sunward side and forms a tail many times this distance on the
side away from the Sun.
... ... Rickard Lundin (Institute of Space Physics Kiruna, SE)
presents a commentary on current research on the solar wind on
planetary atmospheres, the author making the following points:
1) The author points out that the planets in the Solar
System are exposed to a variable but persistent bombardment by
the solar wind, with the intensity of the flux dependent on how
close a planet is to the Sun. The Earthlike planets, Mercury,
Venus, Earth, and Mars, are thus the planets most affected by the
solar wind. The power per unit area of the solar wind is 6 orders
of magnitude less than that of the solar electromagnetic
radiation, but the solar wind is much more effective in removing
planetary atmospheres. Thus, an important question is why there
is so little loss from Earth.
2) The author points out that there are two solar-induced
atmosphere erosion processes: a) thermal ("Jeans escape"); and b)
nonthermal ("plasma escape"). In a low-gravity environment close
to the Sun, thermal escape may be the major loss process, but in
a strong-gravity environment it is usually less important, except
for Mercury, which orbits very close to the Sun. The erosion of
most planetary atmospheres is thus dominated by nonthermal
escape. Like comets, planets have extended and highly-structured
plasma tails, but the rate of erosion is much lower and the
planetary tails in the Solar System are invisible to ground-based
optical telescopes.
3) The author points out that at present the Earth loses
matter at a rate of 1 to 3 kilograms per second, the rate and
composition varying with solar cycle (sunspot cycle). Recent
measurements (K. Seki et al, Science 291:1939 2001) suggest the
rate is lower than this, but even with a net loss of 3 kilograms
per second, it would take 50 billion years to deplete the Earth's
atmosphere and at least another 15 trillion years to evacuate the
oceans. For comparison, the total lifetime of the Sun is only
approximately 10 billion years.
4) The author (Lundin) suggests that loss processes may
account for differences in atmospheric and surface properties
between the Earthlike planets. Earth is the only planet covered
with oceans. Venus has an extremely hot and dense atmosphere.
Mars may have had a hydrosphere in the past, but it is bare
today. If the four Earthlike planets aggregated from the same
dust cloud in the early *solar nebula, which seems likely given
their similar mass density and core properties, why did they
evolve in such a different way? The author suggests that an
important part of the answer is that there was (and continues to
be) a substantial difference in the rate of atmospheric and
hydrospheric erosion between the planets. Mercury, the smallest
of the planets and the closest to the Sun, is essentially void of
volatiles, which have been eroded away by the intense heat and
solar wind close to the Sun. Earth has a strong intrinsic
magnetic field that fends off the solar wind well outside the
atmosphere, so that the magnetic field acts as a shield against
solar wind erosion. Venus and Mars both lack an intrinsic
magnetic field, and their interaction with the solar wind
resembles that of comets.
5) The author concludes: "Assuming no chemical differences
between the Earthlike planets during formation, an important
requirement for an Earthlike planet to retain its atmosphere and
hydrosphere is thus to have a protection mechanism against the
eroding solar wind. A strong intrinsic magnetic dynamo, like that
of Earth, appears to help."
-----------
Rickard Lundin: Erosion by the solar wind.
(Science 9 Mar 01 291:1909)
QY: Rickard Lundin: rickard.lundin@irf.se
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Text Notes:
... ... *solar nebula: The current theory of the origin of the
Solar System is that the Sun and the planets were born from a
rotating disk of cosmic gas and dust (the "solar nebula").
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Summary & Notes by SCIENCE-WEEK http://scienceweek.com 27Apr01
For more information: http://scienceweek.com/swfr.htm
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Related Background:
ASTROPHYSICS: ON THE NEAR DISAPPEARANCE OF THE SOLAR WIND
... On 11 May 1999, the particle density of the solar wind
unexpectedly decreased to a remarkably low value -- approximately
0.2 particles per cubic centimeter, compared with a normal value
of 10 particles per cubic centimeter, and a special meeting of
the American Geophysical Union was devoted to the phenomenon in
December 1999.
... ... Alan J. Lazarus (Massachusetts Institute of Technology,
US) presents an account of the special meeting, the author making
the following points:
1) Other instances of low-density solar wind are known, but
this period of more than 27 hours on 11 May 1999 was the longest
known period having a density below 1 particle per cubic
centimeter. At 360 kilometers per second, the speed of the wind
was near its typical value of approximately 400 kilometers per
second, but the pressure exerted by the wind was so low that the
shock front formed by the interaction between the incoming
supersonic wind and the magnetic field of Earth moved outward
from its usual location (approximately 15 Earth-radii in front of
Earth as measured along the Earth-Sun line) to at least 60 Earth-
radii and near the Moon.
2) Normally, the completely ionized solar wind *plasma
compresses the dayside magnetic field of Earth because of the
relatively high conductivity of the wind. The resulting pressure
flattens the magnetic field on the sunward side and drags it out
on the night side into a tail many Earth-radii long. On 11 May
1999, the unusually low pressure resulting from the low-density
wind allowed the magnetic field of Earth to reassert its dipolar
shape over a larger volume.
3) The term "strahl" refers to a particular region of the
solar wind electron velocity distribution, the region forming a
beam that streams in a narrow cone along the magnetic field
lines. On 11 May 1999, the strahl dominated the electron velocity
distribution. Because the solar wind density was so low, the
strahl electrons were relatively unscattered by collisions in the
solar wind, and they arrived near Earth in an unusually intense
and narrow beam that penetrated into the north polar region. The
electron collisions with the atmosphere of Earth over the north
polar region generated the strongest x-ray emissions ever seen
from the polar cap.
4) Why periods of very low density solar wind occur remains
unknown, but such low wind flux periods tend to appear on the
ascending portion of the *solar activity cycle, which is the
period we are in now. The author concludes: "Discussion of low
solar wind flux periods will undoubtedly occupy solar
astrophysicists for years to come."
-----------
Alan J. Lazarus: The day the solar wind almost disappeared.
(Science 24 Mar 00 287:2172)
QY: Alan J. Lazarus: ajl@space.mit.edu
-----------
Text Notes:
... ... *plasma: In general, in this context, a "plasma" is a
fully ionized gas consisting of ions and electrons moving freely.
... ... *solar activity cycle: The term "solar activity" refers
to all active phenomena on the surface of the Sun, including
sunspots, active prominences, flares, active regions, etc. In
general, solar activity is strongly associated with magnetic
fields which are believed to arise from a dynamo action within
the Sun. Solar activity increases and decreases in a cycle
lasting approximately 11 years.
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Summary & Notes by SCIENCE-WEEK http://scienceweek.com 9Jun00
For more information: http://scienceweek.com/swfr.htm
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2. PLANETARY SCIENCE:
ON AMINO ACIDS IN THE ORGUEIL AND IVUNA METEORITES
What is called the "Orgueil meteorite" has an interesting
history. For more than 100 years this object was involved in
controversy concerning organic matter in meteorites as evidence
of extraterrestrial life. The Orgueil meteorite fell on May 14,
1864 on the village of Orgueil (which is near Toulouse) in
France. Approximately 20 fragments (total 11 kilograms) were
recovered, and examination of the meteorite soon after the fall
indicated that the meteorite contained hydrogen, carbon, and
oxygen, and that its substance resembled peat and lignite, both
of which are produced on Earth from living matter. In 1869, a
second examination revealed the presence of hydrocarbons similar
to those found in petroleum. In 1962, further examination of
parts of the Orgueil meteorite that had apparently lain sealed in
a museum case for almost a century revealed an attempted hoax:
seeds, pieces of a European reed species, and fragments of gravel
and coal were found embedded in the meteorite, these particles
apparently carefully placed there, since the substance of the
meteorite had been restored around the particles to rule out
contamination. The apparent fakery is believed to be related to a
debate that raged in France in 1864 concerning the possibilities
that life might evolve from nonliving matter. It was only a month
before the fall of the meteorite, in fact, that Louis Pasteur
(1822-1895) reported the experiments that decisively disproved
the doctrine of spontaneous generation of living matter.
(Pasteur's notes indicate that he examined the Orgueil meteorite
and found no evidence of bacteria.) In 1964 and 1975, it was
definitively established that some of the Orgueil hydrocarbons
considered to be "organized elements" resembling fossilized algae
were terrestrial contaminants that included ragweed pollen.
The Orgueil meteorite is a carbonaceous chondrite. What are
called "stony meteorites" (aerolites) are meteorites formed
solely of rock-forming silicates, and chondrites are a type of
stony meteorite consisting of an agglomeration of millimeter-
sized globules (chondrules) that are thought to be unchanged
since the original condensation out of the nebula from which the
Sun and Solar System formed. A "carbonaceous chondrite" is a
chondritic meteorite that contains a relatively large amount of
carbon, with a resultant dark appearance. Carbonaceous chondrites
are divided into 4 subgroups on the basis of composition: CI, CM,
CO, CV. Meteorites in the CI subgroup are the most primitive,
with the lowest density, the highest content of volatiles and
carbon, and compositions most closely resembling that of the Sun.
Despite the name "chondrite", CI carbonaceous chondrites contain
no chondrules. The CM-type carbonaceous chondrites contain less
than 15 percent chondrules with average diameter approximately
0.3 millimeters.
What is called the "Ivuna meteorite" is another CI
carbonaceous chondrite meteorite that fell in Tanzania on
December 16, 1938. The Ivuna meteorite is unique in that thus far
it is the only carbonaceous chondrite found to contain a
distribution of polycyclic aromatic hydrocarbons.
Neither the Orgueil meteorite nor the Ivuna meteorite have
been much studied using modern techniques.
In contrast to the CI-type carbonaceous chondrites, the CM-
type carbonaceous chondrites Murchison and Murray, which fell in
Australia in 1969 (Murchison) and in Kentucky in 1950 (Murray),
have been analyzed extensively for organic compounds using modern
methods.
... ... P. Ehrenfreund et al (5 authors at 3 installations, NL,
US) present a report of analyses of the Orgueil and Ivuna
meteorites, the authors making the following points:
1) The authors point out that carbonaceous chondrites
provide some of the most primitive Solar System material
available for study and are known to contain a wide variety of
organic compounds. In particular, the CI group of meteorites,
which apparently have been altered extensively by water on their
parent body, have been found to contain high abundances of
organic carbon. The last amino acid analysis of the Orgueil
meteorite was reported in 1972: *gas chromatography analysis
indicated that the meteorite contained D- and L- amino acids,
including amino acids generally not present in living terrestrial
organisms.
2) The authors report that amino acid analyses using *high-
pressure liquid chromatography of pristine interior pieces of the
Orgueil and Ivuna meteorites reveal beta-alanine, glycine, and
gamma-amino-n-butyric acid as the most abundant amino acids in
these two meteorites, with concentrations ranging from
approximately 600 to 2000 parts per billion. Other alpha-amino
acids such as alanine, alpha-amino-n-butyric acid, alpha-
aminoisobutyric acid, and isovaline are present only in trace
amounts (less than 200 parts per billion) [*Note #1]. Carbon
isotope measurements of beta-alanine and glycine in the Orgueil
meteorite suggest that these amino acids are extraterrestrial in
origin. In contrast to the CM-type Murchison and Murray
meteorites, the amino acid composition of the CI-type Orgueil and
Ivuna meteorites is strikingly distinct, and the authors suggest
that the Orgueil and Ivuna meteorites derive from a different
type of parent body, possibly an extinct comet.
3) The authors conclude: "The simple amino acid mixture
found in CI carbonaceous chondrites is interesting in the sense
that generally it has been thought that a wide variety of amino
acids were required for the origin of life. However, among the
candidates for the first genetic material is peptide nucleic
acid, a nucleic acid analogue in which the backbone does not
contain sugar or phosphate moieties. For the peptide nucleic acid
backbone, achiral amino acids such as glycine and beta-alanine,
possibly delivered by CI-type carbonaceous chondrites to the
early Earth, may have been the only amino acids needed for the
origin of life."
-----------
P. Ehrenfreund et al: Extraterrestrial amino acids in Orgueil and
Ivuna: Tracing the parent body of CI type carbonaceous
chondrites.
(Proc. Natl. Acad. Sci. US 27 Feb 01 98:2138)
QY: Jeffrey L. Bada: jbada@ucsd.edu
-----------
Text Notes:
... ... *gas chromatography: In general, the term
"chromatography" refers to any technique for separating the
components of a mixture by differential adsorption of compounds
to adsorbents, and the term "gas chromatography" refers to any
form of chromatography in which the mobile phase is a gas.
... ... *high-pressure liquid chromatography: (high-performance
liquid chromatography) A rapid technique of column chromatography
that provides high resolution. The liquid is forced at high
pressure through a column, allowing the use of small adsorbent-
support particles in the column, which greatly improves
chromatographic resolution.
... ... *Note #1: Editor's note: It is instructive to consider
the analytical technique used in such investigations. The sample
processing technique in this study was as follows: A large
interior piece of the Orgueil meteorite (6.3 grams obtained from
the National Museum of Paris (FR) and several small interior
chips (0.3 grams) of the Ivuna meteorite obtained from the
Smithsonian Museum of Natural History (US), were crushed
separately into fine powders using a mortar and pestle in a
positive pressure clean room. A portion of the Orgueil (133
milligrams) and Ivuna (94 milligrams) meteorite samples were then
sealed separately in clean test tubes with 1 milliliter of
double-distilled water and boiled at 100 degrees centigrade for
24 hours, and the water supernatants were subjected to a 6 molar
hydrochloric acid vapor procedure. The acid-hydrolyzed hot-water
extracts of the meteorites were desalted with a cation-exchange
resin, and the amino acids were then analyzed by high-pressure
liquid chromatography coupled with UV fluorescence detection. For
comparison, powdered samples of the Murchison (92 milligrams) and
Murray (121 milligrams) meteorites were analyzed simultaneously
with the Orgueil and Ivuna meteorites. As controls, 108
milligrams of crushed serpentine (a hydrated magnesium silicate)
that had been heated at 500 degrees centigrade for 3 hours and a
procedural blank were carried through the same processing
procedure as the meteorite samples.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 27Apr01
For more information: http://scienceweek.com/swfr.htm
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3. HISTORY OF PHYSICS: ON X-RAYS AND X-RAY DIFFRACTION
There are several popular distortions concerning the history
of the discovery of x-rays, one of which concerns the origin of
the "Crookes tube", and the other of which concerns the
scientific talents of the discoverer of x-rays, Wilhelm Roentgen
(1845-1923).
Concerning the Crookes tube, the term "cathode rays" refers
to streams of electrons emitted at the cathode in an evacuated
tube containing a negative electrode (cathode) and a positive
electrode (anode) that are part of a closed circuit containing a
source of electromotive force. Cathode-ray investigations began
in 1854 when Heinrich Geissler, a glassblower in the laboratory
of the physicist Julius Pluecker (1801-1868) improved the vacuum
tube. Pluecker discovered cathode rays in 1858 by sealing two
electrodes inside a vacuum tube, evacuating the air, and forcing
electric current between the electrodes. He observed a green glow
on the wall of the glass tube and he attributed the glow to rays
emanating from the cathode. In 1869, with improved vacuums,
Pluecker's student Johann W. Hittorf (1824-1914) (of later fame
as an electrochemist) saw a shadow cast by an object placed in
front of the cathode. The shadow proved that the cathode rays
originated from the cathode. The physicist and chemist William
Crookes (1832-1919) investigated cathode rays in 1879 and
discovered the rays were bent by a magnetic field, with the
direction of deflection suggesting the rays were negatively-
charged particles. As the luminescence of the rays did not depend
on what gas had been in the vacuum or on what metal formed the
electrodes, Crookes suggested the rays were a property of the
electric current itself. As a result of Crookes's work,
investigations of cathode rays intensified, and the evacuated
tubes came to be called "Crookes tubes". But if any name should
be associated with evacuated glass tubes containing two
electrodes, perhaps the deserved name would be "Pluecker tubes".
Concerning the scientific talents of Wilhelm Roentgen, it is
a popular myth that Roentgen was a second-rate scientist lifted
from obscurity by a completely accidental discovery. Details of
the discovery are provided below, but concerning Roentgen's
talents, he was apparently one of the best experimental
physicists of his generation. Apart from his discovery of x-rays
(for which he received the first Nobel Prize in Physics in 1901),
Roentgen is perhaps best known for his experiments verifying the
prediction of Oliver Heaviside (1850-1925) that magnetic effects
would be produced by a dielectric rotated rapidly between the
plates of a charged condenser. Roentgen also made a classic
determination of the ratio of specific heats of gases, and
investigated pyro- and piezo-electrical phenomena. It is
instructive to note that Crookes had actually observed x-rays
before Roentgen, but Crookes was not astute enough to realize
what he was looking at. Roentgen was an extremely sharp
experimentalist, and he immediately realized the significance of
his discovery. It is a tribute to Roentgen's mastery of
experimental physics that most of the currently accepted basic
properties of x-rays were described in the series of papers
(1895-1897) in which his discovery was first announced.
Roentgen's discovery provoked intense interest, and in 1896 over
400 papers on x-rays were published by other physicists. Roentgen
won the Rumford Medal in 1896 and the Nobel Prize five years
later. But he refused ennoblement by the king of Bavaria, and he
made no attempt to patent any aspect of x-ray production or to
make any financial gain from his work. He maintaining that his
discovery should be used for the benefit of mankind. When he died
in 1923 at the age of 78, Roentgen was completely impoverished
and suffering from the effects of radiation disease due to years
of x-ray exposure.
In Roentgen's first paper (Sitzungsber. der Wuerzburg. Ges.
1895; Ann. Physik. 64:1 1898), he reported the following
observations concerning x-rays:
1) All substances are more or less transparent to x-rays.
For example, wood 2 to 3 centimeters thick is very transparent.
Aluminum 15 millimeters thick "weakens the effect considerably,
although it does not entirely destroy the fluorescence." Lead
glass is quite opaque, but other glass of the same thickness is
much more transparent. "If the hand is held between the discharge
tube and the screen, the dark shadow of the bones is visible
within the slightly dark shadow of the hand."
2) Many other substances besides barium-platino-cyanide
fluoresce: calcium compounds, uranium glass, rock salt, etc.
3) Photographic plates and films "show themselves
susceptible to x-rays." Hence, photography provides a valuable
method of studying the effects of x-rays.
4) X-rays are neither reflected nor refracted (so far as
Roentgen could discover). Hence, "x-rays cannot be concentrated
by lenses."
5) Unlike cathode rays, x-rays are not deflected by a
magnetic field. They travel in straight lines (as Roentgen
demonstrated by pinhole photographs).
6) X-rays discharge electrified bodies, whether the
electrification is positive or negative.
7) X-rays are generated when the cathode rays of the
discharge tube strike any solid body. A heavier element, such as
platinum, however, is much more efficient as a generator of x-
rays than is a lighter element such as aluminum.
In terms of modern physics, x-rays comprise electromagnetic
radiation of wavelengths shorter than the ultraviolet, with x-
rays produced by bombardment of atoms by high-quantum-energy
particles. The range of x-ray wavelengths is 10^(-11) meters to
10^(-9) meters. Atoms of all the elements emit a characteristic
x-ray spectrum when they are bombarded by electrons, the x-ray
photons being emitted when the incident electrons knock an inner
orbital electron out of an atom. When this occurs, an outer
electron falls into the inner shell to replace the lost electron,
the outer electron losing potential energy. The wavelength of the
emitted photon will be given by l = ch/(deltaE), where (l) is the
wavelength of the emitted photon, (c) is the velocity of light,
(h) is Planck's constant, and (deltaE) is the potential energy
loss.
The term "x-ray diffraction" refers to the diffraction of x-
rays by a crystal. The wavelengths of x-rays are comparable in
size to the distances between atoms in most crystals, and the
repeated pattern of the crystal lattice acts as a diffraction
grating for x-rays.
The term "x-ray powder diffraction" refers in general to x-
ray diffraction of a collimated monochromatic beam by a sample (a
powder) containing a large number of tiny crystals having random
orientations.
The term "x-ray crystallography" refers to the use of x-ray
diffraction to determine the structure of crystals or molecules.
In general, the technique involves directing a beam of x-rays at
a crystalline sample and recording the diffracted x-rays on a
photographic plate. The x-ray diffraction pattern consists of a
pattern of spots on the plate, and the crystal structure can be
determined from the positions and intensities of the diffraction
spots. Since x-rays are diffracted by the electrons in a
molecule, if molecular crystals of a compound are used, the
electron density distribution in the molecule can be determined.
X-ray diffraction is currently one of the most important
tools of solid-state physics and chemistry, the technique used
for differentiation between crystalline and amorphous materials,
determination of the structure of crystalline materials,
determination of electron distributions within atoms and
throughout unit cells of a crystal, determination of the
orientation of single crystals, determination of the texture of
polygrained materials, etc. In the biological sciences, x-ray
diffraction has contributed greatly to our understanding of
processes occurring in living cells, with the technique revealing
the structures of proteins and nucleic acids and their
constituent monomers, and also of drugs, hormones, and vitamins.
Knowledge of 3-dimensional structures of various biomolecules has
had a profound impact on the whole of biology.
... ... Ron Jenkins (International Center for Diffraction Data,
US) presents a review of the history of x-ray diffraction
instrumentation, the author making the following points
concerning the early years of x-ray diffraction experiments:
1) The author points out that in 1888 Roentgen became head
of the physics department in the University of Wurzburg, and like
many other scientists of his day, he studied the visible light
emitted in a closed glass tube (Crookes tube) under reduced
pressure when high voltage is applied. On November 8, 1895,
Roentgen wrapped his glass tube in black paper to prevent escape
of the light, and darkened the room. He used fluorescent screens
in his experiments, and to his great surprise, when voltage was
applied across the glass tube, the fluorescent screen showed
flashes of light. Roentgen attempted to block the fluorescent
screen with his hand, and he then saw the shadow of his hand upon
the screen. He soon discovered that the radiation had great
penetrating power, was absorbed by lead, and could blacken a
photographic plate. On the 28th of December 1895, Roentgen
published his first paper, in which he included a radiograph of
the hand of his wife. The impact of his paper was enormous.
2) The author points out that the main application of x-rays
at the end of the 19th century was in radiography, and several
German organizations, notably Siemens, began the commercial
development of x-ray tubes and high-voltage generators. In 1912,
there was much discussion at the University of Munich about
whether x-radiation could best be described as wave or particle
radiation. Max von Laue (1879-1960) wagered a bet that x-rays
were electromagnetic waves that could be refracted by crystals.
Two of von Laue's students then obtained a diffraction pattern
from a single crystal. Soon after that, von Laue proposed a
theory to describe the conditions for diffraction, and in
England, the Braggs, William Henry Bragg (1862-1942) and William
Lawrence Bragg (1890-1971), father and son, began their work on
crystal structure analysis. Using a much improved x-ray tube that
produced a significant amount of characteristic radiation, and an
ionization chamber as a detector, the Braggs demonstrated that x-
rays are diffracted only in specific directions. The elder Bragg
derived a simple equation relating the diffraction angle to the
diffracted wavelength and the interplanar spacing of the crystal.
The development of various x-ray methods followed rapidly between
1914 and 1937, including the beginning of the use of powder
diffraction in 1916.
-----------
Ron Jenkins: Landmarks in the development of powder diffraction
instrumentation.
(J. Chem. Educ. 5 May 01 78:601)
QY: Ron Jenkins: International Center for Diffraction Data,
Newtown Square, PA 19073-3273 (US).
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 27Apr01
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
4. EVOLUTIONARY BIOLOGY:
ON THE EVOLUTION OF SIZE IN LIVING SYSTEMS
A long view of the evolutionary history of life on Earth suggests
that living systems tend to evolve into larger and more complex
forms. However, some of the most successful living systems are
relatively small and have remained small. Is there a pattern in
the evolution of size? And if there is a pattern, what are the
forces responsible for the pattern and how do we explain the
exceptions?
... ... Sean B. Carroll (University of Wisconsin, US) presents a
review of current ideas concerning the evolution of morphological
complexity and diversity, the author making the following points
concerning the evolution of size:
1) The author points out that for the first 2.5 billion
years of life on Earth, most species rarely exceeded 1 millimeter
in size and were generally much smaller. The earliest reported
bacterial microfossils from approximately 3.5 billion years ago
averaged approximately 5 microns in diameter. Early *eukaryotic
microfossils (*acritarchs), while considerably larger, still
ranged generally from approximately 40 to 200 microns in size
(with a few larger exceptions) for much of their first 600 to 800
million year history. Organismal size increased appreciably with
the evolution of multicellular forms. In bacterial and algal
forms with cell walls, one of the simplest ways to become
multicellular was for the products of cell division to remain
together to form long filaments. Many early multicellular
eukaryotes were millimeter-scale, linear or branched, filamentous
forms.
2) The author points out that the size and shape of life did
not expand appreciably until the late *Proterozoic. Radially
symmetric impressions and trace fossils indicate the presence of
millimeter scale multicellular organisms (metazoans) around 550
million years ago. The puzzling *Ediacaran fauna comprised of
tubular, frond-like, radially symmetric forms generally reached
several centimeters in size (although some forms approached 1
meter in size), as did macroscopic algae. Organismal sizes
expanded considerably in the *Cambrian, including *bilaterians up
to 50 centimeters in size, as well as sponges and algae up to 5
to 10 centimeters. Maximal body lengths of animals increased
subsequently by another 2 orders of magnitude, as did algal size
(e.g., *kelp).
3) The authors point out that the largest existing
organisms, giant fungi and trees, evolved from independent small
ancestors. Land plants are believed to have evolved from
*charophyte green algae, and both green algae and plants
apparently evolved from a unicellular *flagellate ancestor.
Fossil spores indicating the earliest evidence of plant life date
from the *mid-Ordovician. The oldest plant-body fossil
(Cooksonia) suggests that early land plants were small, and on
the basis of molecular phylogenetic analyses are believed to be
comparable in organization and life cycle to *liverworts. Many of
the principal groups of land plants have evolved large (> 10
meters) species at some point in their history. Thus, increases
in both mean and maximal organismal size apparently occurred in
the evolution of bacteria, eukaryotes, and within the algal,
fungal, and animal lineages.
4) The author points out that there is a long history of
support for the general notion of overall evolutionary trends
toward increases in size, complexity, and diversity. However,
there are two fundamentally distinct mechanisms that have been
proposed to explain these trends. One proposed mechanism is a
random and passive tendency to evolve away from the initial
minima of organismal size, complexity, and diversity through an
overall increase in variance ("there is no where to go but up").
The second proposed mechanism is a non-random, active or "driven"
process that biases evolution towards increased size or
complexity. What must be noted is that there are relationships
between size and complexity and between complexity and diversity
that are intuitive apparent. Increases in organismal size through
increases in cell number create the potential for increases in
diversity of cell type, and as a result, increases in anatomical
complexity. Increases in morphological complexity then may lead
to expansions into previously unoccupied "ecospace" and an
accompanying expansion of species diversity.
-----------
Sean B. Carroll: Chance and necessity: The evolution of
morphological complexity and diversity.
(Nature 22 Feb 01 409:1102)
QY: Sean B. Carroll: sbcarrol@facstaff.wisc.edu
-----------
Text Notes:
... ... *eukaryotic: Cells (or organisms composed of such cells)
containing internal membrane-bound organelles such as a nucleus.
... ... *acritarchs: Unicellular microfossils of unknown or
uncertain biological origin that occur abundantly in strata from
the Precambrian and Paleozoic (see next note).
... ... *Proterozoic: The complete geological time-scale is as
follows:
Time-Frame Starting Date (Millions of Years Ago)
---------- -------------------------------------
Hadean 4600
Archaean 4000
Proterozoic 2500
Cambrian 570
Ordovician 510
Silurian 439
Devonian 408.5
Carboniferous 362.5
Permian 290
Triassic 245
Jurassic 208
Cretaceous 145.6
Paleocene 65
Eocene 56.5
Oligocene 35.4
Miocene 23.3
Pliocene 5.2
Pleistocene 1.64
Holocene 0.01
... ... *Ediacaran: The term "Ediacaran" refers to an assemblage
(until recently the oldest) of soft-bodied marine animals, the
assemblage first discovered in the Ediacara Hills in Australia.
... ... *Cambrian: See time-scale above. The most outstanding
aspect of the Cambrian was the rather sudden appearance of
numerous invertebrate fossils, so numerous that some researchers
have termed the Cambrian an explosion of evolutionary processes.
Many of the life forms that existed during the Cambrian are long
extinct, but their fossils are numerous, and through their
fossils the various Cambrian species have been the subject of
much study by paleobiologists. The Cambrian explosion of life
forms has been a long-standing puzzle for paleobiologists, and at
present there is apparently no single generally accepted
explanation.
... ... *bilaterians: The "Bilateria" are a major division of the
animal kingdom comprising all forms with bilateral symmetry, and
the term "bilaterians" refers to the first such forms appearing
after the emergence of protozoa.
... ... *kelp: A group of large brown "seaweeds", actually algae,
growing in large structures that may be as long as 60 meters.
... ... *charophyte green algae: In general, "green algae" are
algae in which chlorophyll is not masked by another pigment.
Charophyte green algae (also known as "stoneworts"), are a type
of green algae usually found in fresh or brackish water.
... ... *flagellate: Possessing one or more flagella. A flagellum
is a long threadlike extension providing locomotion for a cell.
... ... *mid-Ordovician: See time-scale above.
... ... *liverworts: (Hepaticopsida) A group of lower plants in
which the dominant generation is the sexual phase of the plant
(gametophyte phase).
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 27Apr01
For more information: http://scienceweek.com/swfr.htm
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5. CHEMICAL ECOLOGY:
DEFENSIVE FUNCTION OF HERBIVORE-INDUCED PLANT VOLATILE EMISSIONS
"Ecology" (environmental biology) is the study of the
relationship between organisms and their environment, with the
term "environment" including both other organisms and the
physical surroundings. The word "ecology" was introduced by the
zoologist Ernst Haeckel (1834-1919), who applied the term
"oekologie" to "the relation of the animal both to its organic as
well as its inorganic environment."
Of major importance in ecology is the categorization of
organisms as "autotrophs" or "heterotrophs". All biological
communities have a basic structure of interaction that forms a
"trophic pyramid". The trophic pyramid consists of trophic
levels, with food energy passed from one level to the next along
the food chain. The base of the pyramid is composed of
autotrophs, the primary producers of the ecosystem, those
biological organisms (e.g., plants) that do not obtain energy and
nutrients by eating other organisms. Instead, autotrophs harness
solar energy by photosynthesis (photoautotrophs) or, more rarely,
they harness chemical energy by oxidation (chemoautotrophs), both
cases involving the production of organic substances from
inorganic substances. All other organisms in the ecosystem are
consumers called "heterotrophs", which either directly or
indirectly depend on the autotrophs for food energy.
In many land-based ecosystems, the major autotrophs are
plants, and the major heterotrophs feeding on plants are insects.
One of the more intriguing ecological relationships in nature is
that of the "trophic triangle", an often remarkable instance of
co-evolution among different species. One example is the insect-
plant-insect trophic triangle, in which a species of plant, prey
to a predatory insect species that destructively feeds on it,
evolves a defense mechanism that sends out a signal that attracts
another species of insect that feeds on the first species of
insect. Such co-evolutionary triangles, delicate minuets of prey
and predation, have become an important focus of experimental
ecology.
... ... A. Kessler and I.T. Baldwin (Max Planck Institute for
Chemical Ecology Jena, DE) present a report on an insect-plant-
insect trophic triangle, the authors making the following points:
1) The authors point out that plants defend themselves
against plant-eating animals (herbivores) with chemical and
physical defenses that directly influence herbivore performance
and indirectly influence such performance through traits that
attract the natural enemies of herbivores. One such indirect
defense, the release of volatile organic compounds specifically
after herbivory, is known to attract parasitoids and predators to
actively feeding larvae in the laboratory, and evidence from
agricultural systems suggests a role for herbivore-induced
volatile organic compounds in increasing predation evolutionary
selection pressure. But exclusive evidence has been lacking, and
it is not even known whether plants growing in natural
populations increase volatile organic compound emissions after
herbivore attack.
The authors report a study in which they quantified volatile
emissions from Nicotania attenuata plants growing in natural
populations during attack by 3 species of leaf-feeding herbivores
(the caterpillars of Manduca quinquemaculata [Lepidoptera]; the
leaf bug Dicyphus minimus [Heteroptera]; the flea beetle Epitrix
hirtipennis [Coleoptera]). In the experiment, the authors
mimicked the individual release of 5 commonly emitted volatiles.
Three compounds (cis-3-hexen-1-ol; linalool; cis-alpha-
bergamotene) increased egg predation rates by a generalist
predator (Geocoris pallens [Heteroptera; a bug]). Linalool and
the complete blend decreased lepidopteran egg-deposition rates.
As a consequence, a plant could reduce the number of herbivores
by more than 90 percent by releasing volatiles. The authors
suggest these results confirm that indirect defenses can operate
in nature.
-----------
A. Kessler and I.T. Baldwin: Defensive function of herbivore-
induced plant volatile emissions in nature.
(Science 16 Mar 01 291:2141)
QY: Ian T. Baldwin: baldwin@ice.mpg.de
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 27Apr01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
INITIAL CHEMICAL SIGNAL IN INSECT-PLANT-INSECT TROPHIC TRIANGLES
Both corn and cotton plants, when attacked by plant-eating
insects, release a volatile substance that specifically attracts
other insects that are the natural predators of the plant-eating
insects. A group led by J. H. Tumlinson (U.S. Department of
Agriculture), studying the trophic triangle of the beet armyworm
caterpillar (Spodoptera exigua Hubner), corn seedlings (Zea mays
L.), parasitic wasp (Cotesia marginiventris), have isolated and
synthesized the chemical substance responsible for the initial
signal. They have named the substance volicitin. It is present
in the oral secretions of the caterpillar, and it induces the
damaged corn seedlings to release a volatile blend of terpenoids
and indole, which calls in the parasitic female wasps that are
the natural enemies of the caterpillars. The wasps lay eggs in
the caterpillars, and the hatched larvae destroy the
caterpillars by eating them. Mechanically damaged plants exposed
to synthetic volicitin, in the absence of caterpillar attack,
release the usual volatiles that attract the wasps. Plants
mechanically damaged but not exposed to volicitin do not release
the volatiles. (Science 9 May 97) (Science-Week 15 May 97)
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
6. MEDICAL BIOLOGY: ON CUTANEOUS SQUAMOUS-CELL CARCINOMA
The skin is one of the largest organs of the human body in
surface area and weight, in adults covering an area of
approximately 2 square meters and weighing 4.5 to 5 kilograms.
The thickness of the skin ranges from 0.5 to 4.0 millimeters,
depending on location. Structurally, the skin consists of 2
principal parts: a) the outer and thinner portion, composed of
epithelium, is called the "epidermis"; b) the inner and thicker
connective tissue part is called the "dermis". The epidermis is
composed of layers of flat cells (stratified squamous epithelium)
and contains various types of cells, including keratinocytes,
which produce the protein keratin that helps waterproof skin, and
melanocytes, which produce the pigment melanin. In most regions
of the body, the epidermis is 0.1 millimeters thick and has 4
layers; where exposure to friction is greatest, such as the palms
and the soles, the epidermis is 1 to 2 millimeters thick and has
5 layers.
Synthesis of vitamin D, a vitamin essential to human
biological function, begins with activation of a precursor
molecule in the skin by ultraviolet rays in sunlight. Enzymes in
the liver and kidneys then modify the activated precursor and
finally produce calcitriol, the most active form of vitamin D.
During most of the year, an hour per week in the sunlight with
the hands, arms, and face exposed meets the requirements of the
body for activation of the vitamin D precursor. Additional
sunlight exposure merely increases the risk of skin cancer,
especially in fair-skinned persons.
Skin cancer is the most common type of cancer in humans,
resulting in significant mortality. Primary skin cancers are of
two types: a) epidermal cancers, which originate in
keratinocytes, melanocytes, or skin appendages such as sweat
glands; b) dermal cancers, which originate in neural, vascular,
and other tissues of the skin. Malignant tumors originating from
keratinocytes and melanocytes are the most frequent skin cancers.
In general, a "carcinoma" is any malignancy derived from
epithelial tissue. In animals, epithelial cells compose the cell
layers that form the interface between a tissue and the external
environment, for example, the cells of the skin, the lining of
the intestinal tract, and the lung airway passages. In contrast,
a "sarcoma" is a connective tissue neoplasm, usually highly
malignant.
The skin cancer called "basal-cell carcinoma", rare in
Blacks and Asians, is the most common malignant skin tumor in
Whites. This cancer arises from the undifferentiated basal
keratinocytes of the epidermis. Such cancers rarely metastasize
but may be highly invasive locally, when they are called "rodent
ulcers". The lesions are prevalent in fair-skinned persons and on
areas of skin that receive the greatest exposure to sunlight.
Treatment with inorganic arsenical drugs and exposure to ionizing
radiation (e.g., x-rays) may also be contributing factors.
The skin cancer called "squamous-cell carcinoma" is less
common than basal-cell carcinoma but has a higher rate of
metastasis. Squamous-cell carcinoma is common in children with
xeroderma pigmentosum, who are unable to repair DNA damage caused
by ultraviolet radiation. In most persons, such an inability to
repair DNA damage is due to a deficiency of an endonuclease
enzyme. In adults, squamous-cell carcinoma rarely occurs in the
absence of an external cause, and protracted exposure to sunlight
is the usual cause of this disease. Chronic scarring from burns,
as well as reactions to vaccinations, radiation dermatitis, and
chronic ulceration may also be risk factors for squamous-cell
carcinoma of the skin.
In general, cancer genes have been divided into 2 classes,
proto-oncogenes and tumor suppressor genes. Proto-oncogenes are
genes that sustain activating changes in human cancer. These
changes may take the form of point mutations or gene
rearrangements that lead to increased or uncontrolled activity of
the encoded protein, or they may take the form of gene
amplification, which results in increased levels of protein
expression. In contrast, tumor suppressor genes are characterized
by inactivating changes in human cancer, typically point
mutations that result in truncation or functional inactivation of
the encoded protein, or gross deletions of chromosomal fragments
carrying these genes. Studies have identified genes that
sense damaged DNA and cause the arrest of the cell cycle, which
allows time for the molecular defect to be repaired. These genes
operate at several specific "checkpoints" in the cell cycle as a
means of ensuring genomic integrity before DNA is synthesized.
The most critical checkpoint gene yet identified that is
related to cancer in humans is the tumor suppressor gene _p53_.
This gene is not essential for cell viability, but it is critical
for monitoring damage to DNA. Inactivation of _p53_ is an early
step in the development of many kinds of tumors.
... ... M. Alam and D. Ratner (Columbia University, US) present a
review of the etiology and clinical aspects of squamous-cell
carcinoma of the skin, the authors making the following points:
1) The authors point out that non-melanoma skin cancer is
the most common cancer in the US, with over 1.3 million cases
expected to occur in the year 2001. Approximately 80 percent of
non-melanoma skin cancers are basal-cell carcinomas, and 20
percent are squamous-cell carcinomas. Squamous-cell carcinoma is
the second most common cancer among Whites. Unlike almost all
basal-cell carcinomas, cutaneous squamous-cell carcinomas are
associated with a substantial risk of metastasis.
2) The authors point out that in 1994 in the US, the
lifetime risk of squamous-cell carcinoma was 9 to 14 percent
among men and 4 to 9 percent among women. A sharp rise in
incidence during the past two decades has been documented.
According to longitudinal studies in both the US and Canada, the
age-adjusted incidence of squamous-cell carcinoma has grown by 50
to 200 percent over the past 10 to 30 years. In addition, the
incidence doubles with each 8- to 10-degree decrement in
geographic latitude and is highest at the Equator. The age-
adjusted incidence of this neoplasm among Whites is 100 to 150
per 100,000 persons per year, and the age-specific incidence
among persons over the age of 75 years is approximately 10 times
that rate.
3) The authors point out that exposure to ultraviolet
radiation is the most common cause of squamous-cell carcinoma of
the skin. Ultraviolet B radiation (wavelength 290 to 320
nanometers) from sunlight is principally responsible, with
ultraviolet A radiation (320 to 400 nanometers) adding to the
risk. Ultraviolet radiation produces mutations in DNA, usually
the formation of thymidine dimers in the p53 tumor-suppressor
gene. Failure to repair these mutations may result in tumor
formation.
5) The authors point out that lifestyle changes during the
past 50 years have led to increased voluntary exposure to
sunlight. Fair-skinned people are at the highest risk for the
development of skin cancer. A history of exposure to sunlight
during childhood, particularly a history of sunburns, may be the
most important behavioral risk factor for squamous-cell carcinoma
of the skin. Occupational exposure to ultraviolet radiation has
also been implicated. The relative risk of squamous-cell
carcinoma is 3 times as high among people born in areas that
receive high amounts of ultraviolet radiation from the Sun as
among people who move to those areas in adulthood; 2 to 5 times
as high in those with light skin, hazel or blue eyes, and blonde
or red hair as in those with darker features; 5 times as high
among those with outdoor occupations as among those who work
indoors.
6) The authors conclude: "Non-melanoma skin cancers,
including squamous-cell carcinoma, are largely preventable.
Unfortunately, changing leisure habits involving greater exposure
to sunlight have resulted in epidemic increases in the incidence
of cutaneous squamous-cell carcinoma. Physicians should emphasize
to their patients the prophylactic benefits of sun avoidance and
protection from sunlight, beginning in childhood, to minimize the
risk that this potentially life-threatening cancer will develop."
-----------
M. Alam and D. Ratner: Cutaneous squamous-cell carcinoma.
(New England J. Med. 29 Mar 01 344:975)
QY: Desiree Ratner: dr221@columbia.edu
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 27Apr01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
MEDICAL BIOLOGY: PATHOGENESIS OF UV INDUCTION OF MELANOMAS
Skin cancers, predominantly *basal-cell and *squamous-cell
*carcinomas, have accounted for an estimated 40 percent of all
cancers in the US in recent years, and the frequency of these
cancers has been increasing. The most common fatal skin cancer is
*melanoma, and this has also increased, indeed by a factor of
approximately 15 in the past 60 years. In 1997, more than 40,000
new cases of melanoma were diagnosed in the US, with more than
7200 fatalities. ... ... B.A. Gilchrest et al (Boston University,
US) present an extensive review of current research concerning
the pathogenesis of melanoma induced by UV radiation, the authors
making the following points:
1) The factors underlying the rapid increase in the
incidence of skin cancer are incompletely understood, but
increased total exposure to the Sun, and in the case of melanoma,
altered patterns of exposure, are strongly implicated.
2) The risk of melanoma is higher in fair-skinned people,
especially those with blond or red hair who sunburn and freckle
easily, than in people with darker complexions. The incidence of
melanoma among whites is inversely related to the latitude of
residence, with the world's highest incidence in Australia, a
subtropical country with a largely Celtic population. Conversely,
melanomas are uncommon in darker-skinned people.
3) The epidemiologic evidence implicating solar exposure in
the causation of melanoma is supported by biologic evidence that
damage caused by ultraviolet radiation, particularly damage to
DNA, plays a central part in the pathogenesis of these tumors.
4) Age plays a major part in vulnerability to
photocarcinogenesis. Aging (the passage of time) provides more
opportunities for the initiation of tumor formation (the
induction of mutations by exposure to UV radiation) and for the
promotion of tumor formation (the reparative cell proliferation
after exposure to UV radiation or after other skin injury). There
is an age-associated decrease in the capacity to repair DNA, and
a consequent increase in the rate of mutations of DNA. Moreover,
the rate of removal of UV-radiation-induced DNA photoproducts
from UV-irradiated skin decreases with age, especially during the
first two decades of life.
5) There has been considerable research addressing the
relative contributions of UV-B wavelengths (290-320 nm) and UV-A
wavelengths (320-400 nm) to photocarcinogenesis, particularly to
the development of melanoma. UV-B radiation is overwhelmingly
responsible for the formation of the principle DNA lesions,
cyclobutane pyrimidine dimers and pyrimidine (6-4) pyrimidone
photoproducts, whose incorrect repair leads to mutations.
However, UV-A radiation is far more abundant in sunlight than UV-
B radiation, and UV-A radiation causes oxidative DNA damage that
is also potentially mutagenic. UV-A radiation is also believed to
contribute substantially to immunosuppression, thus preventing
immunologic rejection of nascent UV-induced skin cancers.
6) Unlike the more common skin cancers, which are associated
with total cumulative exposure to UV radiation, melanomas are
associated with intense intermittent exposure. Thus, basal-cell
and squamous-cell carcinomas occur most commonly in maximally
solar-exposed areas of the body (e.g., face, back of hands,
forearms, and in persons with almost daily and substantial
lifetime exposure to UV radiation, such as farmers and sailors).
In contrast, melanoma occurs most commonly in areas of the body
exposed to the Sun intermittently, such as the back in men and
the lower legs in women, with relative sparing of more frequently
exposed sites such as the face, hands, and forearms. Melanoma is
most common in persons with predominantly indoor occupations
whose exposure to the Sun is limited to weekends and vacations.
Indeed the large increase in the incidence of melanoma in recent
decades has been attributed in part to the ability of large
numbers of people to travel long distances to obtain intense
exposure to the Sun in winter.
7) The authors propose a hypothesis to explain the
epidemiology of melanoma as compared with non-melanoma skin
cancer, the authors suggesting that the data predict that a high-
dose first exposure to the Sun after a prolonged period of Sun
avoidance will cause substantial damage to DNA in *melanocytes
and *keratinocytes, both of which at that time have a relatively
low base-line capacity for DNA repair and a low melanin content.
The authors propose that the final effect of UV radiation is not
attributable simply to the cumulative dose -- the arithmetic sum
of all individual exposures over a lifetime -- but rather may be
strongly influenced by the dose per exposure and by the pattern
of exposures.
8) The authors conclude: "Protection from the Sun is
critical to the prevention of both melanoma and non-melanoma skin
cancers, and protection is most effective when it is begun in
early childhood. It is especially important to protect against
intermittent Sun exposures, in order to reduce genomic damage at
a time of maximal cellular vulnerability and to reduce the risk
of melanoma."
----------
B.A. Gilchrest et al: The pathogenesis of melanoma induced by
ultraviolet radiation.
(New England J. Med. 29 Apr 99 340:1341)
QY: Barbara A. Gilchrest [bgilchre@bu.edu]
-----------
Text Notes:
... ... *basal-cell: A basal cell is a cell of the deepest layer
of stratified epithelium. In animals, including humans,
epithelial cells (epithelium) compose the cell layers that form
the interface between a tissue and the external environment, for
example, the cells of the skin, the lining of the intestinal
tract, and the lung airway passages. The term "stratified
epithelium" is a general term for epithelium composed of several
layers of different cell types.
... ... *squamous-cell: The cells of the epithelium are for the
most part closely packed cells with little extracellular material
between adjacent cells, the cells arranged in continuous sheets
in either single or multiple layers. The cells may be flat,
cubelike, columnar, or a combination of shapes, and "squamous"
cells are flattened and scalelike.
... ... *carcinomas: In general, a carcinoma is any malignancy
derived from epithelial tissue.
... ... *melanoma: Melanomas are a group of skin cancers
involving *melanocytes, cells found throughout the lower layers
of the skin.
... ... *melanocytes: (melanodendrocytes) Pigment-producing cells
located in the deepest (basal) layer of the skin (epidermis) with
branching processes by means of which melanin-containing bodies
(melanosomes) are transferred to epidermal cells with a resultant
pigmentation of the epidermis.
... ... *keratinocytes: "Keratinocyte" is a generic term for any
mammalian epithelial cell that produces keratin, a group of
proteins present in cuticular structures (e.g., hair, nails).
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 9Jul99
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
BIOLOGY OF CANCER: ON SQUAMOUS CELL CARCINOMA
Epithelial tissue (also called epithelium) forms the outer layer
of the skin and the outer layers of some internal organs, the
inner lining of blood vessels, ducts, body cavities, and the
interiors of the respiratory, digestive, urinary, and
reproductive systems. The cells of the epithelium are for the
most part closely packed cells with little extracellular material
between adjacent cells, the cells arranged in continuous sheets
in either single or multiple layers. The cells may be flat,
cubelike, columnar, or a combination of shapes, and "squamous"
cells are flattened and scalelike. One of the most common forms
of skin cancer is squamous cell carcinoma. These cells usually
arise from preexisting lesions on sun-exposed skin, and they have
a variable tendency to metastasize. Approximately 90 percent of
the cells in the skin are keratinocytes, cells that produce the
protein keratin, which helps waterproof and protect the skin and
underlying tissues. ... ... N.M. Wikonkal and D.E. Brash present
a review of the cellular biology of squamous cell carcinoma, and
the authors make the following points: 1) Ultraviolet light
absorbed by DNA molecules in keratinocytes is the inciting cause
of squamous cell carcinoma of the skin. 2) Failure of DNA repair
enzymes to correct the UV-induced damage can lead to mutations
when cells divide. Particular mutations in the *tumor suppressor
gene are characteristic of UV damage and are commonly found
in squamous cell carcinoma. 3) If mutations result in defective
p53 protein, the functions of this protein in *cell cycle control
and facilitation of *apoptosis will be absent, and this allows
*clonal expansion and eventual cancer. 4) A remarkable feature of
this type of cancer is that tumors closely resembling the human
clinical entity can be generated in the laboratory in mice,
allowing experiments that are not possible in humans. The
visibility and accessibility of squamous cell carcinoma of the
skin have made this tumor one of the best understood human
malignancies.
-----------
N.M. Wikonkal and D.E. Brash (Yale University, US)
Squamous Cell Carcinoma
(Science & Medicine Sep/Oct 1998)
QY: Norbert M. Wikonkal, Yale University, 203-432-4771.
-----------
Text Notes:
... ... *tumor suppressor gene: Tumor suppressor genes code for
proteins that apparently either prevent cell division or provoke
cell death in defective cells.
... ... *cell cycle control: In this context, the term "cell
cycle" refers to the entire life history of a single cell from
mitosis to mitosis, including the sequence of intervening phases.
... ... *apoptosis: In general, the term "apoptosis" refers to
programmed cell death, whether as a part of normal tissue
differentiation and development, or as a program activated in a
defective cell. In the molecular biology of cancer, apoptosis is
the name given to the programmed cell death provoked by the
proteins expressed by tumor suppressor genes. Thus, malignant
cells are defective cells with a deactivated apoptosis program,
and this allows malignant cells to survive and replicate.
... ... *clonal expansion: This refers to the expansion of a
population of cells all derived from repeated replications of
progeny of a single cell.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 13Novl98
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
SUNLIGHT EXPOSURE AND THE RISK OF EYE LENS OPACITIES
Despite advances in surgical procedures and targeted programs,
*cataract remains the leading cause of visual loss worldwide. In
the US, for example, cataract surgery is responsible for 12
percent of the Medicare budget (data for 1991). Exposure to *UV-B
radiation in sunlight has been shown to increase the risk of
cataract formation in high-risk occupational groups, but the risk
to the general population has not yet been quantified.
... ... West et al (7 authors at Johns Hopkins University, US)
report a study to determine the ocular exposure to UV-B radiation
in sunlight for a population of older persons, and to determine
the association between UV-B radiation and lens opacities. The
research group consisted of a total of 2520 community-dwelling 65
to 84 year old adults in Salisbury, Maryland (US) from 1993 to
1995. The study involves detailed quantitative assessment of all
apparently relevant factors, including the correlation variables.
The authors report that the odds of lens opacity increased with
increasing ocular exposure to UV-B (odds ratio = 1.10). Analysis
of the ocular dose by each age group after the age of 30 years
(based on questionnaires concerning past subject job history)
showed no vulnerable age group, suggesting damage is based on
cumulative exposure. The authors suggest their results add to the
growing body of knowledge that indicates even low levels of UV-B
radiation can harm the lens of the eye. The authors suggest
measures to avoid ocular exposure to UV-B radiation are simple:
the wearing of plastic glasses or sunglasses confers excellent
protection, and the mere wearing of a hat with a brim decreases
ocular exposure by 30 to 50 percent. The authors conclude: "These
measures should be part of any public health program to increase
awareness of sun damage and avoid unhealthy consequences."
QY: Sheila K. West: swest@dcpom.med.jhu.edu
(J. Amer. Med. Assoc. 26 Aug 98 280:714)
(Science-Week 11 Sep 98)
-------------------
Related Background:
... ... *cataract: In general, cataract is any developmental or
degenerative opacity of the lens of the eye. The cause of
degenerative cataract may be aging, exposure to x-rays, heat from
infrared exposure, systemic disease (e.g., diabetes mellitus),
various inflammations of the internal eye, and systemic
medication (e.g., corticosteroids).
... ... *UV-B radiation: In medical science, the ultraviolet part
of the spectrum is partitioned into 3 regions: UV-A is radiation
of wavelength 320-400 nanometers, causing skin tanning but only
weakly sunburn producing or carcinogenic; UV-B is radiation of
wavelength 290-320 nanometers, most effectively causing sunburn
and tanning, and carcinogenic for fair skin; UV-C is radiation of
wavelength 200-290 nanometers, and this radiation does not reach
the surface of the Earth from sunlight. UV-C radiation produced
by mercury arc lamps is germicidal, causes sunburn, and may cause
photokeratitis (light-induced inflammation of the cornea).
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 11Sep98
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ALARM AT INCREASED INCIDENCE OF MELANOMA CANCERS
Melanocytes, cells which synthesize inclusions of the dark
pigment melanin, are found in the skin, choroid of the eye, and
hair. Melanocyte cancers are called melanomas, and if they are of
the rapidly proliferating type, and not caught early, they have a
high fatality rate. At the present time, the incidence of
melanoma in the U.S. is increasing faster than any other cancer,
and the statistics seem incredible: 0.0007 in 1935, 0.004 in
1977, 0.01 in 1997. According to projections, in the U.S. this
year, 40,300 new cases will be diagnosed, and 7,300 people will
die from the disease. The majority of oncologists and
epidemiologists believe this striking increase in incidence is
real and not a function of ambiguous diagnostic techniques. They
point to the simple fact that deaths from melanoma have increased
along with the incidence, even though early diagnosis and
improved treatment methods have increased the 5-year survival
rate. The projected U.S. incidence of melanoma in the year 2000
is 0.013, which means at that time 1 in 75 people will expect to
be diagnosed with the disease.
(New York Times 6 Aug 97) (Science-Week 15 Aug 97)
For more information: http://scienceweek.com/swfr.htm
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7. IN FOCUS: ON THE DEVELOPMENT OF GENE CLONING
"In the late 1960s there was a sense of frustration among
scientists working in the field of molecular biology. Research
had developed to the point where progress was being hampered by
technical constraints, as the elegant experiments that had helped
to decipher the genetic code could not be extended to investigate
the gene in more detail. However, a number of developments
provided the necessary stimulus for gene manipulation to become a
reality. In 1967, the enzyme "DNA ligase" was isolated. This
enzyme can join two strands of DNA together, a prerequisite for
the construction of recombinant molecules, and can be regarded as
a sort of molecular glue. This was followed by the isolation of
the first "restriction enzyme" in 1970, a major milestone in the
development of genetic engineering. Restriction enzymes are
essentially molecular scissors, which cut DNA at precisely
defined sequences. Such enzymes can be used to produce fragments
of DNA that are suitable for joining to other fragments. Thus, by
1970, the basic tools required for the construction of
recombinant DNA were available. The first recombinant DNA
molecules were generated at Stanford University in 1972,
utilizing the cleavage properties of restriction enzymes
(scissors) and the ability of DNA ligase to join DNA strands
together (glue). The importance of these first tentative
experiments cannot be overestimated. Scientists could now join
different DNA molecules together, and could link the DNA of one
organism to that of a completely different organism. The
methodology was extended in 1973 by joining DNA fragments to the
plasmid pSC101, which is an extrachromosomal element isolated
from the bacterium Escherichia coli. These recombinant molecules
behaved as "replicons", i.e., they could replicate when
introduced into E. coli cells. Thus, by creating recombinant
molecules in vitro, and placing the construct in a bacterial cell
where it could replicate in vivo, specific fragments of DNA could
be isolated from bacterial colonies that formed clones (colonies
formed from a single cell, in which all cells are identical) when
grown on agar plates. This development marked the emergence of
the technology which became known as "gene cloning".
-----------
Desmond S.T. Nicholl: _An Introduction to Genetic Engineering_
(Cambridge University Press, Cambridge UK 1994, p.4)
-------------------
SCIENCE-WEEK http://scienceweek.com 27Apr01
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8. SCIENCEWEEK NOTES:
EARTH SCIENCE: THE MASSES OF EARTH'S COMPONENTS
1) In terms of mass, how does the biosphere rank as a component
of this planet? Here are the approximate numbers in kilograms:
mantle: 4.1 x 10^(24) kilograms
core: 1.9 x 10^(24)
continental crust: 1.6 x 10^(22)
oceanic crust: 7.0 x 10^(21)
hydrosphere: 1.4 x 10^(21)
atmosphere: 5.3 x 10^(18)
biomass: 1.0 x 10^(15)
2) And in terms of crust composition? Where does carbon rank in
crust composition in parts per million? Carbon, in fact, is
almost a trace element. Here are the approximate numbers, Earth's
crust composition in parts per million:
O 456,000
Si 273,000
Al 83,600
Fe 62,600
Ca 46,600
Mg 27,640
Na 22,700
K 18,400
Ti 6,320
H 1,520
P 1,120
F 544
S 340
C 180
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9. FROM THE SCIENCEWEEK ARCHIVE:
ASTROPHYSICS: ON THE COMPLEXITY OF THE DEATH OF STARS
In astronomy, the term "nebula" was originally applied to any
astronomical object that appeared fuzzy and extended in a
telescope, and over 100 such objects had already been catalogued
in the 18th century. The majority of these objects were later
identified as galaxies and star clusters. At the present time,
the term "nebula" (i.e., cloud) refers to a region of
interstellar gas and dust. "Emission nebulae" are bright diffuse
nebulae that emit light and other radiation as a result of
ionization and excitation of gas atoms by ultraviolet radiation,
the source of the UV usually one or more hot stars. A "planetary"
nebula is a nebula formed when a *red giant or *supergiant star
sheds its outer layers in the last stage of its evolution,
leaving a hot core that ionizes the expunged gas. The size of
planetary nebulae range from approximately the diameter of our
solar system to a light year across. Their lifetime is only about
10,000 years, and they are all expanding with speeds of
approximately 20 kilometers/second. In this context, the term
"planetary" has nothing to do with planets: the term is
historical, the first planetary nebulae discovered so named
because they gave the impression of planetary disks around stars
when viewed in small telescopes. The term "protoplanetary nebula"
has two separate meanings. In the context of this report, the
term refers to an early mass of gas and dust that will become a
planetary nebula. In the context of studies of planetary
formation, the term refers to the mass of gas and dust
surrounding young stars, the material from which planets will be
formed.
... ... Yervant Terzian (Cornell University) presents a review of
current research concerning planetary nebulae produced by the
death of stars, the author making the following points:
1) The images of planetary and protoplanetary nebulae
provided by the *Hubble Space Telescope have revealed previously
unsuspected morphological complexities of nebulae, and these new
complexities represent a challenge for researchers who study the
mechanisms of star death.
2) When a star similar to the Sun, with a mass of up to a
few solar masses, reaches the last stages of its evolution, the
star expands and becomes a relatively cool (e.g., 2500 degrees
kelvin) red giant, with a size so large that its outer perimeter
would include the orbit of Mars. Such a star loses mass in the
form of *stellar wind, followed by a more intense mass loss that
results from a "superwind". The star loses a substantial fraction
of its mass, and the ejected material forms a planetary nebula.
Following this, the stellar core contracts and becomes a "white
dwarf star", a star approximately the size of the Earth, with a
density of approximately 10^(7) times the density of the Earth,
and with a surface temperature of approximately 10^(5) degrees.
3) The material ejected from such a dying star initially
consists for the most part of atomic and molecular gas, which
partly coalesces shortly after ejection to form warm dust
particles. Eventually, the ultraviolet radiation of the hot white
dwarf ionizes the nebula, and the nebula becomes an emission
nebula. Within a few tens of thousands of years, the expanding
planetary nebula diffuses into the interstellar medium with a
velocity of approximately 20 kilometers per second.
4) The general stages of late stellar evolution are
reasonably well understood, but the mechanisms by which these old
stars eject their envelopes remain unknown, and researchers have
been unable to understand what factors contribute to creating any
particular nebular morphology. Not one of the images provided by
the Hubble Space Telescope shows a simple expanding bubble: most
objects have complex bipolar structures with central torii,
multipolar bubbles, jet-like filaments, globules, and in some
cases sets of circular rings. Many nebula show a detailed point
and mirror symmetry that can extend as much as 100,000
*astronomical units from the central star.
5) Hydrodynamical and magnetohydrodynamical simulations of
such dying stars show a multitude of morphologies, but the models
require magnetic field strengths and stellar rotations that are
not well understood in the context of present theory.
6) The author concludes: "The Hubble Space Telescope
observations have greatly enriched our knowledge about the death
of sun-like stars. The new discoveries illustrate the complexity
of stellar explosions near their deaths and pose a multitude of
new questions for observers and theorists alike."
-----------
Yervant Terzian: The complexity of stellar death
(Science 15 Oct 99 286:425)
QY: Yervant Terzian: terzian@astrosun.tn.cornell.edu
-----------
Text Notes:
... ... *red giant: A "red giant star" is a star in a late
stage of evolution, the star having exhausted the hydrogen fuel
in its core. It has a surface temperature of less than 4700
degrees Kelvin and a diameter 10 to 100 times that of the Sun.
... ... *supergiant star: A supergiant star is an extremely
luminous star of large diameter and low density. The diameter can
be as large as 1000 times that of our Sun.
... ... *Hubble Space Telescope: The Hubble Space Telescope was
launched from a space shuttle in 1990 into a 600-kilometer
low-Earth orbit and has been providing extensive imaging and
spectroscopic observations critical for the development of
astronomy and astrophysics. The new information has concerned hot
stars, stellar chromospheres and coronas, the interstellar
medium, galaxies and galactic clusters, quasars, etc. -- all of
it information uncorrupted by the Earth's atmosphere, which is
the problem for ground based telescopes.
... ... *stellar wind: In general, the term "stellar wind" refers
to the outflow of gas from the surface of a star. The Sun, for
example, loses approximately 10(-14) of its mass each year via
such a wind (solar wind).
... ... *astronomical units: (AU) 1 AU = the mean distance from
the Sun to the Earth = approximately 93 million miles, and
exactly 149,597,870 kilometers.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 24Dec99
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON PLANETARY NEBULA AND THE DEATH OF STARS
... In a review of planetary nebulae, Sun Kwon (University of
Calgary, CA) makes the following points: 1) During the final
10,000 years of their life, stars with masses up to 8 times the
mass of the Sun pass through a stage in which they produce
planetary nebulae. Such nebulae are not only bright in visible
light, but they are also radio, infrared, and x-ray sources.
Immediately preceding the first planetary nebula formation, there
is a progenitor phase called the "protoplanetary nebula", and
this entity has recently come into its own as a focus of
research. 2) A full-grown planetary nebula is completely ionized
by the UV light from its central hot star. The central star of a
protoplanetary nebula, however, is relatively cool and does not
emit UV radiation, so the nebula is not ionized and shines by
reflected light only (i.e., the protoplanetary nebula is not yet
an emission nebula). 3) The first protoplanetary nebulae were
observed in the 1970s, when it became apparent that many terminal
stage stars are obscured by dust and can be found only by
searching for emissions at mid-infrared wavelengths. More than
2000 mid-infrared stars in our Galaxy were thus identified. 4)
The classification of planetary nebula is based not on appearance
by on their emission spectra. Because planetary nebula expand
with time, their radio surface brightness decreases as the nebula
ages and becomes more diffuse. The youngest planetary nebula are
thus small and radio bright. 5) While all protoplanetary nebula
have similar infrared characteristics, they differ greatly from
one another in their visual brightness. But since protoplanetary
nebula are not ionized, whatever brightness they possess arises
from starlight reflected off the surrounding dust. A bright
central star typically outshines the small faint protoplanetary
nebula, so that protoplanetary nebulae are best identified when
the system is observed edge-on. 6) Planetary nebula often have
bipolar shapes, and the origin of this form has been a focus of
research. In 1978, Kwon et al proposed a stellar fast wind
hypothesis which has been successful in simulations and has had
some observational support. The author concludes: "While
planetary nebulae have been well-known objects for more than 200
years and have fascinated generations of astronomers, the nature
of their immediate progenitors [protoplanetary nebulae] was not
known until recently. At last, we are now filling in this
missing-link in our understanding of stellar evolution."
QY: Sun Kwon, University of Calgary, CA.
(Sky & Telescope October 1998) (Science-Week 4 Sep 98)
-----------
Text Notes:
... ... *red giant: See notes to previous report.
... ... *supergiant star: See notes to previous report.
-------------------
Related Background:
BIRTH AND EARLY EVOLUTION OF A PLANETARY NEBULA
White dwarf stars are extremely dense and compact stars that have
undergone gravitational collapse. White dwarfs are of great
interest to cosmologists, because it is believed their masses and
luminosities have little variance and they can thus be used as
"standard candles" to estimate distances. The final expulsion of
a gas by a star as it forms a planetary nebula (the ionized shell
of gas often observed surrounding a young white dwarf star) is
one of the most poorly understood stages of stellar evolution.
Particularly puzzling is how a spherical star can produce a
highly asymmetric nebula with collimated outflows (outflows
aligned parallel to a particular axis). ... ... Bobrowsky et al
(4 authors at 4 installations, US IN ES) now report optical
observations of the nebula surrounding the star He3-1357 (called
by the authors the "Stingray nebula"), a nebula that has
evidently become an ionized planetary nebula within the past few
decades. The authors find that the collimated outflows are
already evident, and they have identified the nebular structure
that focuses the outflows, and have also found a companion star,
which reinforces previous suspicions that binary companions play
an important role in shaping planetary nebulas and in changing
the direction of successive outflows. The authors suggest the
Stingray nebula demonstrates how far the nebular structure can
develop by the time the nebula becomes ionized, and that no other
planetary nebula in this phase of its evolution has been
previously identified.
QY: Matthew Bobrowsky: mattb@cta.com
(Nature 2 Apr 98) (Science-Week 24 Apr 98)
For more information: http://scienceweek.com/swfr.htm
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