ScienceWeek

SCIENCEWEEK

ScienceWeek
September 5, 2003
Vol. 7 Number 36A

An Online Digest of Research in the Sciences

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How fortunate for us that not every point
on the vault of heaven is beaming sunlight
upon the Earth!
-- H.W.M. Olbers (1758-1840)

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Section 1

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Part A - Symposium: Astrophysics: Supernovas

1. Introduction
2. Supernova Explosions in the Universe
3. Type Ia Supernovas: A Hydrogen Anomaly
4. Type II Supernovae and Interstellar Dust
5. Supernovas and Cosmic Rays
6. A Very Energetic Supernova Associated with a Gamma-Ray Burst
7. Supernovas and Cosmic Expansion
8. On Thermonuclear Supernovas
9. On the Enigma of the Beginning of the Supernova Explosion

Notices and Subscription Information

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Section 2

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1. INTRODUCTION

ON NOVAS AND SUPERNOVAS

From the name, you might think that a supernova is like a nova,
only bigger. That's true in a way -- but only in the same way
that a hydrogen bomb is like a firework, only bigger. Novae got
their name because to the astronomers of ancient times they
appeared to be "new" stars that suddenly flared into existence.
In fact, we now know that novae are temporary outbursts of much
fainter stars, which can often be seen using telescopes, and are
not new at all.

In a typical nova outburst, a star brightens by a factor of about
one hundred thousand in a few days, then fades away back to its
former level over a few months. In an ordinary galaxy like our
Milky Way, there are about twenty-five novae each year. They
occur in binary systems, where a white dwarf with a mass well
below the Chandrasekhar limit is in orbit around a red giant.
Material from the tenuous outer layers of the red giant is
attracted by the gravity of the white dwarf and falls onto its
surface, at a rate equivalent to about a billionth of the mass of
the Sun each year. There, the hydrogen and helium mixture from
the red giant builds up a layer on the surface of the white dwarf
until the pressure at the bottom of the layer is great enough to
cause an outburst of nuclear reactions, blowing the material away
into space as the star flares up. The process can then repeat
itself.

Impressive though the release of energy in a nova is by human
standards, it is peanuts compared with a supernova, which
releases a million times as much energy and briefly shines as
brightly as all the stars in a galaxy like the Milky Way put
together. It is literally, for a few weeks, as bright as a
hundred billion suns. Supernovae are much more rare than novae --
Tycho Brahe (1546-1601) saw one in our Galaxy in 1572, and
Johannes Kepler (1571-1630) saw one just thirty-two years later,
in 1604, but none has been seen in our Galaxy since then,
although in 1987 a supernova was observed in the Large Magellanic
Cloud, a small galaxy close to our Milky Way.

Indeed, supernovae are so rare that astronomers began to
appreciate their true nature only in the mid-1920s, with their
initial realization of just how big the Universe is. Until then,
it had still been possible to argue that the system we now call
the Milky Way Galaxy, a flattened disk of stars about 90 to 100
thousand light years across and containing a few hundred million
stars, was the entire Universe. Fuzzy patches of light in the
sky, called "nebulae", had been noticed long before this, but in
the early twentieth century nobody could establish unambiguously
whether these fuzzy blobs were clouds of material inside the
Milky Way, relatively small star systems (like clusters) in orbit
around the Milky Way, or (the most extreme possibility) entire
galaxies of stars, like the Milky Way, but so remote from us that
individual stars could not be picked out in them even with the
most powerful telescopes available.

Adapted from: John Gribbin: Stardust: Supernovae and Life -- The
Cosmic Connection. Yale University Press 2000, p.155.
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THE SUPERNOVA OF THE YEAR 1054

On July 4, in the year 1054, Chinese astronomers recorded what
they called a "guest star" in the constellation of Taurus, the
Bull. A star never before seen became brighter than any star in
the sky. Halfway around the world, in the American Southwest,
there was then a high culture, rich in astronomical tradition,
that also witnessed this brilliant new star. From carbon 14
dating of the remains of a charcoal fire, we know that in the
middle eleventh century some Anasazi, the antecedents of the Hopi
of today, were living under an overhanging ledge in what is today
New Mexico. One of them seems to have drawn on the cliff
overhang, protected from the weather, a picture of the new star.
Its position relative to the crescent Moon would have been just
as was depicted. There is also a handprint, perhaps the artist's
signature.

This remarkable star, 5000 light-years distant, is now called the
Crab Supernova, because an astronomer centuries later was
unaccountably reminded of a crab when looking at the explosion
remnant through his telescope. The Crab Nebula is the remains of
a massive star that blew itself up. The explosion was seen on
Earth with the naked eye for three months. Easily visible in
broad daylight, you could read by it at night. On the average, a
supernova occurs in a given galaxy about once every century.
During the lifetime of a typical galaxy, about ten billion years,
a hundred million stars will have exploded -- a great many, but
still only about one star in a thousand.

Adapted from: Carl Sagan: Cosmos. Random House 1980, p.235.
More information at:
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THE SUPERNOVAS OF THE 16TH AND 17TH CENTURIES

A great empirical challenge to Aristotle's cosmological hegemony
came with the opportune appearance, in the late sixteenth and
early seventeenth centuries, of two violently exploding stars --
what we today call supernovas. A star that undergoes such a
catastrophic detonation can increase a hundred million times in
brightness in a matter of days. Since only a tiny fraction of the
stars in the sky are visible without a telescope, supernovae
almost always seem to have appeared out of nowhere, in a region
of the sky where no star had previously been charted; hence the
name nova, for "new". Supernovae bright enough to be seen without
a telescope are rare; the next one after the seventeenth century
did not come until 1987, when a blue giant star exploded in the
Large Magellanic Cloud, a neighboring galaxy to the Milky Way, to
the delight of astronomers in Australia and the Chilean Andes.

The two supernovae that graced the Renaissance caused quite a
stir, inciting not only new sights but new ideas. Tycho Brahe
(1546-1601) spotted the supernova of 1572 on the evening of
November 11, while out taking a walk before dinner, and it
literally stopped him in his tracks. As he recalled the moment:

"Amazed, and as if astonished and stupefied, I stood still,
gazing for a certain length of time with my eyes fixed intently
upon it and noticing that same star placed close to the stars
which antiquity attributed to Cassiopeia. When I had satisfied
myself that no star of that kind had ever shone forth before, I
was led into such perplexity by the unbelievability of the thing
that I began to doubt the faith of my own eyes."

The next supernova came only thirty-two years later, in 1604.
Johannes Kepler (1571-1630) observed it for nearly a year before
it faded from view, and Galileo Galilei (1564-1642) lectured on
it to packed halls in Padua.

Scrutinized week by week through the pinholes and lens-less
sighting-tubes of the sixteenth- and seventeenth-century
astronomers, the two supernovae stayed riveted in the same spot
in the sky, and none revealed any shift in perspective when
triangulated by observers at widely separated locations. Clearly
the novae, too, belonged to the starry realm that Aristotle had
depicted as inalterable. Wrote Tycho of the 1572 supernova:

"That it is neither in the orbit of Saturn, nor in that of
Jupiter, nor in that of Mars, nor in that of any one of the other
planets, is hence evident, since after the lapse of several
months it has not advanced by its own motion a single minute from
that place in which I first saw it; which it must have done if it
were in some planetary orbit. Hence this new star is located
neither below the Moon, nor in the orbits of the seven wandering
stars but in the eighth sphere, among the other fixed stars."

The shock dealt to the Aristotelian world view could not have
been greater had the stars bent down and whispered in the
astronomers' ears. Clearly there was something new, not only
under the sun but beyond it.

Adapted from: Timothy Ferris: Coming of Age in the Milky Way.
William Morrow 1988, p.70.
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SUPERNOVA 1987A

On 23 February 1987, a four-century vigil by astronomers came to
an end. A star exploded right on our cosmic doorstep, as a
supernova bright enough to be seen by the unaided eye. But more
important, it was close enough to be accessible to all the
instruments of the new astronomy. The last naked eye supernova
was seen in 1604, a few years before Galileo turned his telescope
to the sky.

Despite the high-tech follow-up, this supernova was discovered by
nothing more sophisticated than the human eyeball and the
photographic plate. At the Las Campanas Observatory in Chile,
Canadian astronomer lan Shelton found the supernova as an unusual
star on a photographic plate of the Large Magellanic Cloud, the
Milky Way's neighbour galaxy.

The first signals from the supernova had actually reached the
Earth several hours earlier. They were detected in two huge
underground tanks of water, in Ohio and in Japan. At 41 seconds
past 7:35 am (UT) on 23 February 1987, a fusillade of flashes
began to fire in both tanks. Over the next 12 seconds, the
Japanese Kamiokande instrument recorded 12 flashes, while eight
were observed by the American Irvine-Michigan-Brookhaven (IMB)
tank.

The flashes were caused by a flood of penetrating neutrinos,
which hardly ever interact with matter. For the IMB detector to
stop eight neutrinos, a total of 300 million million neutrinos
must have passed through the tank.

Their energies revealed that these neutrinos were born in an
inferno with a temperature of 50,000 million degrees. They
provided the first direct view of the central core of a star
collapsing to become a neutron star. For the few seconds it
lasted, the flood of neutrinos from Supernova 1987A carried away
almost as much energy as the combined light of all the stars in
the observable Universe.

The energy from the collapsing core ripped through the star in a
matter of hours, blowing its outer layers into space. Astronomers
swung all the world's major telescopes and astronomy satellites
into action to observe this supernova.

Adapted from: N. Henbest and M. Marten: The New Astronomy. 2nd
Edition. Cambridge University Press 1996, p.94
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NOTES AND TERMINOLOGY

White dwarf stars are extremely dense and compact stars that have
undergone gravitational collapse. They are the final stage in the
evolution of low-mass stars after they have lost their outer
layers. White dwarf stars are about the size of Earth, but with a
mass about that of the Sun.

Chandrasekhar limit: The remnant mass after the blow-off during
the terminal stage of the life of a star determines the ultimate
fate of the star. If the remnant mass is less than 1.44 solar
masses (the Chandrasekhar limit for a star with no hydrogen
content), the star collapses into a white dwarf. If the remnant
mass is greater than 1.44 solar masses, depending on the remnant
mass, the star collapses into either a neutron star or a black
hole. Named after Subrahmanyan Chandrasekhar (1910-1995), who
first proposed the modern theory of stellar gravitational
collapse, and who received the Nobel Prize in Physics 1983.

If, following its terminal stages, the remnant mass of a star is
between 1.4 and 2 to 3 solar masses, the star will collapse into
a neutron star, a body with a radius of 10 to 15 kilometers, with
a core so dense that its component protons and electrons have
merged into neutrons. The average density of a neutron star is
10^(15) grams per cubic centimeter, and the weight of an object
on the surface of a neutron star would be 10^(11) its weight on
the surface of the Earth. Neutron stars apparently have an outer
shell of iron, but it is iron like no Earth iron, an iron of 4
orders of magnitude greater density.

If the terminal stages of star death leave a remnant star mass
greater than 3 solar-masses, the ultimate gravitational collapse
will produce a black hole, a relativistic singularity. A black
hole is a localized region of space from which neither matter nor
radiation can escape. The "trapping" occurs because the requisite
escape velocity, which can be calculated from the relevant
equations, exceeds the velocity of light and is therefore
unattainable. Another view of a black hole

Gamma rays are radiation of high energy, from about 10^(5)
electronvolts to more than 10^(14) electronvolts -- radiation
with the shortest wavelengths and highest frequencies, the gamma
ray region of the electromagnetic spectrum merging into the
adjacent lower energy x-ray region.

electronvolts: (eV) A unit of energy defined as the energy
acquired by an electron in falling through a potential difference
of 1 volt. 1 electronvolt = 1.602 x 10^(-19) joule.

In general, in this context, the term "standard candles" refers
to astronomical objects whose intrinsic brightness is known and
whose distance can therefore be calculated from apparent
brightness.

Redshift (symbol: z) is a lengthening of the wavelengths of
electromagnetic radiation from a source caused either by the
movement of the source (Doppler effect) or by the expansion of
the universe (cosmological redshift). Redshift is defined as the
change in wavelength of a particular spectral line divided by the
unshifted wavelength of that line. Large redshifts imply large
radial velocities (which imply large distances, according to
current cosmological theory), but at redshifts greater than about
0.2 there is a relativistic divergence from a linear relation. A
redshift of 4.0 corresponds to an object receding with a radial
velocity 92% that of the velocity of light. The largest
astrophysical redshifts so far observed are of the order of z =
5.

Hubble flow: In general, the outward motion of galaxies resulting
from the uniform expansion of the Universe, with all motions
lying in a radial direction from the observer, and with
velocities proportional to the distance of the galaxies. (Because
of mutual gravitational interactions between galaxies, the actual
pattern of galaxy motions is not precisely of this form.)

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2. SUPERNOVA EXPLOSIONS IN THE UNIVERSE

The following points are made by Adam Burrows (Nature 2000
403:727):

1) Supernovae are crucial to the dynamical and morphological
development of the Universe. They are also at the nexus of many
of the great debates now raging among astronomers. One subtype of
supernovae, the so-called type Ia, is now arguably astronomy's
most accurate probe of the scale and geometry of the Universe. An
unknown fraction of another subtype, the core-collapse
supernovae, may be the source of gamma-ray bursts. As major
sources of the elements of existence, supernovae themselves are
primary agents of stellar and galactic evolution. Supernovae and
gamma-ray bursts share the distinction of being the most powerful
explosions in the cosmos, and recent observational and
theoretical breakthroughs and a renewed appreciation of the
manifold roles of supernovae have inaugurated a new era in their
study.

2) It is by its death that the "purpose" of a massive star is
most clearly revealed. All stars are born, have thermonuclear
lives, and die, leaving behind tiny fossils. For most stars,
including our Sun, these fossils are or will be carbon/oxygen
white dwarfs with radii near that of the Earth, masses near 0.5
to 1.0 solar masses (sol-M), and central densities 10^(7) times
that of tungsten. Low-mass stars die and white dwarfs are born
slowly over hundreds to thousands of years through the ejection
of the dying star's heavy outer mantle. There is no explosion.
The dense white-dwarf residue, if in isolation, then cools into
obscurity.

3) In contrast, a star more massive than 8 sol-M does not go with
a whimper. Without the quietus of gentle mantle ejection, the
white dwarf that such a star creates in its core during its last
thermonuclear stages continues to evolve in composition and grow
in mass and density until it achieves the so-called
"Chandrasekhar" mass near 1.4 sol-M (1,2). At this mass, an iron
or oxygen–neon–magnesium white dwarf, normally supported against
gravity by electron degeneracy pressure, becomes sufficiently
unstable to collapse. Owing to the Pauli exclusion principle, at
the high densities achieved by massive white dwarfs, their
electrons are relativistic.

4) Unlike a non-relativistic gas, a relativistic gas has a soft
equation of state and is easily compressed by the ineluctable
forces of persistent gravity. Within one second, the core of a
star that may have lived for ten million years, cooking its
hydrogen into progressively heavier elements, implodes from
something the size of our planet to something the size of a city,
achieving densities in excess of that of the atomic nucleus and
velocities one-fourth the speed of light. At nuclear densities
(10^(13) times that of tungsten), matter is barely compressible
and the core bounces(3), rebounding into the infalling inner
mantle and, like a piston, generating a strong shock front that
with effort and a short delay overcomes the confining tamp of
imploding mantle mass in order to launch a supernova explosion.

5) The violent explosion disassembles the massive star, litters
the interstellar medium with freshly synthesized heavy elements
(such as oxygen, carbon, magnesium, silicon, calcium, sulfur, and
radioactive 56Ni), blows a many-parsec-sized hole in the
surrounding galactic gas, and announces itself with a luminous
display that can rival that of its parent galaxy for months. Its
fossil is most often a neutron star, twenty kilometers wide, with
an average density near that of an atomic nucleus, spinning with
a period of milliseconds to seconds, and possessing a surface
magnetic field of 10^(12) gauss. With the right combination of
period and field, this object is a radio pulsar. Astronomers have
discovered more than 1,000 such radio beacons in the Galaxy, each
of which was born in a supernova explosion(4). The famous Crab
pulsar is one such result, now pulsing in the radio, optical, and
X-ray frequencies with a period of 30 milliseconds and surrounded
by an X-ray emitting remnant of the supernova explosion, that
itself was witnessed by humans in AD 1054 to be as bright as
Venus in the night sky.(5)

6) In summary: During the lifetime of our Milky Way galaxy, there
have been something like 100 million supernova explosions, which
have enriched the Galaxy with the oxygen we breathe, the iron in
our cars, the calcium in our bones and the silicon in the rocks
beneath our feet. These exploding stars also influence the birth
of new stars and are the source of the energetic cosmic rays that
irradiate us on the Earth. The prodigious amount of energy
(10^(51) ergs, or 2.5 x 10^(28) megatons of TNT equivalent) and
momentum associated with each supernova may even have helped to
shape galaxies as they formed in the early Universe. Supernovae
are now being used to measure the geometry of the Universe, and
have recently been implicated in the decades-old mystery of the
origin of the gamma-ray bursts. Together with major conceptual
advances in our theoretical understanding of supernovae, these
developments have made supernovae the centre of attention in
astrophysics.

References (abridged):

1. Woosley, S. E. & Weaver, T. A. The evolution and explosion of
massive stars. II. Explosive hydrodynamics and nucleosynthesis.
Astrophys. J. Suppl. 101, 181-235 (1995)

2. Nomoto, K. & Hashimoto, M. Pre-supernova evolution of massive
stars. Phys. Rep. 163, 13-36 (1988)

3. Bethe, H. A., Brown, G. E., Applegate, J. & Lattimer, J. M.
Equation of state in the gravitational collapse of stars. Nucl.
Phys. A324, 487-533 (1979)

4. Taylor, J. H. & Cordes, J. M. Pulsar distances and the
galactic distribution of free electrons. Astrophys. J. 411, 674-
684 (1993)

5. Nomoto, K. Evolution of 8-10 solar mass stars toward electron
capture supernovae. I--Formation of electron-degenerate O + Ne +
Mg cores. Astrophys. J. 277, 791-805 (1984)

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3. TYPE IA SUPERNOVAS: A HYDROGEN ANOMALY

The following points are made by Eddie Baron (Nature 2003 424:628):

1) Over the past decade or so, type Ia supernovae have served as
useful cosmic beacons. Using them, astronomers have measured(1-3)
the value of the Hubble constant (which determines the rate of
expansion of the Universe) to an accuracy of 10%, and discovered
the existence of "dark energy"(4,5) (which is thought to drive
the acceleration of the rate of expansion). Even so, astronomers
are uncertain about the origins of these objects -- what
progenitor system leads to the creation of a type Ia supernova?

2) Supernovae are categorized according to the different chemical
elements identified in the spectra of the radiation they emit.
Typically, type Ia supernovae are bright, and their spectra show
no evidence of hydrogen. But Hamuy et al (Nature 2003 424:651)
report that they have detected hydrogen in the spectrum of the
supernova SN2002ic, which was discovered in November last year
(2002). Although in other respects SN2002ic fits the type Ia
classification, the presence of hydrogen in its vicinity sets it
apart from this class, but perhaps offers a clue to its
progenitor system.

3) Astronomers believe that type Ia supernovae are produced by
the thermonuclear explosion of a white dwarf -- a star that has
evolved so far that its electrons are cold and densely packed, a
phenomenon known as degeneracy. A white dwarf can explode when
its mass grows to a limiting value, called the Chandrasekhar mass
(roughly 1.4 times the mass of the Sun). To reach this limit, a
white dwarf must have a companion star to feed it. In the favored
picture -- the "single-degenerate scenario" -- this companion
star is an ordinary star in a late stage of evolution, probably
donating material in the form of hydrogen or helium.

4) An alternative theory (the "double-degenerate scenario")
postulates that the companion is another white dwarf, made of
carbon and oxygen, which donates material as it is ripped apart
by the tidal forces exerted by its more massive neighbor. In this
case, no hydrogen should be seen around the eventual supernova,
and until now this was true for all observed type Ia supernovae.
But detailed calculations have raised doubts about whether the
double-degenerate system would actually explode in a supernova,
and the single-degenerate scenario is now preferred by most
astronomers. The detection of hydrogen around SN2002ic lends
further support to the single-degenerate scenario.

References (abridged):

1. Parodi, B. R., Saha, A., Sandage, A. & Tammann, G. A.
Astrophys. J. 540, 634-651 (2000)

2. Freedman, W. L. et al. Astrophys. J. 553, 47-72 (2001)

3. Saha, A. et al. Astrophys. J. 562, 314-336 (2001)

4. Riess, A. et al. Astron. J. 116, 1009-1038 (1998)

5. Perlmutter, S. et al. Astrophys. J. 517, 565-586 (1999)

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AN ASYMPTOTIC-GIANT-BRANCH STAR IN THE PROGENITOR SYSTEM OF A
TYPE IA SUPERNOVA

The following points are made by Mario Hamuy et al (Nature 2003
424:651):

Stars that explode as supernovae come in two main classes. A type
Ia supernova is recognized by the absence of hydrogen and the
presence of elements such as silicon and sulfur in its spectrum;
this class of supernova is thought to produce the majority of
iron-peak elements in the Universe. They are also used as precise
"standard candles" to measure the distances to galaxies. While
there is general agreement that a type Ia supernova is produced
by an exploding white dwarf star, no progenitor system has ever
been directly observed. Significant effort has gone into
searching for circumstellar material to help discriminate between
the possible kinds of progenitor systems, but no such material
has hitherto been found associated with a type Ia supernova.

The authors report the presence of strong hydrogen emission
associated with the type Ia supernova SN2002ic, indicating the
presence of large amounts of circumstellar material. The authors
infer from this that the progenitor system contained a massive
asymptotic-giant-branch star that lost several solar masses of
hydrogen-rich gas before the supernova explosion.

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4. TYPE II SUPERNOVAE AND INTERSTELLAR DUST

The following points are made by Loretta Dunne et al (Nature 2003
424:285):

1) Large amounts of dust (>10^(8) sol-M) have recently been
discovered in high-redshift quasars(1,2) and galaxies(3-5)
corresponding to a time when the Universe was less than one-tenth
of its present age. The stellar winds produced by stars in the
late stages of their evolution (on the asymptotic giant branch of
the Hertzsprung–Russell diagram) are thought to be the main
source of dust in galaxies, but they cannot produce that dust on
a short enough timescale (<1 Gyr) to explain the results in the
high-redshift galaxies.

2) Supernova explosions of massive stars (type II) are also a
potential source, with models predicting 0.2 to 4 sol-M of dust.
As massive stars evolve rapidly, on timescales of a few million
years, these supernovae could be responsible for the high-
redshift dust.

3) Over the past three decades, many searches for dust in
supernova remnants have been made in the mid and far-infrared (6
to 100 microns). Remnants must be studied when they are young,
before they have swept up large masses of interstellar material
which makes it difficult to distinguish dust formed in the ejecta
from that present in the interstellar medium (ISM) prior to the
explosion. The handful of Galactic remnants which are both young
and close enough (Cas A, Kepler and Tycho) have been studied with
the Infrared Astronomical Satellite (IRAS) and the Infrared Space
Observatory (ISO), but although dust at 100 to 200 K has been
detected, the dust mass deduced is only 10^(-7) to 10^-(3) sol-M,
many orders of magnitude lower than the solar mass quantities
predicted.

4) The authors report the detection of 2 to 4M of cold dust in
the youngest known Galactic supernova remnant, Cassiopeia A. This
observation implies that supernovae are at least as important as
stellar winds in producing dust in our Galaxy and would have been
the dominant source of dust at high redshifts.

References (abridged):

1. Bertoldi, F. et al. Dust emission from the most distant
quasars. Astron. Astrophys. Lett. (in the press); also available
at http://www.arXiv.org/astro-ph/0305116 (2003)

2. Archibald, E. N. et al. A submillimeter survey of the star
formation history of radio galaxies. Mon. Not. R. Astron. Soc.
323, 417-444 (2001)

3. Hughes, D. H. et al. High-redshift star formation in the
Hubble Deep Field revealed by a submillimeter-wavelength survey.
Nature 394, 241-247 (1998)

4. Smail, I., Ivison, R. J. & Blain, A. W. A deep submillimeter
survey of lensing clusters: A new window on galaxy formation and
evolution. Astrophys. J. 490, L5-L8 (1997)

5. Dunne, L., Eales, S. A. & Edmunds, M. G. A census of metals at
high and low redshifts and the connection between submillimeter
sources and spheroid formation. Mon. Not. R. Astron. Soc. 341,
589-598 (2003)

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5. SUPERNOVAS AND COSMIC RAYS

The following points are made by Felix Aharonian (Nature 2002
416:797):

1) Cosmic rays are energetic particles, mainly protons, that
arrive at Earth from all directions in space. They were first
detected by Victor Hess (1883-1964) in 1912. But, despite
considerable experimental data and extensive theoretical efforts
over past decades, there is still no generally accepted view on
where cosmic rays are produced, or how they are accelerated to
such high energies, although there have been recent suggestive
observations(1).

2) The acceleration, accumulation and effective mixing of cosmic
rays, through their diffusion and convection in galactic magnetic
fields, produce a "sea" of energetic particles that pervades our
Galaxy. The corresponding pressure exerted by this sea is
surprisingly (but presumably not accidentally) close to the
pressure of the galactic magnetic field, as well as to that of
the interstellar gas. This implies that galactic cosmic rays have
an important role in the dynamical balance of our Galaxy and
influence interstellar chemistry through the heating and
ionization of interstellar gas.

3) But where do the cosmic rays come from, and how are they
accelerated? In 1933, Walter Baade (1893-1960( and Fritz Zwicky
(1898-1974) (2) suggested a link between cosmic rays and
supernovae. The main argument is that the energy needed to
maintain the galactic population of cosmic rays is estimated as
at least a few per cent of the total mechanical energy released
by supernova explosions in our Galaxy. This is an important, but
not sufficient, argument, given that other potential sources,
such as pulsars or young stars with energetic mechanical winds,
might also meet this energy requirement.

4) The second argument in favor of supernova remnants hinges on
the strong shock waves that emanate from them after the collapse
of the star. According to the diffusive-shock acceleration
mechanism, these shock waves could transfer 10–30% of the
mechanical energy of the supernova explosion (which is around
10^(51) erg; 1 erg = 10^(-7) joule) to protons, electrons and
nuclei in the surrounding interstellar gas. This mechanism can
explain the energy distribution of cosmic rays that has been
measured around the Earth(3-5).

References (abridged):

1. Enomoto, R. et al. Nature 416, 823-826 (2002)

2. Baade, W. & Zwicky, F. Phys. Rev. 45, 138 (1934)

3. Drury, L. O'C. et al. Space Sci. Rev. 99, 329-352 (2001)

3. Drury, L. O'C. et al. Space Sci. Rev. 99, 329-352 (2001)

4. Tanimori, T. et al. (CANGAROO collaboration) Astrophys. J.
497, L25-L28 (1998)

5. Aharonian, F. A. et al. (HEGRA collaboration) Astron.
Astrophys. 370, 112-120 (2001)

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6. A VERY ENERGETIC SUPERNOVA ASSOCIATED WITH A GAMMA-RAY BURST

The following points are made by Jens Hjorth et al (Nature 2003
423:847):

1) Over the past five years evidence has mounted that long-
duration (>2 s) gamma-ray bursts (GRBs) -- the most luminous of
all astronomical explosions -- signal the collapse of massive
stars in our Universe. This evidence was originally based on the
probable association of one unusual GRB with a supernova(1), but
now includes the association of GRBs with regions of massive star
formation in distant galaxies(2,3), the appearance of supernova-
like "bumps" in the optical afterglow light curves of several
bursts(4,5), and lines of freshly synthesized elements in the
spectra of a few X-ray afterglows. These observations support,
but do not yet conclusively demonstrate, the idea that long-
duration GRBs are associated with the deaths of massive stars,
presumably arising from core collapse.

2) The authors report evidence that a very energetic supernova (a
"hypernova") was temporally and spatially coincident with a GRB
at redshift z = 0.1685. The timing of the supernova indicates
that it exploded within a few days of the GRB, strongly
suggesting that core-collapse events can give rise to GRBs,
thereby favoring the "collapsar" model.

References (abridged):

1. Galama, T. J. et al. An unusual supernova in the error box of
the gamma-ray burst of 25 April 1998. Nature 395, 670-672 (1998)

2. Paczynski, B. Are gamma-ray bursts in star-forming regions?
Astrophys. J. 494, L45-L48 (1998)

3. Fruchter, A. S. et al. Hubble Space Telescope and Palomar
imaging of GRB 990123: Implications for the nature of gamma-ray
bursts and their hosts. Astrophys. J. 519, L13-L16 (1999)

4. Bloom, J. S. et al. The unusual afterglow of the -ray burst of
26 March 1998 as evidence for a supernova connection. Nature 401,
453-456 (1999)

5. Castro-Tirado, A. J. & Gorosabel, J. Optical observations of
GRB afterglows: GRB 970508 and GRB 980326 revisited. Astron.
Astrophys. Suppl. Ser. 138, 449-450 (1999)

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7. SUPERNOVAS AND COSMIC EXPANSION

Robert P. Kirshner (Science 2003 300:1914)

1) Observations of exploding stars halfway back to the Big Bang
reveal a surprising phenomenon: The expansion of the universe has
been speeding up in the past 7 billion years. We attribute this
effect to the presence of a dark energy, whose energy density
helps make the universe flat and whose negative pressure produces
cosmic acceleration. On the basis of observations of supernova
brightness, of the dark matter that makes galaxies cluster, and
of the angular scale of primordial freckles in the glow from the
cosmic microwave background (CMB), we infer that about 28% of the
universe is matter and 72% is dark energy. In the self-proclaimed
age of "precision cosmology", we know the amount of each
component to a few percent, but in the spirit of "honest
cosmology" we also have to admit we do not know precisely what
either of them is. But we are not helpless. We can observe light
emitted by supernova explosions to trace the history of cosmic
expansion to learn more about the invisible forces that shape the
universe.

2) Evidence for the nature of the dark energy comes from the
observed brightness of a particular class of supernova explosions
called type Ia supernovae (SN Ias). Defined empirically from
their spectra (1), these events mark the thermonuclear
destruction of white dwarf stars. A white dwarf, stable when
solitary up to 1.4 solar masses, can accrete matter from a
companion when it is in a binary system. A white dwarf in a
binary will explode violently, destroying the star, when accreted
mass provokes the carbon and oxygen in its interior to erupt in a
runaway thermonuclear explosion (2,3).

2) SN Ias are infrequent events, erupting roughly once per
century in a galaxy, and found in all types of galaxies. SN Ias
are useful for probing the history of cosmic expansion and the
nature of dark energy because they are very bright, typically
about 4 x 10^(9) times the luminosity of the Sun. With careful
measurements of the color and the apparent brightness during the
month when an SN Ia shines most brightly, the distance to an
individual explosion can be derived to better than 10% (4,5).

3) This precision makes SN Ias the best standard candles in
extragalactic astronomy: Observations of nearby and bright SN Ias
help determine the present rate of cosmic expansion, the Hubble
constant. Observations of the brightness and spectra of these
objects measure the relation between distance and redshift for
the Universe. The redshifts of supernovae at different distances
reveal changes in the rate of cosmic expansion that have
developed while the light was in flight to us from explosions
over 7 billion light-years away.

4) In summary: Supernova observations show that the expansion of
the Universe has been speeding up. This unexpected acceleration
is ascribed to a dark energy that pervades space. Supernova data,
combined with other observations, indicate that the Universe is
about 14 billion years old and is composed of about 30% matter
and 70% dark energy. New observational programs can trace the
history of cosmic expansion more precisely and over a larger span
of time than has been done to date to learn whether the dark
energy is a modern version of Einstein's cosmological constant or
another form of dark energy that changes with time. Either
conclusion is an enigma that points to gaps in our fundamental
understanding of gravity.

References (abridged):

1. A. V. Filippenko, Ann. Rev. Astron. Astrophys. 35, 209 (1997)

2. J. Whelan, I. Iben, Astrophys. J. 186, 1007 (1973)

3. K. Nomoto, F.-K. Thielemann, J. C. Wheeler, Astrophys. J. 297,
L23 (1984)

4. M. M. Phillips, Astrophys. J. 413, L105 (1993)

5. A. G. Riess, W. H. Press, R. P. Kirshner, Astrophys. J. 473,
88 (1995)

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8. ON THERMONUCLEAR SUPERNOVAS

The following points are made by V.N. Gamezo et al (Science 2002
299:77):

1) According to observations and models, many stars that steadily
burn their nuclear fuel for millions or billions of years
suddenly end their lives with a powerful explosion that produces
a bright object called a supernova. A supernova explosion can be
powered either by the gravitational energy released during the
core collapse of a massive star or by the nuclear energy released
by explosive thermonuclear burning of a star. The authors focus
on thermonuclear supernovae that belong to the type Ia (SN Ia) in
the observation-based classification (1-3).

2) Thermonuclear supernovae are produced by explosions of white
dwarfs (WDs), small dense stars composed of carbon and oxygen
nuclei and detached degenerate electrons (1, 3-5). The term
"degenerate" means that electrons occupy all possible quantum
states below a certain energy. The hydrostatic equilibrium in a
WD is supported for the most part by the degenerate electron
pressure that does not depend on temperature. WDs form at the end
of the evolution of stars whose original masses are less than 8
solar masses (sol-M). A star can lose a large fraction of its
material by ejecting outer layers into space at the final stages
of evolution.

3) The mass of a remaining WD is always less than the
Chandrasekhar limit, 1.4 sol-M, above which a hydrostatic
equilibrium of degenerate matter is impossible. An isolated
carbon-oxygen WD is stable and almost inert, because its
temperature is not high enough to induce any substantial nuclear
reactions. This isolated dead star can exist almost indefinitely,
slowly cooling down as it radiates its energy into space.
Observations show, however, that more than 50% of all stars are
not isolated. They belong to groups of two or more stars that
orbit a common center of mass. In a close binary system, a WD can
increase its own mass by accreting material from its companion
star. Such systems are considered to be the most probable SN Ia
progenitors, even though the exact nature of the companion star
and the details of the mass accretion are still unclear (1, 3-5.

4) When the mass of the WD approaches the Chandrasekhar limit,
any small mass increase results in a substantial contraction of
the star, and the material near its center is compressed. This
increases the temperature and accelerates thermonuclear reactions
near the center. The released energy further increases the
temperature, thus further accelerating thermonuclear reactions.
This process is slowed by the neutrino cooling and by the
convective and conductive heat exchange. Nevertheless, the
temperature in the WD center rises and reaches the point where
the energy release overwhelms the energy outflow. In an ordinary
nondegenerate star, the energy release would be stabilized by a
thermal expansion accompanied by the work against gravity.

5) In a WD, however, the initial temperature increase does not
affect the degenerate electron pressure and therefore does not
lead to any substantial expansion that could slow down
thermonuclear reactions and prevent the runaway process.
Eventually, the temperature increases to the level where the
thermal and the degenerate electron pressure components become
comparable, and the material begins to expand. At that time,
however, the expansion is already unable to quench the fast
thermonuclear burning ignited in the center of a WD. The
thermonuclear runaway mechanism in degenerate matter was first
described in (11) and now is a key component of all plausible SN
Ia scenarios (1, 4-9). Depending on the scenario, the ignition
may occur near the center, off center, or in outer layers of the
star.

References (abridged):

1. J. C. Wheeler and R. P. Harkness, Rep. Prog. Phys. 53, 1467
(1990)

2. A. V. Filippenko, Annu. Rev. Astron. Astrophys. 35, 309 (1997)

3. J. C. Wheeler, Am. J. Phys., in press.

4. S. E. Woosley and T. A. Weaver, Annu. Rev. Astron. Astrophys.
24, 205 (1986)

5. D. Branch and A. M. Khokhlov, Phys. Rep. 256, 53 (1995)

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9. ON THE ENIGMA OF THE BEGINNING OF THE SUPERNOVA EXPLOSION

H-Th. Janka (Science 2002 297:1134)

1) Once every second, somewhere in the Universe a massive star is
disrupted in a supernova explosion. Visible even at cosmic
distances, these stellar catastrophes provide valuable
information about the history of star formation in the Universe.
Ejecting several solar masses of stellar debris, they enrich the
interstellar medium with heavy elements from millions of years of
quiescent nuclear burning, and with radioactive nuclei that are
freshly synthesized during the star's violent death.

2) As brilliant as it may be, a supernova explosion is only a
weak side effect of a much more energetic event. Theory suggests
that as the iron core of the exploding star collapses to form a
neutron star or black hole, most of the gravitational binding
energy is carried away by neutrinos. This prediction was
confirmed by the detection of two dozen of the 10^(58) neutrinos
from Supernova 1987A in the underground experiments of
Kamiokande, Irvine-Michigan-Brookhaven, and Baksan. Typically,
only 1% of the released energy goes into kinetic energy of the
ejecta, and only a small fraction of this energy is converted to
electromagnetic radiation.

3) How is energy transferred from the collapsing compact remnant
to the matter that gets ejected? Understanding this driving force
of the explosion is crucial for predicting remnant masses,
explosion energies, and nucleosynthetic yields. It is thus
essential for establishing the theoretical link between the
properties of massive stars and the observables of supernova
explosions. Unfortunately, observations have so far been unable
to constrain the processes that take place in the collapsed core
of a star.

4) Future measurement platforms may provide the required data by
allowing thousands of neutrinos and possibly gravitational waves
to be measured in a future supernova in our Galaxy. But current
knowledge is based mainly on numerical simulations and analytic
analysis. Despite more than 30 years of research and increasingly
detailed computer models, there is still no satisfactory
understanding of the start of the explosion.(1-5)

References (abridged):

1. S. A. Colgate, R. H. White, Astrophys. J. 143, 626 (1966)

2. H. A. Bethe, J. R. Wilson, Astrophys. J. 295, 14 (1985)

3. J. R. Wilson, R. W. Mayle, Phys. Rep. 227, 97 (1993)

4. R.W. Mayle et al, Astrophys. J. 418, 398 (1993)

5. M. Herant et al., Astrophys. J. 435, 339 (1994)

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