ScienceWeek

ASTRONOMY AND ASTROPHYSICS

EDWIN HUBBLE (1889-1953)

At the time of his death, Edwin Hubble (1889-1953) was the most eminent and celebrated observational astronomer in the world, a man as well-known to the public as to the scientific community. Essentially, Hubble made 3 major contributions during his scientific career: 1) He provided the first evidence (1923) of the existence of galaxies other than our own; 2) he proposed (in the 1920s) a classification of galaxies as ellipticals, spirals, and *barred spirals, a classification scheme still in use; 3) he provided observational evidence (1928) for the recession of galaxies (the expansion of the Universe), and he proposed (1929) what is known as "Hubble's Law" relating the observed recession rate of galaxies to their distance. The first and last of these contributions have certainly been a most important part of the foundation of 20th century cosmology. In much of his telescope observational work, Hubble was assisted by Milton Humason (1891-1972), an accomplished astronomer who was something of a Michael Faraday in American astronomy: Humason had little formal education and he first worked for Hubble as a janitor at the Mount Wilson Observatory (*Note #1).

The following points are made by Gale E. Christianson (Astronomy February 1999):

1) Hubble was 6 feet 2 inches tall, weighed 190 pounds, constantly smoked a pipe, favored a tightly-belted military trenchcoat, sported an English accent acquired during his days as a Rhodes scholar at Oxford University (where he studied law, not astronomy), and had a special observatory wardrobe consisting of knickers, jodhpurs, high-topped military boots, and a *Norfolk jacket. The author, in fact, states that Hubble was disappointed when he arrived in France during World War I only days before the November armistice, "ending his dreams of leading men into battle."

2) Hubble settled at the Mount Wilson Observatory in 1919, and immediately began working with the 100-inch Hooker telescope. In 1923, working with the Hooker telescope, Hubble discovered *Cepheid variable stars in the Andromeda *nebula M31 (later to be called the Andromeda galaxy). Using the well-known period-luminosity relationship established for Cepheid variables, Hubble estimated the distance of the Andromeda galaxy as 300,000 *parsecs from Earth, a distance which apparently astounded everyone.

3) Following the Andromeda galaxy work, in search of an understanding of the formation and evolution of galaxies, Hubble proposed what is now known as the "tuning fork diagram", a galaxy classification scheme: the handle of the tuning fork consists of an evolving sequence of elliptical galaxies (from spherical to true elliptical), with one arm of the tuning fork diagramming the evolution of spiral galaxies, and the other arm of the tuning fork diagramming the evolution of the "barred" spiral galaxies.

4) In 1928, Hubble and Humason began a systematic study of the *redshifts of galaxies, and in 1929 this resulted in the proposal that the distances and recessional speeds of the nebulae (galaxies) are in direct proportion to each other ("Hubble's Law"): double the distance to a galaxy and the speed of recession doubles; triple the distance and the speed triples [*Note #2].

5) In 1931, Einstein visited Hubble in Pasadena, and apparently when Hubble was working at the observatory, Grace Hubble, Edwin Hubble's wife, drove Einstein to his meetings and appointments. The author concludes: "He [Einstein] was silent sometimes, and sometimes he would talk in French or English, for Grace knew no German. One afternoon he broke his silence to say, 'Your husband's work is beautiful.'" (Editor's note: It was Hubble's demonstration of the apparent expansion of the Universe that caused Einstein to call his rather ad hoc introduction of the so-called "cosmological constant" into his relativity equations "the greatest aesthetic blunder of my life." Einstein's relativity model proposed a non-expanding Universe essentially held static by the cosmological constant term.).

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Notes:

barred spirals: In general, a barred spiral galaxy is a type of galaxy with spiral arms extending from an almost rectangular or cigar-shaped bar of stars across its central region.

Norfolk jacket: From Norfolk UK (c. 1866). A loose-fitting belted single-breasted jacket with box pleats.

Cepheid variable stars: These are variable stars that pulsate periodically, expanding and contracting with as much as a 30% change in size in each cycle, with a typical average luminosity about 10,000 times that of our Sun. In 1912, Henrietta Swan Leavitt discovered a simple relationship between the period of light variation and the absolute magnitude of a Cepheid variable. This relationship, called the "period-luminosity law", enabled the calculation of distances to the stars in our own galaxy and to the stars in other galaxies. In 1952 it was discovered that there are two types of Cepheid variables, which meant an error had been introduced in the earlier calculations of distances, and when the correction was made, the apparent size of the universe abruptly doubled. During 1908-1912, Leavitt (1868-1928), a graduate of Radcliffe College on the staff of the Harvard Observatory, discovered 2400 variable stars, doubling the number known in her time. In the early years of stellar spectroscopy, particularly at the Harvard Astronomical Observatory, nearly all the data was catalogued and analyzed by female astronomers, called "computers", who were forbidden because of their sex to use the telescopes. It is an irony of the social history of science that the work of such female astronomers as Henrietta Swan Leavitt and Annie Jump Cannon (1863-1941) came to be of greater significance than the work of many of the male astronomers who considered these female astronomers to be no more than menial assistants.

nebula: Before Hubble, all of the fuzzy astronomical objects that appeared in telescopes, many of which are now known to be galaxies, were thought to be clumps of gas and dust. There are indeed clumps of gas and dust everywhere in the Universe, and they are correctly termed "nebulae". But since the work of Hubble, which means since the 1920s, the use of the term "nebula" for a galaxy is obsolete and incorrect.

parsecs: 1 parsec equals 3.262 light-years, or 30.86 x 10^(12) kilometers.

Note #1: In general, the Hubble tuning fork diagram contains 10 galaxy categories. An 11th category, "irregular galaxies", is usually diagrammed as an extending tine between the two tuning fork arms. The result is a 3-prong "pitchfork" diagram, rather than a "tuning fork" diagram.

redshifts: 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 = 4.9. The furthest galaxy on record is at a redshift z=4.92), which implies a distance of approximately 13 billion light years.

Note #2: Humason measured the speed of recession of approximately 800 galaxies. In 1956, three years after the death of Hubble, Humason and others refined Hubble's Law, making the proportionality constant (Hubble constant) in the relationship between recession velocity and distance essentially time-dependent. The "Hubble constant" is thus more properly called the "Hubble parameter".

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PLANETARY SCIENCE

The following developments in the history of planetary science are noted by David J. Stevenson (Science 11 Feb 00 287:997):

Ancient times: Observers notice that although most of the stars seen in the sky at night move en masse, certain stars appear to wander. These stars were called "the wanderers", which yields from the Greek the term "planets".

1519: Ferdinand Magellan (1480-1521) circumnavigates the globe.

1543: In his /De Revolutionibus Orbium Coelestium/, Nicolaus Copernicus (1473-1543) proposes that Earth and the other planets travel around the Sun.

1600: The Inquisition burns Giordano Bruno (1548-1600) at the stake, perhaps partly for his belief that the Earth revolves around the Sun, and partly for his belief in an infinite number of inhabited worlds.

1608: Hans Lippershey (1570-1619) invents the telescope.

1609: Johannes Kepler (1571-1630) publishes /Astronomia Nova/containing the first two laws of planetary motion.

1610: Galileo Galilei (1564-1642) discovers the moons of Jupiter, the "handles" (rings) of Saturn, the phases of Venus, and sunspots.

1671: Jean Dominique Cassini (1625-1712) calculates distances from the Sun of all planets then known.

1687: Isaac Newton (1642-1727) publishes the _Principia Mathematica_, establishing the three laws of motion and the law of universal gravitation.

1755: Immanuel Kant (1724-1804) proposes the Solar System arose from a vast nebula of material.

1781: William Herschel (1738-1822) discovers the planet Uranus.

1795: James Hutton (1726-1797) publishes /Theory of the Earth/, in which he argues that all apparent geological features emerge from observable changes unfolding over great expanses of time. The theory is called "uniformitarianism".

1797: James Hall (1761-1832) demonstrates that igneous rock forms crystalline rock upon cooling.

1798: Henry Cavendish (1731-1810) determines the mass of the Earth as 6.6 x 10^(21) tons.

1830: Charles Lyell (1797-1875) publishes the first volume of his uniformitarianism work /The Principles of Geology/.

1837: Louis Agassiz (1807-1873) proposes the idea of an ice age, that at one time glaciers covered Europe.

1846: Johann Galle (1812-1910) discovers the planet Neptune, the discovery based on earlier calculated predictions by others.

1857: James Clerk Maxwell (1831-1879) demonstrates theoretically that the rings of Saturn consist of small particles that do not coalesce.

1859: Gustav Kirchhoff (1824-1887) and Robert Bunsen (1811-1899) introduce spectroscopy to chemistry and use it to infer the chemistry of the Sun.

1907: Bertram Boltwood (1870-1927) combines information on the half-life of uranium and the proportion of lead found within uranium deposits to estimate the age of the Earth at 2.2 billion years.

1912: Alfred Wegener (1880-1930) proposes the idea of "continental drift".

1919: Joseph Larmor (1857-1942) develops the idea of self-exciting dynamos inside the Earth and the Sun to account for their magnetic fields.

1930: Clyde Tombaugh (1906-1997) discovers the planet Pluto.

1931: Harold Urey (1893-1981) deduces that hydrogen probably has isotopes, and then discovers deuterium with a spectroscopic technique that becomes important for cosmochemical studies.

1937: Grote Reber constructs the first radio telescope (9.4 meters in diameter).

1950: Jan Oort (1900-1992) suggests that a distant shell of comets surrounds the Solar System.

1958: James van Allen demonstrates the value of satellite-based studies when he uses data from a particle counter on Explorer IV to discover Earth's magnetosphere.

1960s: The US and the Soviet Union begin an epoch of planetary exploration using satellites that eventually reaches every object in the Solar System larger than the Moon.

1969: Human beings land on the Moon.

1973: First images of Jupiter transmitted from close vicinity by Pioneer 10.

1974: First images of Mercury transmitted from close vicinity by Mariner 10.

1976: Viking space probes land on Mars.

1979: Voyagers 1 and 2 relay flyby data from Jupiter and some of its moons, and from Saturn, Uranus, Neptune, and some of their moons.

1986: Space shuttle Challenger explodes soon after launch, killing all 7 crew members.

1990: Hubble Space Telescope is placed in orbit, and soon begins to produce "a relentless stream of spectacular imagery."

1992: Alexander Wolszczan and Dale Frail discover two Earth-sized planets orbiting a pulsar.

1994: Comet Shoemaker-Levy crashes into Jupiter.

1995: Michel Mayor and Didier Queloz discover the first planet around a Sun-like star -- 51 Pegasi. An era of extrasolar planetary discovery begins.

1999: Evidence for extrasolar planets accumulates.

The author suggests that the future of planetary science is likely to emerge from three intertwined trends: 1) the search for extrasolar planets; 2) the search for life elsewhere, and for the origin of life; and 3) the search for a fully integrated view of planets in general and our planet in particular.

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ON PULSARS AND MAGNETARS

In astrophysics, the term "nova" refers to a class of exploding stars whose luminosity temporarily increases from several thousand to as much as 10^(5) times its normal level. Most novas are thought to involve special double-star systems ("close binaries"), one star a red giant and the other star a white dwarf. If an expansion of the red giant encroaches the gravitational domain of the white dwarf, the intense gravitational field of the white dwarf pulls material from the red giant, and this material accumulates on the surface of the white dwarf until a nuclear explosion occurs.

The term "supernova" refers to a completely different phenomenon: a supernova is any of a class of violently exploding stars whose luminosity after eruption suddenly increases millions or billions of times its normal level, the supernova explosion, unlike a nova explosion, a cataclysmic event associated with the essential end of the active (energy-generating) life of the star. In general, the cause of the explosion is believed to be as follows:

a) When a star exhausts its nuclear fuel, the star undergoes gravitational collapse, this collapse resulting in one of 3 possible objects, depending on the mass of the collapsing star: black hole, neutron star, white dwarf. If the mass is greater than approximately 3 solar-masses, a black hole will result; if the mass if less than 3 but more than approximately 1.4 solar-masses, a neutron star will result; is the mass is less than approximately 1.4 solar-masses, a white dwarf star will result.

b) The core of a collapsing star of intermediate mass (1.4 to 3 solar-masses) soon consists almost entirely of neutrons, the core with a diameter of only approximately 20 kilometers, and with a mass equal to at least several solar-masses -- the result a core of enormous density.

c) It is believed that a supernova explosion occurs when material falling in from the outer layers of the star rebounds off the dense core, which has stopped collapsing and now presents a hard surface (iron of enormous density) to the infalling gases. The shock wave generated by this collision propagates outward and blows off the outer layers of the star. When a star "goes supernova", material equaling the material of several Suns may be blasted into space with enough energy so that the supernova outshines its entire home galaxy.

The existence of neutron stars was first proposed by Lev Landau (1908-1968) in 1932, and their relationship to supernovas was first suggested by W. Baade (1893-1960) and F. Zwicky (1898-1974) in 1934, who wrote the following famous sentence in a short paper: "With all reserve we advance the view that a supernova represents the transition of an ordinary star into a neutron star consisting mainly of neutrons." It took 33 years for the first apparent evidence of neutron stars to be obtained -- the objects called "pulsars" -- a discovery made by A. Hewish and J. Bell in 1967 [see *Note #1 on (Susan) Jocelyn Bell]. Meanwhile, in 1939, J. Robert Oppenheimer (1904-1967) and others developed the idea of neutron star production in the cores of supernovas into an important theory of stellar evolution.

A "pulsar" (pulsating radio star) is any of a class of cosmic objects that emit extremely regular pulses of radio waves, with several such objects known to emit pulses of visible light, x-rays, and gamma-rays as well. In general, pulsars are believed to be rapidly rotating neutron stars. It is believed that neutrons at the surface of the neutron star decay into protons and electrons, and as these charged particles are released from the surface, they enter an intense magnetic field surrounding the star and rotate with the star. The particles accelerate to speeds approaching the velocity of light, and the particles give off electromagnetic radiation by "synchrotron emission" -- the electromagnetic radiation emitted by charged particles moving in a magnetic field at a velocity close to that of light. This radiation is released as intense beams from the magnetic poles of the pulsar, and since the magnetic poles do not coincide with the rotational poles, the emitted radiation beam is rotated and sweeps regularly past the Earth with each complete rotation (like the rotating beam of a light-house), the result an evenly-spaced series of pulses detected by ground-based telescopes. At present, approximately 600 pulsars have been identified.

On August 27, 1998, a tremendous burst of gamma-rays and x-rays, the burst lasting approximately 5 minutes, impacted the Earth, the burst powerful enough to produce noticeable ionization of the Earth's atmosphere. The x-rays were found to vary with a 5.16 second period, precisely the same as that of a known active x-ray source in a galaxy 20,000 light-years from Earth in the constellation Aquila. Such x-ray sources are believed to be highly magnetic rotating neutron stars, and it was suggested that the burst was caused by a "starquake" on a neutron star with an intense magnetic field possibly 10^(15) times larger than that of Earth. Such stellar objects were named "magnetars", and one proposal is that a magnetar's enormous magnetic field occasionally cracks open the crust of the star, and this leads in some way to the production of energetic charged particles and gamma-rays.

The following points are made by Jim Cordes (Nature 18 Jan 01 409:296):

1) The author points out that most neutron stars are detected as "ordinary" radio pulsars with magnetic fields of 10^(12) gauss --10^(12) times as strong as the magnetic field of Earth -- and spin periods between 16 milliseconds and 8.5 seconds. The rapid rotation combines with the strong magnetic field to produce electric forces that generate particles moving near the speed of light. These relativistic particles radiate intense electromagnetic waves directed along the magnetic poles of the neutron star, which appear to an observer as pulses of radiation as the star rotates. The magnetic field also slows the rotation of the pulsar down through "magnetic braking", an effect caused by the radiation carrying away the angular momentum of the star. Ordinary pulsars remain radio "loud" for approximately 10 million years, the time it takes for the pulsar to slow down to a spin rate at which particle creation stops.

2) During the past 5 years, considerable interest has focused on objects that appear to be even more highly magnetized than typical radio pulsars. These are the so-called "magnetars", objects with magnetic fields that range from approximately 10^(13) to 10^(15) gauss. A few magnetars have been identified in x-ray and gamma-ray observations. Magnetars spin down much more rapidly than radio pulsars, on timescales of 10,000 years.

3) A combination of data from two x-ray satellites recently led to the discovery of a very young pulsar with an unusually high magnetic field (E. Gotthelf et al: Astrophys. J. Lett. 542:37 2000). This x-ray pulsar is associated with the supernova remnant Kesteven 75. Although neutron stars are thought to be created in supernova explosions, there are surprisingly few clear examples of this. Gotthelf et al suggest that the unusual x-ray pulsar observed by them may be a missing link between different classes of pulsars, and that this new pulsar could provide important clues to understanding how particles are created and radiate in the magnetospheres surrounding neutron stars. Cordes states: "It may turn out that many of the neutron stars in our Galaxy are born with properties similar to this pulsar rather than to the bulk of previously known neutron stars."

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Notes:

Note #1: It is generally agreed that (Susan) Jocelyn Bell, who was 24 years old and Hewish's graduate student at the time (1967), made the actual discovery of the first pulsar by noticing unexplained pulses in radio telescope data contained in 100-foot lengths per day of paper charts, and that her discovery was instrumental in Hewish winning his Nobel Prize in Physics in 1974, which he shared with Martin Ryle (1918-1984), a prime figure in the development of radio-telescope astronomy. The Bell discovery was made while Bell, Hewish, and Ryle were at Cambridge University (UK), and the astronomer Martin Rees, who was of the faculty at Cambridge at that time, writes of Jocelyn Bell as follows: "Jocelyn Bell received less than her fair share of credit for the discovery of pulsars. This happened, I think, because of the social pressures which (then even more than now) impeded women's careers, and lowered their scientific aspirations. After getting her PhD, Jocelyn Bell left active research for several years -- giving priority to her husband's career seemed at that time the 'natural' thing to do. Had she instead continued, and acquired 'visibility' by joining the small cohort of radio astronomers who, over the next few years [after 1967] consolidated our knowledge of pulsars and discovered many more -- as, almost certainly, a /man/ with her extraordinary initial record would have done -- it is hard to believe her achievements would have been slighted to the same extent." [Martin Rees: Before the Beginning, (1997) p.263]. It has been suggested that in an earlier age CP-1919, the first observed pulsar, would have been called "Bell's Star". No matter the name of the first observed pulsar, it was discovered by Susan Jocelyn Bell (now Susan Jocelyn Bell Burnell), and there are many who believe that the Nobel Prize in Physics of 1974 should read Ryle, Hewish, and Bell. Bell Burnell, however, disagrees, and she has stated: "Nobel prizes are based on long-standing research, not on flash-in-the-pan observation by a research student. The award to me would have debased the prize."