ASTROPHYSICS

Astrophysics is the branch of physics that attempts to understand the structure and evolution, appearance and behavior, of astronomical objects, especially those outside the solar system, including stars and galaxies, assemblages of these, and the material between them. Study of the universe as a whole is sometimes included but is more often given the separate name cosmology. Subdisciplines include (1) nuclear astrophysics, focused on nuclear reactions as energy sources in stars and as synthesizers of the chemical elements, nearly all of which are made in stars and stellar explosions; (2) high-energy astrophysics, which studies objects such as supernovae, pulsars, quasars, X-ray sources, radio galaxies, and astrophysical black holes (where the energy per particle or photon or the total energy is much larger than in typical stars and galaxies, and where strong magnetic fields, motions at close to the speed of light, and strong gravitational fields often occur); (3) particle astrophysics, whose main topic is the evidence for the existence and behavior of particles other than protons, neutrons, electrons, and photons (light) in stars, galaxies, and the universe; and (4) plasma astrophysics, which is concerned with low-density, very hot gases in the coronae of our sun and other stars, in the ejecta from supernovae and other cosmic explosions, and in interplanetary, interstellar, and intergalactic medium.

Stars and Nucleosynthesis

Stars, of which the Sun is quite typical, derive most of their energy from nuclear reactions, predominately the fusion of hydrogen to helium. They have lifetimes ranging from a few million years, for the biggest and brightest, to about 10 billion years for solar type stars, and up to 100 billion years or more for the tiniest and faintest. Most stars, indeed most of the matter in the universe, is, by weight, about three-quarters hydrogen and one-quarter helium, with only 1 or 2 percent of all the other elements. Oxygen, carbon, neon, magnesium, silicon, iron, sulfur, and nitrogen are the most abundant (notice they are also the biologically important ones), and uranium, thorium, and tantalum are among the rarest. The vast majority of stars are smaller and fainter than the Sun. The ones seen on a dark night are not a fair sample and include many of the rare, very bright stars, which are far away from the Earth.

Stars form, usually in groups or clusters, from clouds of gas and dust in the interstellar medium of galaxies. The gas is mostly molecular hydrogen, and the dust is mostly carbon compounds, silicates, and ices. The clouds are turbulent and pervaded by magnetic fields. The details of star formation involve complex interactions among rotation, gas flow, turbulence, and magnetic fields and are not completely understood.

In contrast, theoretical understanding of the structure and evolution of stars, once they have formed, is on quite solid ground, based in laboratory measurements of nuclear reaction rates and of how atoms and molecules behave when light shines on them. The structure is expressed in a set of differential equations, which can be solved numerically and integrated forward in time to describe changes in the brightness and colors of stars as hydrogen is gradually transformed to helium. The Sun is currently about 30 percent brighter than when it formed and began hydrogen fusion or burning, and about five billion years in the future, it will become still brighter but redder (a red giant). It will end by fusing helium to carbon (C) and oxygen (O) and puffing off its outer layer to leave a dense core of C and O, called a white dwarf.

More massive stars continue on to the fusion of carbon, neon, oxygen, and silicon, producing cores of nickel, iron, cobalt, and other intermediate mass elements, which then collapse, leaving a neutron star (so-called because it is made mostly of neutrons, with only about 1 percent protons and electrons) or a black hole (so-called because its escape velocity exceeds the speed of light c , and light cannot get out).

The most abundant elements, hydrogen and helium, are (mostly) left from the early universe. Those from carbon to germanium in the periodic table are made by fusion reactions from which evolved, massive stars derive some of their energy. Lithium, beryllium, and boron are produced when cosmic rays strike atoms of carbon, nitrogen, and oxygen in the interstellar medium. And all of the elements heavier than iron and its neighbors in the periodic table come from the capture of neutrons by iron "seeds," followed by beta decays, late in the lives of stars and in their explosions. Energy is absorbed in these reactions, and the elements so made are all rather rare (hence the price of gold).

Stellar astrophysics is, for the most part, a consumer of information from atomic, nuclear, and particle physics, though it has been a donor in the past. For instance, it showed that fusion reactions must be possible, and it also showed that atoms like carbon have particular properties, before these properties had been measured in the laboratory. During the first half of the twentieth century, several elementary particles were recognized among cosmic ray secondaries before they had been produced on Earth. Close to the end of the century, it became clear that the deficit of neutrinos coming to the Earth from the Sun, relative to the numbers calculated, had been trying for more than 30 years say something about neutrinos, rather than about the Sun.

White Dwarfs, Neutron Stars, and Stellar Black Holes

White dwarfs, neutron stars, and black holes are the three expected end points of stellar evolution. Table 1 compares some of their properties. There is observational evidence for all of them, including single white dwarfs and neutron stars, and ones with normal stars as binary companions, bound to them by gravitation, and for black holes in binary systems. Single black holes must surely exist, but unless one came very close to the solar system or passed across our line of sight to a normal star and bent the light rays coming to us, we would not be aware of them. There have been tentative detections of a few such gravitational lensing events, implying black holes with masses about six times that of the Sun, close to what is found in the black hole binaries.

Single neutron stars are most conspicuous when they are young and so have strong magnetic fields and short rotation periods. These cause beams of radio waves (and sometimes visible light and X rays) to sweep around like searchlight beams, which sometimes intersect the Earth. They are called pulsars. The best known is part of the remnant of a supernova explosion, seen in 1054 c.e., and called the Crab Nebula.

When white dwarfs, neutron stars, or black holes have binary companions, their masses can be measured, using Newton's laws of gravity, the same way as ordinary stars. Indeed having a mass too large for a neutron star (more than about three times that of the Sun) is part of the "signature" of a black hole. In addition, because the three are all very compact, material falling down onto or into them gets very hot and can radiate brightly, often at X-ray wavelengths.

A binary companion can supply gas to be accreted, some of which may also undergo nuclear reactions. The combination of accretion energy and nuclear energy is responsible for the many kinds of astronomical events and sources that occur in these binaries. Examples are nova explosions, X-ray binaries, some kinds of supernovae, and (probably) gamma-ray bursters. All of these are bright enough to be studied throughout our own galaxy and in those nearby; the supernovae and gamma-ray bursters can be seen even when they are in very distant galaxies (but they are rare).

The measured masses, sizes, rotation periods, and surface temperatures of some neutron stars are very close to the maximum or minimum possible according to calculations. It has been suggested that

TABLE 1

the more extreme cases may be made not of pure neutrons but, at least at the centers, primarily of pions or even strange quarks.

Galaxies and Dark Matter

Galaxies as now seen are assemblages of stars—anything from a million to 10¹² or more—often with 5 to 25 percent residual gas that is still forming stars. Galaxies come in a couple of characteristic shapes, called elliptical and spiral, and many others are irregular in appearance, either because there has not been time for gravitational processes to smooth them out or because they have been involved in a collision or near miss with another galaxy in the past few hundred million years. Both sorts of irregular galaxies are seen to be more common when looking back into the past by looking far away. This and other evidence indicate that galaxy formation was largely complete billions of years ago, though interactions (and star formation) are still going on. Small elliptical galaxies (called dwarf spheroidals) are the commonest sort of galaxy. Large spirals are the prettiest and so the kind most often shown in pictures.

Our galaxy is the Milky Way, a large spiral galaxy. It is part of a group including another large spiral (the Andromeda Nebula) and about three dozen smaller galaxies, called the Local Group (LG). The LG, in turn, is on the outskirts of a supercluster, whose center is a cluster of more than 1,000 large galaxies. It can be seen by looking through the star pattern of Virgo and so is called the Virgo Cluster. Hierarchical structure of galaxies, groups, clusters, and superclusters is typical. There seem to be no isolated galaxies (nor, for that matter, no more than a very few isolated stars, probably kicked out of their galaxies).

The largest structures have sizes about ten times our distance from the Virgo cluster (100 to 200 megaparsecs in the units actually used in astrophysics or 300 to 600 light-years in the units of science fiction and some well-meaning introductory books). The large-scale distribution of galaxies and clusters is more like sheets and filaments in a honeycomb, sponge, or foam than like matzoh balls in chicken soup. The space between the galaxies is very empty, with a density less than that of the space between the stars by a factor of 100,000 or more. On the other hand, while stars in the Milky Way are separated by distances that are millions of times their own sizes, galaxies in clusters are separated by only about ten times their diameters.

The process of galaxy formation is much less well understood than is star formation because there is a critical part of the physics about which very little is known. From 1922 onward, astronomers have gradually become aware, and forced themselves to accept, that the total masses of galaxies, clusters, and superclusters are very much larger than would have been supposed by adding up the masses of the stars and gas that contribute to their emission of light, radio waves, and X rays. This so-called dark matter reveals itself by exerting gravitational forces on the stars and galaxies we see, on the gas between them, and even on the very light rays that pass through clusters of galaxies (a process called gravitational lensing). The evidence therefore comes from measuring the motions of stars and galaxies, from the temperatures of X-ray emitting gas in elliptical galaxies and clusters, and from images of lensed galaxies and quasars (Figure 1).

The evidence is both internally consistent and quite strong for quasars. Alternative interpretations of the data have been attempted, but involve significant changes in the behavior of gravitation, which would have to differ from the familiar Newtonian and Einsteinian (general relativistic) versions very profoundly and in ways that probably disagree with other kinds of astrophysical data. The gradual increase in ratio of dark to luminous material as one looks on larger and larger scales, from cores of galaxies to their outskirts and to pairs, groups, clusters, and superclusters, means that the dark stuff is spread more uniformly through space than is the luminous stuff. Indeed, it would have been reasonable to guess that there would be still more dark matter entirely outside the largest structures seen. This does not seem to be the case.

Strangely, although the evidence for dark matter is very compelling, very little is known of its nature or behavior. This is a major stumbling block in trying to model the formation of galaxies and larger-scale structures, since these must arise by gravitational amplification of subtle variations in the density of matter when the universe was young. And most of the gravitational

FIGURE 1

attraction will come from the dark matter, whatever it is, because there is more of it than there is of ordinary matter. Thus it is possible to provide a table of dark matter candidates (Table 2), but no actual laboratory examples of dark matter, though searches for various sorts have been under way since about 1980.

Of course old white dwarfs and brown dwarfs (like stars but too small in mass for nuclear reactions to light them up) also exist, but there are too few of them to constitute most of the dark matter. Similarly, the evidence for small, but nonzero, rest mass for neutrinos is quite strong, but they are also not most of the dark matter. This leaves the field for the "cold" dark matter candidates (axions or WIMPs, also called neutralinos, lowest-mass-supersymmetric-particles, etc.), topological defects like monopoles, and the cosmological constant (also called quintessence).

Quasars, Other Active Galaxies, and Galactic Black Holes

The light of normal galaxies comes largely from stars and so is emitted by the whole body of the galaxy. About 1 percent of galaxies (at present, but much larger billions of years ago) display much larger luminosities in light, X rays, radio waves, or some combination, that either comes directly from a tiny core or nucleus of the galaxy or is clearly powered by streams of fast-moving particles from the nucleus. These are the active galaxies.

The main types of active galaxies are radio galaxies (ellipticals with blobs of radio-emitting plasma far outside their visible limits, connected to the nuclei by jets), quasars (also strong radio sources but with cores emitting more visible light than all the rest of the galaxy by a factor of 100 or more), quasi-stellar objects (QSO) (like quasars but without radio emission), blazars (QSOs with very rapid changes in luminosity and structure), and Seyfert galaxies (spirals with cores about as bright as the rest of the galaxy).

When the radio galaxies and quasars were first recognized between 1954 and 1963, it was suggested that they might be collisions between normal galaxies and galaxies made of antimatter. This would have been very important as evidence about the symmetry between matter and antimatter, but it was the wrong answer because it predicted strong emission in gamma rays, which is not seen. The rapid firing of many supernova explosions in galactic centers also did not fit all the observations. What has turned out to work is the presence of a massive black hole at the center of an active galaxy, accreting stars and gas from its surroundings and using some of the energy that is released by the accretion to accelerate relativistic particles and amplify magnetic fields. These, in turn, radiate the photons we see.

Evidence for the black holes and their masses (from about a million to perhaps as much as 10 billion solar masses) comes from (1) the large luminosities, (2) the rapid variability, (3) the strong concentration of light in a cusp at the very center, and (4) the large velocities of stars and gas in that central cusp. Simple calculations show that a quasar can last for at most 1 percent of the age of the universe, and observations show that they were much

TABLE 2

commoner in the past. Thus "dead quasars" (that is, black holes that are not currently accreting very much and so are not very bright) are expected inside many normal galaxies. The strongest evidence comes from the Milky Way, where the motions of the stars and gas near the center indicate that there is a central core of about three million solar masses, too compact to be anything except a single, massive black hole.

See also:Big Bang; Cosmic Microwave Background Radiation; Cosmic Rays; Cosmological Constant; Cosmology; Hubble Constant; Inflation; Neutrino, Solar; Supernovae

Bibliography

Cox, A. N. Allen's Astrophysical Quantities, 4th ed. (Springer-Verlag, New York, 2000).

Lang, K. R. Astrophysical Data, 2nd ed. (Springer-Verlag, New York, 1992).

Lang, K. R. Astrophysical Formulae, 3rd ed. (Springer-Verlag, New York, 1999).

Trimble, V. "Existence and Nature of Dark Matter in the Universe." Annual Review of Astronomy and Astrophysics25 , 425–472 (1987).

Virginia Trimble