The Milky Way and Beyond (38 page)

Although the first 20 years of quasar studies were noted more for controversy and mystery than for progress in understanding, subsequent years finally saw a solution to the questions raised by these strange objects. It is now clear that quasars are extreme examples of energetic galaxy nuclei. The amount of radiation emitted by such a nucleus overwhelms the light from the rest of the galaxy, so only very special observational techniques can reveal the galaxy's existence.

A quasar has many remarkable properties. Although it is extremely small (only the size of the solar system), it emits up to 100 times as much radiation as an entire galaxy. It is a complex mixture of very hot gas, cooler gas and dust, and particles that emit synchrotron radiation. Its brightness often varies over short periods—days or even hours. The galaxy underlying the brilliant image of a quasar may be fairly normal in some of its properties except for the superficial large-scale effects of the quasar at its centre. Quasars apparently are powered by the same mechanism attributed to radio galaxies. They demonstrate in an extreme way what a supermassive object at the centre of a galaxy can do.

With the gradual recognition of the causes of the quasar phenomenon has come an equally gradual realization that they are simply extreme examples of a process that can be observed in more familiar objects. The black holes that are thought to inhabit the cores of the quasar galaxies are similar to, though more explosive than, those that appear to occur in certain unusual nearer galaxies known as Seyfert galaxies. The radio galaxies fall in between. The reason for the differences in the level of activity is apparently related to the source of the gas and stars that are falling into the centres of such objects, providing the black holes with fuel. In the case of quasars, evidence suggests that an encounter with another galaxy, which causes the latter to be tidally destroyed and its matter to fall into the centre of the more massive quasar galaxy, may be the cause of its activity. As the material approaches the black hole, it is greatly accelerated, and some of it is expelled by
the prevailing high temperatures and drastically rapid motions. This process probably also explains the impressive but lower-level activity in the nuclei of radio and Seyfert galaxies. The captured mass may be of lesser amount—i.e., either a smaller galaxy or a portion of the host galaxy itself. Quasars are more common in that part of the universe observed to have redshifts of about 2, meaning that they were more common about 10 or so billion years ago than they are now, which is at least partly a result of the higher density of galaxies at that time.

In the 1970s a new type of object was identified as using orbiting gamma-ray detectors. These “gamma-ray bursters” are identified by extremely energetic bursts of gamma radiation that last only seconds. In some cases the bursters are clearly identified with very distant galaxies, implying immense energies in the bursts. Possibly these are the explosions of “hypernovae,” posited to be far more energetic than supernovae and which require some extreme kind of event, such as the merging of two neutron stars.

These galaxies and galaxy clusters are some of the most astronomically important. They range from the Small Magellanic Cloud to the vast Great Attractor.

The great spiral Andromeda Galaxy (M31) is the nearest external galaxy (except for the Magellanic Clouds, which are companions of the Milky Way Galaxy, in which Earth is located). The Andromeda Galaxy is one of the few visible to the unaided eye, appearing as a milky blur. It is located about 2,480,000 light-years from Earth; its diameter is approximately 200,000 light-years; and it shares various characteristics with the Milky Way system. It was mentioned as early as 965 CE, in the
Book of the Fixed Stars
, by the Islamic astronomer
ạ
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, and rediscovered in 1612, shortly after the invention of the telescope, by the German astronomer Simon Marius, who said it resembled the light of a candle seen through a horn. For centuries astronomers regarded the Andromeda Galaxy as a component of the Milky Way Galaxy—i.e., as a so-called spiral nebula much like other glowing masses of gas within the local galactic system (hence the misnomer Andromeda Nebula). Only in the 1920s did the American astronomer Edwin Powell Hubble determine conclusively that the Andromeda was in fact a separate galaxy beyond the Milky Way.

The Andromeda Galaxy has a past involving collisions with and accretion of other galaxies. Its peculiar close companion, M32, shows a structure that indicates that it was formerly a normal, more massive galaxy that lost much of its outer parts and possibly all of its globular clusters to M31 in a past encounter. Deep surveys of
the outer parts of the Andromeda Galaxy have revealed huge coherent structures of star streams and clouds, with properties indicating that these include the outer remnants of smaller galaxies “eaten” by the giant central galaxy, as well as clouds of M31 stars ejected by the strong tidal forces of the collision.

The Coma cluster is the nearest rich cluster of galaxies; it contains thousands of systems. The Coma cluster lies about 33 million light-years away, about seven times farther than the Virgo cluster, in the direction of the constellation Coma Berenices. The main body of the Coma cluster has a diameter of about 2.5 10
⁷
light-years, but enhancements above the background can be traced out to a supercluster of a diameter of about 2 10
⁸
light-years. Ellipticals or S0s constitute 85 percent of the bright galaxies in the Coma cluster; the two brightest ellipticals in Coma are located near the centre of the system and are individually more than 10 times as luminous as the Andromeda Galaxy. These galaxies have a swarm of smaller companions orbiting them and may have grown to their bloated sizes by a process of “galactic cannibalism” like that hypothesized to explain the supergiant elliptical cD systems.

The spatial distribution of galaxies in rich clusters such as the Coma cluster closely resembles what one would expect theoretically for a bound set of bodies moving in the collective gravitational field of the system. Yet, if one measures the dispersion of random velocities of the Coma galaxies about the mean, one finds that it amounts to almost 900 km per second (500 miles per second). For a galaxy possessing this random velocity along a typical line of sight to be gravitationally bound within the known dimensions of the cluster requires Coma to have a total mass of about 5 10
¹⁵
solar masses. The total luminosity of the Coma cluster is measured to be about 3 10
¹³
solar luminosities; therefore, the mass-to-light ratio in solar units required to explain Coma as a bound system exceeds by an order of magnitude what can be reasonably ascribed to the known stellar populations. A similar situation exists for every rich cluster that has been examined in detail. When Swiss astronomer Fritz Zwicky discovered this discrepancy in 1933, he inferred that much of the Coma cluster was made of nonluminous matter. The existence of nonluminous matter, or “dark matter,” was later confirmed in the 1970s by American astronomers Vera Rubin and W. Kent Ford.

Cygnus A is the most powerful cosmic source of radio waves known, lying in the northern constellation Cygnus about 500,000,000 light-years (4.8 × 10
²¹
km [3 × 10
²¹
miles]) from Earth. It has the appearance of a double galaxy. For a time it was thought to be two galaxies in collision, but the energy output is too large to be accounted for in that way. Radio energy is emitted from Cygnus A at an estimated 10
⁴⁵
ergs per second, more than 10
¹¹
times the rate at which energy of all kinds is emitted by the Sun. The source of the energy of Cygnus A remains undetermined.

The Great Attractor is a proposed concentration of mass that influences the movement of many galaxies, including the Milky Way. In 1986 a group of astronomers observing the motions of the Milky Way and neighbouring galaxies noted that the galaxies were moving toward the
Hydra-Centaurus superclusters in the southern sky with velocities significantly different from those predicted by the expansion of the universe in accordance with the Hubble law. One possible explanation for this perturbation in the Hubble flow is the existence of the so-called Great Attractor—a region or structure of huge mass (equivalent to tens of thousands of galaxies) exerting a gravitational pull on the surrounding galaxies. It is estimated that the Great Attractor would have a diameter of about 300 million light-years and that its centre would lie about 147 million light-years away from Earth.

The Magellanic Clouds are two satellite galaxies of the Milky Way Galaxy, the vast star system of which Earth is a minor component. These companion galaxies were named for the Portuguese navigator Ferdinand Magellan, whose crew discovered them during the first voyage around the world (1519–22).

The Magellanic Clouds are irregular galaxies that share a gaseous envelope and lie about 22° apart in the sky near the south celestial pole. One of them, the Large Magellanic Cloud (LMC), is a luminous patch about 5° in diameter, and the other, the Small Magellanic Cloud (SMC), measures less than 2° across. The Magellanic Clouds are visible to the unaided eye in the Southern Hemisphere, but they cannot be observed from the northern latitudes. The LMC is about 160,000 light-years from Earth, and the SMC lies 190,000 light-years away. The LMC and SMC are 14,000 and 7,000 light-years in diameter, respectively, and are smaller than the Milky Way Galaxy, which is about 140,000 light-years across.

The Magellanic Clouds were formed at about the same time as the Milky Way Galaxy, approximately 13 billion years ago. They are presently captured in orbits around the Milky Way Galaxy and have experienced several tidal encounters with each other and with the Galaxy. They contain numerous young stars and star clusters, as well as some much older stars. The Magellanic Clouds serve as excellent laboratories for the study of very active stellar formation and evolution. With the Hubble Space Telescope it is possible for astronomers to study the kinds of stars, star clusters, and nebulae that previously could be observed in great detail only in the Milky Way Galaxy.

The M81 group of more than 40 galaxies is found at a distance of 12 million light-years from Earth, one of the nearest galaxy groups to the Local Group (the group of galaxies that includes the Milky Way Galaxy). The dominant galaxy in the M81 group is the spiral galaxy M81. Much like the Andromeda and Milky Way galaxies, M81 is of Hubble type Sb and luminosity class II.

There are two subgroups in the M81 group: one group is associated with
M81 and another is associated with the spiral galaxy NGC 2403. These two subgroups are moving toward each other. The total mass of the M81 group has been determined from the motion of galaxies within it to be 1 10
¹²
solar masses. M81 has a mass of 6.7 10
¹¹
solar masses.

The M81 group also has a few galaxies with classifications similar to those of galaxies in the Local Group, and it was noticed by some astronomers that the linear sizes of the largest H II regions (which are illuminated by many OB stars) in these galaxies had about the same intrinsic sizes as their counterparts in the Local Group. This led American astronomer Allan Sandage and the German chemist and physicist Gustav Tammann to the (controversial) technique of using the sizes of H II regions as a distance indicator, because a measurement of their angular sizes, coupled with knowledge of their linear sizes, allows an inference of distance.

The two galaxies Maffei I and II are relatively close to the Milky Way Galaxy but were unobserved until the late 1960s, when the Italian astronomer Paolo Maffei detected them by their infrared radiation. Studies in the United States established that the objects are galaxies. Lying near the border between the constellations Perseus and Cassiopeia, they are close to the plane of the Milky Way, where obscuring dust clouds in interstellar space prevent nearly all visible light emitted by external galaxies from reaching Earth.