The Milky Way and Beyond (32 page)

Galaxies differ from one another in shape, with variations resulting from the way in which the systems were formed and subsequently evolved. Galaxies are extremely varied not only in structure but also in the amount of activity observed. Some are the sites of vigorous star formation, with its attendant glowing gas and clouds of dust and molecular complexes. Others, by contrast, are quiescent, having long ago ceased to form new stars. Perhaps the most conspicuous activity in galaxies occurs in their nuclei, where evidence suggests that in many cases supermassive objects—probably black holes—lurk. These central black holes apparently formed several billion years ago; they are now observed forming in galaxies at large distances (and, therefore, because of the time it takes light to travel to Earth, at times in the far distant past) as brilliant objects called quasars.

The existence of galaxies was not recognized until the early 20th century. Since then, however, galaxies have become one of the focal points of astronomical investigation. The notable developments and achievements in the study of galaxies are surveyed here. Included in the discussion are the external galaxies (i.e., those lying outside the Milky Way Galaxy, the local galaxy to which the Sun and Earth belong), their distribution in clusters and superclusters, and the evolution of galaxies and quasars.

The study of the origin and evolution of galaxies and the quasar phenomenon has only just begun. Many models of galaxy formation and evolution have been constructed on the basis of what we know about conditions in the early universe, which is in turn based on models of the expansion of the universe after the big bang (the primordial explosion from which the universe is thought to have originated) and on the characteristics of the cosmic microwave background (the observed photons that show us the light-filled universe as it was when it was a few hundred thousand years old).

According to the big-bang model, the universe expanded rapidly from a highly compressed primordial state, which resulted in a significant decrease in density and temperature. Soon afterward, the dominance of matter over antimatter (as observed today) may have been established by processes that also predict proton decay. During this stage many types of elementary particles may have been present. After a few seconds, the universe cooled enough to allow the formation of certain nuclei. The theory predicts that definite amounts of hydrogen, helium, and lithium were produced. Their abundances agree with what is observed today. About one million years later the universe was sufficiently cool for atoms to form.

When the universe had expanded to be cool enough for matter to remain in neutral atoms without being instantly ionized by radiation, structure apparently had already been established in the form of density fluctuations. At a crucial point in time, there condensed from the expanding matter small clouds (protogalaxies) that could collapse under their own gravitational field eventually to form galaxies.

For the latter half of the 20th century, there were two competing models of galaxy formation: “top-down” and “bottom-up.” In the top-down model, galaxies formed out of the collapse of much larger gas clouds. In the bottom-up model, galaxies formed from the merger of smaller entities that were the size of globular clusters. In both models the angular momentum of the original clouds determined the form of the galaxy that eventually evolved. It is thought that a protogalaxy with a large amount of angular momentum tended to form a flat, rapidly rotating system (a spiral galaxy), whereas one with very little angular momentum developed into a more nearly spherical system (an elliptical galaxy).

The transition from the 20th to the 21st century coincided with a dramatic transition in our understanding of the evolution of galaxies. It is no longer believed that galaxies have evolved smoothly and alone. Indeed, it has become clear that collisions between galaxies have occurred all during their evolution—and these collisions, far from being rare events, were the mechanism by which galaxies developed in the distant past and are the means by which they are changing their structure and appearance even now. Evidence for this new
understanding of galactic evolution comes primarily from two sources: more detailed studies of nearby galaxies with new, more sensitive instruments and deep surveys of extremely distant galaxies, seen when the universe was young.

Recent surveys of nearby galaxies, including the Milky Way Galaxy, have shown evidence of past collisions and capture of galaxies. For the Milky Way the most conspicuous example is the Sagittarius Galaxy, which has been absorbed by our Galaxy. Now its stars lie spread out across the sky, its seven globular clusters intermingling with the globular clusters of the Milky Way Galaxy. Long tails of stars around the Milky Way were formed by the encounter and act as clues to the geometry of the event. A second remnant galaxy, known as the Canis Major Dwarf Galaxy, can also be traced by the detection of star streams in the outer parts of our Galaxy. These galaxies support the idea that the Milky Way Galaxy is a mix of pieces, formed by the amalgamation of many smaller galaxies.

The Andromeda Galaxy (M31) also has a past involving collisions and accretion. 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.

More spectacular are galaxies presently in the process of collision and accretion in the more distant, but still nearby, universe. The symptoms of the collision are the distortion of the galaxies' shape (especially that of the spiral arms), the formation of giant arcs of stars by tidal action, and the enhanced rate of star and star cluster formation. Some of the most massive and luminous young star clusters observed anywhere lie in the regions where two galaxies have come together, with their gas and dust clouds colliding and merging in a spectacular cosmic fireworks display.

A second type of evidence for the fact that galaxies grow by merging comes from very deep surveys of the very distant universe, especially those carried out with the Hubble Space Telescope (HST). These surveys, especially the Hubble Deep Field and the Hubble Ultra Deep Field, found galaxies so far away that the light observed by the HST left them when they were very young, only a few hundred million years old. This enables the direct detection and measurement of young galaxies as they were when the universe was young. The result is a view of a very different universe of galaxies. Instead of giant elliptical galaxies and grand spirals, the universe in its early years was populated with small, irregular objects that looked like mere fragments. These were the building blocks that eventually formed bigger galaxies such as the Milky
Way. Many show active formation of stars that are deficient in heavy elements because many of the heavy elements had not yet been created when these stars were formed.

The rate of star formation in these early times was significant, but it did not reach a peak until about one billion years later. Galaxies from this time show a maximum in the amount of excited hydrogen, which indicates a high rate of star formation, as young, very hot stars are necessary for exciting interstellar hydrogen so that it can be detected. Since that time, so much matter has been locked up in stars (especially white dwarfs) that not enough interstellar dust and gas are available to achieve such high rates of star formation.

An important development that has helped our understanding of the way galaxies form is the great success of computer simulations. High-speed calculations of the gravitational history of assemblages of stars, interstellar matter, and dark matter suggest that after the big bang the universe developed as a networklike arrangement of material, with gradual condensation of masses where the strands of the network intersected. In simulations of this process, massive galaxies form, but each is surrounded by a hundred or so smaller objects. The small objects may correspond to the dwarf galaxies, such as those that surround the Milky Way Galaxy but of which only a dozen or so remain, the rest having presumably been accreted by the main galaxy. Such computer models, called “n-body simulations,” are especially successful in mimicking galaxy collisions and in helping to explain the presence of various tidal arms and jets observed by astronomers.

In summary, the current view of galactic history is that present-day galaxies are a mix of giant objects that accreted lesser galaxies in their vicinities, especially early in the formation of the universe, together with some remnant lesser, or dwarf, galaxies that have not yet come close enough to a more massive galaxy to be captured. The expansion of the universe gradually decreases the likelihood of such captures, so some of the dwarfs may survive to old age—eventually dying, like their giant cousins, when all of their stars become dim white dwarfs or black holes and slowly disappear.

Most nebulae look as amorphous and ephemeral as clouds in the sky. However, in 1845, the Irish astronomer William Parsons discovered that some nebulae had a spiral shape. Why did these objects have such a well-ordered appearance?

The dispute over the nature of what were once termed spiral nebulae stands as one of the most significant in the development of astronomy. On this dispute hinged the question of the magnitude of the universe: were we confined to a single, limited stellar system that lay embedded alone in empty space, or was our Milky Way Galaxy just one of millions of galaxies that pervaded space, stretching
beyond the vast distances probed by our most powerful telescopes? How this question arose, and how it was resolved, is an important element in the development of our prevailing view of the universe.

Up until 1925, spiral nebulae and their related forms had uncertain status. Some scientists, notably Heber D. Curtis of the United States and Knut Lundmark of Sweden, argued that they might be remote aggregates of stars similar in size to the Milky Way Galaxy. Centuries earlier the German philosopher Immanuel Kant, among others, had suggested much the same idea, but that was long before the tools were available to actually measure distances and thus prove it. During the early 1920s astronomers were divided. Although some deduced that spiral nebulae were actually extragalactic star systems, there was evidence that convinced many that such nebulae were local clouds of material, possibly new solar systems in the process of forming.

It is now known that the nearest external galaxies are the Magellanic Clouds, two patchy irregular objects visible in the skies of the Southern Hemisphere. For years, most experts who regarded the Magellanic Clouds as portions of the Milky Way Galaxy system separated from the main stream could not study them because of their position. (Both Magellanic Clouds are too far south to be seen from northern latitudes.) Moreover, the irregular shapes of the objects and their numerous hot blue stars, star clusters, and gas clouds did indeed make them resemble the southern Milky Way Galaxy.

The American astronomer Harlow Shapley, noted for his far-reaching work on the size and structure of the Milky Way Galaxy, was one of the first to appreciate the importance of the Magellanic Clouds in terms of the nature of spiral nebulae. To gauge the distance of the Clouds, he made use of the period-luminosity (P-L) relation discovered by Henrietta Leavitt of the Harvard College Observatory. In 1912 Leavitt had found that there was a close correlation between the periods of pulsation (variations in light) and the luminosities (intrinsic, or absolute, brightnesses) of a class of stars called Cepheid variables in the Small Magellanic Cloud. Leavitt's discovery, however, was of little practical value until Shapley worked out a calibration of the absolute brightnesses of pulsating stars closely analogous to the Cepheids, the so-called RR Lyrae variables. With this quantified form of the P-L relation, he was able to calculate the distances to the Magellanic Clouds, determining that they were about 75,000 light-years from Earth. The significance of the Clouds, however, continued to elude scientists of the time. For them, these objects still seemed to be anomalous, irregular patches of the Milky Way Galaxy, farther away than initially thought but not sufficient to settle the question of the nature of the universe.