Introduction

Galaxies differ from one another in shape, with variations resulting from the way in which the systems were formed. Depending on the initial conditions in the pregalactic gas some 15,000,000,000 years ago, galaxies formed either as slowly turning, smoothly structured, round systems of stars and gas or as rapidly rotating pinwheels of such entities. Other differences between galaxies have been observed and are thought to reflect evolutionary changes. Some galaxies are rife with activity: they are the sites of 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 evolutionary changes in galaxies occur in their nuclei, where evidence suggests that in many cases supermassive objects--probably black holes--formed when the galaxies were young. Such phenomena occurred several billion years ago and are now observed 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. Much attention is also devoted to the Milky Way Galaxy--the local galaxy to which the Sun and Earth belong--and the galaxies and related objects that lie outside it. Included in the discussion are the properties, structures, and major components of the Milky Way system and the external galaxies; the distribution of the latter in clusters and superclusters; and the evolution of galaxies and quasars. 

Historical survey of the study of galaxies

EARLY OBSERVATIONS AND CONCEPTIONS

Up until 1925 spiral nebulas and their related forms had uncertain status. Some scientists, notably of the United States and Knut Lundmark of Sweden, had 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 nebulas were actually extragalactic star systems, there was evidence that convinced many that such nebulas were local clouds of material, possibly new solar systems in the process of forming. 

The problem of the Magellanic Clouds.

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 nebulas. 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 (see below). 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 the 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, farther away than initially thought but not sufficient to settle the question of the nature of the Cosmos. 

Novas in the Andromeda Nebula.

By 1910, however, there was evidence that S Andromeda might have been wrongly identified. Deep photographs were being taken of M31 with the Mount Wilson Observatory's newly-completed 152-centimetre (60-inch) telescope, and the astronomers at the observatory, especially J.C. Duncan and George W. Ritchey, were finding faint objects, just resolved by the longest exposures, which also seemed to behave like novas. These objects, however, were about 10,000 times fainter than S Andromeda. If they were ordinary novas then M31 must be millions of light-years away; but then the nature of S Andromeda became a difficult question. At this vast distance its total luminosity would have to be immense--an incomprehensible output of energy for a single star.

Completion of the 254-centimetre (100-inch) telescope on Mount Wilson in 1917 resulted in a new series of photographs that captured even fainter objects. More novas were found in M31, mainly by Milton L. Humason, who was an assistant at the time to Edwin P. Hubble, one of the truly outstanding astronomers of the day. Hubble eventually studied 63 of these stars, and his findings proved to be one of the final solutions to the controversy 

The scale of the Milky Way Galaxy.

This conception of the Milky Way Galaxy was challenged by Shapley in 1917, when he released the findings of his study of globular clusters. He had found that these spherically symmetrical groups of densely packed stars, as compared to the much closer open clusters, were unusual in their distribution. While the known open clusters are concentrated heavily in the bright belt of the Milky Way, the globulars are for the most part absent from those areas, except in the general direction of the constellation Sagittarius where there is a concentration of faint globulars. Shapley's plot of the spatial distribution of these stellar groupings clarified this peculiar fact: the centre of the globular cluster system--a huge, almost spherical cloud of clusters--lies in that direction, some 30,000 light-years from the Sun. Shapley assumed that this centre must also be the centre of the Milky Way Galaxy. The globular clusters, he argued, form a giant skeleton around the disk of the Milky Way, and thus the system is immensely larger than was previously thought, its total extent measuring nearly 100,000 light-years .

Shapley succeeded in making the first reliable determination of the size of the Milky Way Galaxy largely by using Cepheids and RR Lyrae stars as distance indicators. His approach was based on the period-luminosity relation discovered by Leavitt and on the assumption that all of these variables have the same P-L relation. As he saw it, this assumption was most likely true in the case of the RR Lyrae stars, because all variables of this type in any given globular cluster have the same apparent brightness. If all RR Lyrae variables have the same intrinsic brightness, then it follows that differences in apparent brightness must be due to different distances from the Earth. The final step in developing a procedure for determining the distances of variables was to calculate the distances of a handful of such stars by an independent method so as to enable calibration. Shapley could not make use of the trigonometric parallax method since there are no variables close enough for direct distance measurement. However, he had recourse to a technique devised by the Danish astronomer Ejnar Hertzsprung that could determine distances to certain nearby field variables (i.e., those not associated with any particular cluster) using measurements of their proper motions and the radial velocity of the Sun. Accurate measurements of the proper motions of the variables based on long-term observations were available, and the Sun's radial velocity could be readily determined spectroscopically. Thus, by availing himself of this body of data and adopting Hertzsprung's method, Shapley was able to obtain a distance scale for Cepheids in the solar neighbourhood.

Shapley applied the zero point of the Cepheid distance scale to the globular clusters he had studied with the 152-centimetre telescope at Mount Wilson. Some of these clusters contained RR Lyrae variables, and for these Shapley could calculate distances in a straightforward manner from the period-luminosity relation. For other globulars he made distance determinations using a relationship that he discovered between the brightnesses of the RR Lyrae stars and the brightness of the brightest red stars. For still others he made use of apparent diameters, which he found to be relatively uniform for clusters of known distance. The final result was a catalog of distances for 69 globular clusters, from which Shapley deduced his revolutionary model of the Milky Way--one that not only significantly extended the limits of the galactic system but that also displaced the Sun from its centre to a location nearer its edge.

Shapley's work caused astronomers to ask themselves certain questions: How could the existing stellar data be so wrong? Why couldn't they see something in Sagittarius, the proposed galactic centre, 30,000 light-years away? The reason for the incorrectness of the star count methods was not learned until 1930, when Lick Observatory astronomer Robert J. Trumpler, while studying open clusters, discovered that interstellar dust pervades the plane of the Milky Way Galaxy and obscures objects beyond only a few thousand light-years. This dust thus renders the centre of the system invisible optically and makes it appear that globular clusters and spiral nebulas avoid the band of the Milky Way. 

Shapley's belief in the tremendous size of the local galactic system helped to put him on the wrong side of the argument about other galaxies. He thought that if the Milky Way was so immense, the spiral nebulas must lie within it. His conviction was reinforced by two lines of evidence. One of these has already been mentioned--the nova S Andromeda was so bright as to suggest that the Andromeda Nebula most certainly was only a few hundred light-years away. The second came about because of a very curious error made by one of Shapley's colleagues at Mount Wilson Observatory, Adrian van Maanen. 

The van Maanen rotation.

Shapley seized the van Maanen results as evidence that the spirals must be nearby; otherwise, their true space velocities of rotation would have to be impossibly large. For example, if M51 is rotating at an apparent rate of 0.02 second of arc per year, its true velocity would be immense if it is a distant galaxy. Assuming that a distance of 10,000,000 light-years would lead to an implausibly large rotation velocity of 12,000 kilometres per second (km/sec), Shapley argued that if a more reasonable velocity were adopted, say, 100 km/sec, then the distances would be all less than 100,000 light-years, putting all the spirals well within the Milky Way Galaxy.

It is unclear just why such a crucial measurement went wrong. Van Maanen repeated the measures and obtained the same answer even after Hubble demonstrated the truth about the distances to the spirals (see below). However, subsequent workers, using the same plates, failed to find any rotation. Among the various hypotheses that science historians have proposed as an explanation for the error are two particularly reasonable ideas: (1) possibly the fact that spiral nebulas look like they are rotating (i.e., they resemble familiar rotational patterns that are perceivable in nature) may have influenced the observer subconsciously, and this subtle effect manifested itself in prejudicing the delicate measurements; or (2) possibly the first set of plates was the problem. Many of these plates had been taken in an unconventional manner by Ritchey, who swung the plateholder out of the field whenever the quality of the images was temporarily poor because of atmospheric turbulence. The resulting plates appeared excellent, having been exposed only during times of very fine seeing; however, according to some interpretations, the images had a slight asymmetry that led to a very small displacement of star images compared to nonstellar images. Such an error could look like rotation if not recognized for what it really was. In any case, the van Maanen rotation was accepted by many astronomers, including Shapley, and temporarily sidetracked progress toward recognizing the truth about galaxies. 

The Shapley-Curtis debate.

A careful reading of the documents involved suggests that on the broader topic of the scale of the universe, both men were making incorrect conclusions but for the same reasons--namely, for being unable to accept and comprehend the incredibly large scale of things. Shapley correctly argued for an enormous Milky Way Galaxy on the basis of the period-luminosity relation and the globular clusters, while Curtis incorrectly rejected these lines of evidence, advocating instead a small galactic system. Given a Milky Way system of limited scale, Curtis could argue for and consider plausible the extragalactic nature of the spiral nebulas. Shapley, on the other hand, incorrectly rejected the island universe theory of the spirals (i.e., the hypothesis that there existed beyond the boundaries of the Milky Way comparable galaxies) because he felt that such objects would surely be engulfed by the local galactic system. Furthermore, he put aside the apparent faint novas in M31, preferring to interpret S Andromeda as an ordinary nova, for otherwise that object would have been unbelievably luminous. Unfortunately for him such phenomena--called supernovas--do in fact exist, as was realized a few years later. Curtis was willing to concede that there might be two classes of novas; yet, because he considered the Milky Way to be small, he underestimated their differences. The van Maanen rotation also entered into Shapley's arguments: if spiral nebulas were rotating so fast, they must be within the Milky Way as he conceived it. For Curtis, however, the matter provided less of a problem: even if spiral nebulas did rotate as rapidly as claimed, the small scale of Curtis' universe allowed them to have physically reasonable speeds.

The Shapley-Curtis debate took place near the end of the era of the single-galaxy universe. In just a few years the scientific world became convinced that Shapley's grand scale of the Milky Way Galaxy was correct and at the same time that Curtis was right about the nature of spiral nebulas. Such objects indeed lie even outside Shapley's enormous Milky Way, and they range far beyond the distances that in 1920 seemed too vast for many astronomers to comprehend. 

Hubble's discovery of extragalactic objects.

Hubble then boldly assumed that the P-L relation was universal and derived an estimate for the distance to NGC 6822 using Shapley's most recent (1925) version of the calibration of the relation. This calibration was wrong, as is now known, because of the confusion at that time over the nature of Cepheids. Shapley's calibration included certain Cepheids in globular clusters that subsequent investigators found to have their own fainter P-L relation. (Such Cepheids have been designated Type II Cepheids to distinguish them from the normal variety, which are referred to as Type I.) Thus, Hubble's distance for NGC 6822 was too small: he calculated a distance of only 700,000 light-years. Today it is recognized that the actual distance is closer to 2,000,000 light-years. In any case, this vast distance--even though underestimated--was large enough to convince Hubble that NGC 6822 must be a remote, separate galaxy, much too far away to be included even in Shapley's version of the Milky Way system. Technically, then, this faint nebula can be considered to be the first recognized external galaxy. The Magellanic Clouds continued to be regarded simply as appendages to the Milky Way, and the other bright nebulas, M31 and M33, were still being studied at the Mount Wilson Observatory. Although Hubble did announce his discovery of Cepheids in M31 at a meeting in 1924, he did not complete his research and publish the results for this conspicuous spiral galaxy until five years later.

While the Cepheids made it possible to determine the distance and nature of NGC 6822, some of its other features corroborated the conclusion that it was a separate, distant galaxy. Hubble discovered within it five diffuse nebulas, which are glowing gaseous clouds composed mostly of ionized hydrogen designated H II regions. (H stands for hydrogen and II indicates that most of it is ionized; H I, by contrast, signifies neutral hydrogen.) He found that these five H II regions had spectra like those of gas clouds in the Milky Way system--e.g., the Orion and Eta Carinae nebulas. Calculating their diameters, Hubble ascertained that the sizes of the diffuse nebulas were normal, similar to those of local examples of giant H II regions.

Five other diffuse objects discerned by Hubble were definitely not gaseous nebulas. He compared them with globular clusters (both in the Milky Way Galaxy and in the Magellanic Clouds) and concluded that they were too small and faint to be normal globular clusters. Convinced that they were most likely distant galaxies seen through NGC 6822, he dismissed them from further consideration. Modern studies suggest that Hubble was too hasty. Though probably not true giant globular clusters, these objects are in all likelihood star clusters in the system, fainter, smaller in population, and probably somewhat younger than normal globular clusters.

The Dutch astronomer Jacobus Cornelius Kapteyn showed in the early 20th century that statistical techniques could be used to determine the stellar luminosity function for the solar neighbourhood. (The luminosity function is a curve that shows how many stars there are in a given volume for each different stellar luminosity.) Anxious to test the nature of NGC 6822, Hubble counted stars in the galaxy to various brightness limits and found a luminosity function for its brightest stars. When he compared it with Kapteyn's, the agreement was excellent--another indication that the Cepheids had given about the right distance and that the basic properties of galaxies were fairly uniform. Step by step, Hubble and his contemporaries piled up evidence for the fundamental assumption that has since guided the astronomy of the extragalactic universe, the uniformity of nature. By its bold application, astronomers have moved from a limiting one-galaxy Cosmos to an immense vastness of space populated by billions of galaxies, all grander in size and design than the Milky Way system had once been thought to be. 

The distance to the Andromeda Nebula.

Because M31 is much larger than the field of view of the 152-centimetre and 254-centimetre telescopes at Mount Wilson, Hubble concentrated on four regions, centred on the nucleus and at various distances along the major axis. The total area studied amounted to less than half the galaxy's size, and the other unexplored regions remained largely unknown for 50 years. (Modern comprehensive optical studies of M31 have only been conducted since about 1980.)

Hubble pointed out an important and puzzling feature of the resolvability of M31. Its central regions, including the nucleus and diffuse nuclear bulge, were not well resolved into stars, one reason that the true nature of M31 had previously been elusive. However, the outer parts along the spiral arms in particular were resolved into swarms of faint stars, seen superimposed over a structured background of light. Current understanding of this fact is that spiral galaxies typically have central bulges made up exclusively of very old stars, the brightest of which are too faint to be visible on Hubble's plates. Only in 1944 did the German-born astronomer Walter Baade finally resolve the bulge of M31. Using red-sensitive plates and very long exposures, he managed to detect the brightest red giants of this old population. Out in the arms there exist many young, bright, hot, blue stars, and these are easily resolved. The brightest are so luminous that they can even be seen with moderate-size telescopes.

The most important of Hubble's discoveries was that of M31's population of Cepheid variables. Forty of the 50 variables detected turned out to be ordinary Cepheids with periods ranging from 10 to 48 days. A clear relation was found between their periods and luminosities, and the slope of the relation agreed with those for the Magellanic Clouds and NGC 6822. Hubble's comparison indicated that M31 must be 8.5 times more distant than the Small Magellanic Cloud (SMC), which would imply a distance of 2,000,000 light-years if the modern SMC distance were used (the 1929 value employed by Hubble was about two times too small). Clearly, M31 must be a distant, large galaxy.

Other features announced in Hubble's paper were M31's population of bright, irregular, slowly varying variables. One of the irregulars was exceedingly bright; it is among the most luminous stars in the galaxy and is a prototype of a class of high-luminosity stars now called Hubble-Sandage variables, which are found in many giant galaxies. Eighty-five novas, all behaving very much like those in the Milky Way Galaxy, were also analyzed. Hubble estimated that the true occurrence rate of novas in M31 must be about 30 per year, a figure that was later confirmed by the American astronomer Halton C. Arp in a systematic search.

Hubble found numerous star clusters in M31, especially globular clusters, 140 of which he eventually cataloged. He clinched the argument that M31 was a galaxy similar to the Milky Way by calculating its mass and mass density. Using the velocities that had been measured for the inner parts of M31 by spectrographic work, he calculated (on the basis of the distance derived from the Cepheids) that M31's mass must be about 3,500,000,000 times that of the Sun. Today, astronomers have much better data, which indicate that the galaxy's true total mass must be at least 100 times greater than Hubble's value, but even that value clearly showed that M31 is an immense system of stars. Furthermore, Hubble's estimates of star densities demonstrated that the stars in the outer arm areas of M31 are spread out with about the same density as in the Milky Way system in the vicinity of the Sun. 

RECENT DEVELOPMENTS

During the decade of the 1950s the field began to change. Ever larger optical telescopes became available, and the space program resulted in a sizable increase in the number of astronomers emerging from universities. New instrumentation enabled investigators to explore galaxies in entirely new ways, making it possible to detect their radio radiation, infrared and ultraviolet emissions, and eventually even radiation at X-ray wavelength. Whereas in the 1950s there was only one telescope larger than 254 centimetres (100 inches) and only about 10 astronomers conducting research on galaxies worldwide, by the 1980s the number of large telescopes had grown 10-fold and the number of scientists devoted to galaxy study was in the hundreds. Galaxies are now extensively studied with giant arrays of ground-based radio telescopes, Earth-orbiting X-ray, ultraviolet, and infrared telescopes, and high-speed electronic computers, giving rise to remarkable advances in knowledge and understanding.