Any of the systems of stars and interstellar matter that make up the universe. Many such assemblages are so enormous that they contain hundreds of billions of stars. Nature has provided an immensely varied array of galaxies, ranging from faint, diffuse dwarf objects to brilliant spiral-shaped giants. Virtually all galaxies appear to have been formed soon after the universe began, and they pervade space, even into the depths of the farthest reaches penetrated by powerful modern telescopes. Galaxies usually exist in clusters, some of which in turn are grouped into larger clusters that measure hundreds of millions of light-years across. (A light-year is the distance traversed by light in one year, traveling at a velocity of 300,000 km per second [km/sec], or 650,000,000 miles per hour.) These so-called super-clusters are separated by nearly empty voids, and this causes the gross structure of the universe to look somewhat like a network of sheets and chains of galaxies. 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 super-massive objects probably black holes. 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 super-clusters, and the evolution of galaxies and quasars.
Types of galaxies
Principal schemes of classification
Almost all current systems of galaxy classification are outgrowths of the initial scheme proposed by the American astronomer Edwin Hubble in 1926. In Hubble’s scheme, which is based on the optical appearance of galaxy images on photographic plates, galaxies are divided into three general classes: ellipticals, spirals, and irregulars. Hubble subdivided these three classes into finer groups.
In The Hubble Atlas of Galaxies (1961), the American astronomer Allan R. Sandage drew on Hubble’s notes and his own research on galaxy morphology to revise the Hubble classification scheme. Some of the features of this revised scheme are subject to argument because of the findings of very recent research, but its general features, especially the coding of types, remain viable. A description of the classes as defined by Sandage is given here, along with observations concerning needed refinements of some of the details.
18.104.22.168. Elliptical galaxies
These systems exhibit certain characteristic properties. They have complete rotational symmetry; i.e., they are figures of revolution with two equal principal axes. They have a third smaller axis that is the presumed axis of rotation. The surface brightness of ellipticals at optical wavelengths decreases monotonically outward from a maximum value at the centre, following a common mathematical law of the form:I = I0( r/a +1 )−2, where I is the intensity of the light, I0 is the central intensity, r is the radius, and a is a scale factor. The isophotal contours exhibited by an elliptical system are similar ellipses with a common orientation, each centred on its nucleus. No galaxy of this type is flatter than b/a = 0.3, with b and a the minor and major axes of the elliptical image, respectively. Ellipticals contain neither interstellar dust nor bright stars of spectral types O and B. Many, however, contain evidence of the presence of low-density gas in their nuclear regions. Ellipticals are red in colour, and their spectra indicate that their light comes mostly from old stars, especially evolved red giants.
Subclasses of elliptical galaxies are defined by their apparent shape, which is of course not necessarily their three-dimensional shape. The designation is En, where n is an integer defined byn = 10( a− b)/a. A perfectly circular image will be an E0 galaxy, while a flatter object might be an E7 galaxy. (As explained above, elliptical galaxies are never flatter than this, so there are no E8, E9, or E10 galaxies.)
Although the above-cited criteria are generally accepted, current high-quality measurements have shown that some significant deviations exist. Most elliptical galaxies do not, for instance, exactly fit the intensity law formulated by Hubble; deviations are evident in their innermost parts and in their faint outer parts. Furthermore, many elliptical galaxies have slowly varying ellipticity, with the images being more circular in the central regions than in the outer parts. The major axes sometimes do not line up either; their position angles vary in the outer parts. Finally, astronomers have found that a few ellipticals do in fact have small numbers of luminous O and B stars as well as dust lanes.
- The giant elliptical galaxy M87, also known as Virgo A,
- Giant galaxy NGC 1316 is an elliptical galaxy in the Fornax Cluster.
22.214.171.124. spiral galaxies
SO — SA — SB — SC
SO; NGC 4753 - NGC 5866
SA; Sombrero Galaxy (M104) - NGC 7742
SB; Andromeda Galaxy — M81
SC; Whirlpool Galaxy (M51) - Pinwheel Galaxy (M101)
126.96.36.199. Barred spiral galaxies
Milky Way Galaxy -NGC 1300
188.8.131.52. Irregular galaxies
Large Magellanic Cloud — Small Magellanic Cloud
Spirals are characterized by circular symmetry, a bright nucleus surrounded by a thin outer disk, and a superimposed spiral structure. They are divided into two parallel classes: normal spirals and barred spirals. The normal spirals have arms that emanate from the nucleus, while barred spirals have a bright linear feature called a bar that straddles the nucleus, with the arms unwinding from the ends of the bar. The nucleus of a spiral galaxy is a sharp-peaked area of smooth texture, which can be quite small or, in some cases, can make up the bulk of the galaxy. Both the arms and the disk of a spiral system are blue in colour, whereas its central areas are red like an elliptical galaxy. The normal spirals are designated S and the barred varieties SB. Each of these classes is subclassified into three types according to the size of the nucleus and the degree to which the spiral arms are coiled. The three types are denoted with the lowercase letters a, b, and c. There also exist galaxies that are intermediate between ellipticals and spirals. Such systems have the disk shape characteristic of the latter but no spiral arms. These intermediate forms bear the designation S0.
These systems exhibit some of the properties of both the ellipticals and the spirals and seem to be a bridge between these two more common galaxy types. Hubble introduced the S0 class long after his original classification scheme had been universally adopted, largely because he noticed the dearth of highly flattened objects that otherwise had the properties of elliptical galaxies. Sandage’s elaboration of the S0 class yielded the characteristics described here.
S0 galaxies have a bright nucleus that is surrounded by a smooth, featureless bulge and a faint outer envelope. They are thin; statistical studies of the ratio of the apparent axes (seen projected onto the sky) indicate that they have intrinsic ratios of minor to major axes in the range 0.1 to 0.3. Their structure does not generally follow the luminosity law of elliptical galaxies but has a form more like that for spiral galaxies. Some S0 systems have a hint of structure in the envelope, either faintly discernible armlike discontinuities or narrow absorption lanes produced by interstellar dust. Several S0 galaxies are otherwise peculiar, and it is difficult to classify them with certainty. They can be thought of as peculiar irregular galaxies (i.e., Irr II galaxies) or simply as some of the 1 or 2 percent of galaxies that do not fit easily into the Hubble scheme. Among these are such galaxies as NGC 4753, which has irregular dust lanes across its image, and NGC 128, which has a double, almost rectangular bulge around a central nucleus. Another type of peculiar S0 is found in NGC 2685. This nebula in the constellation Ursa Major has an apparently edge-on disk galaxy at its centre, with surrounding hoops of gas, dust, and stars arranged in a plane that is at right angles to the apparent plane of the central object.
These normal spirals have narrow, tightly wound arms, which usually are visible because of the presence of interstellar dust and, in many cases, bright stars. Most of them have a large amorphous bulge in the centre, but there are some that violate this criterion, having a small nucleus around which is arranged an amorphous disk with superimposed faint arms. NGC 1302 is an example of the normal type of Sa galaxy, while NGC 4866 is representative of one with a small nucleus and arms consisting of thin dust lanes on a smooth disk.
This intermediate type of spiral typically has a medium-sized nucleus. Its arms are more widely spread than those of the Sa variety and appear less smooth. They contain stars, star clouds, and interstellar gas and dust. Sb galaxies show wide dispersions in details in terms of their shape. Hubble and Sandage observed, for example, that in certain Sb galaxies the arms emerge at the nucleus, which is often quite small. Other members of this subclass have arms that begin tangent to a bright, nearly circular ring, while still others reveal a small, bright spiral pattern inset into the nuclear bulge. In any of these cases, the spiral arms may be set at different pitch angles. (A pitch angle is defined as the angle between an arm and a circle centred on the nucleus and intersecting the arm.)
Hubble and Sandage noted further deviations from the standard shape established for Sb galaxies. A few systems exhibit a chaotic dust pattern superimposed upon the tightly wound spiral arms. Some have smooth, thick arms of low surface brightness, frequently bounded on their inner edges with dust lanes. Finally, there are those with a large, smooth nuclear bulge from which the arms emanate, flowing outward tangent to the bulge and forming short arm segments. This is the most familiar type of Sb galaxy and is best exemplified by the giant Andromeda Galaxy.
Many of these variations in shape remain unexplained. Theoretical models of spiral galaxies based on a number of different premises can reproduce the basic Sb galaxy shape, but many of the deviations noted above are somewhat mysterious in origin and must await more detailed and realistic modeling of galactic dynamics.
These galaxies characteristically have a very small nucleus and multiple spiral arms that are open, with relatively large pitch angles. The arms, moreover, are lumpy, containing as they do numerous irregularly distributed star clouds, stellar associations, star clusters, and gas clouds known as emission nebulae.
As in the case of Sb galaxies, there are several recognizable subtypes among the Sc systems. Sandage has cited six subdivisions: (1) galaxies, such as the Whirlpool Galaxy (M51), that have thin branched arms that wind outward from a tiny nucleus, usually extending out about 180° before branching into multiple segments, (2) systems with multiple arms that start tangent to a bright ring centred on the nucleus, (3) those with arms that are poorly defined and that span the entire image of the galaxy, (4) those with a spiral pattern that cannot easily be traced and that are multiple and punctuated with chaotic dust lanes, (5) those with thick, loose arms that are not well defined — e.g., the nearby galaxy M33 (the Triangulum Nebula) — and (6) transition types, which are almost so lacking in order that they could be considered irregular galaxies.
Some classification schemes, such as that of the French-born American astronomer Gerard de Vaucouleurs, give the last of the above-cited subtypes a class of its own, type Sd. It also has been found that some of the variations noted here for Sc galaxies are related to total luminosity. Galaxies of the fifth subtype, in particular, tend to be intrinsically faint, while those of the first subtype are among the most luminous spirals known. This correlation is part of the justification for the luminosity classification
The luminosities, dimensions, spectra, and distributions of the barred spirals tend to be indistinguishable from those of normal spirals. The subclasses of SB systems exist in parallel sequence to those of the latter.
There are SB0 galaxies that feature a large nuclear bulge surrounded by a disklike envelope across which runs a luminous featureless bar. Some SB0 systems have short bars, while others have bars that extend across the entire visible image. Occasionally there is a ringlike feature external to the bar. SBa galaxies have bright, fairly large nuclear bulges and tightly wound, smooth spiral arms that emerge from the ends of the bar or from a circular ring external to the bar. SBb systems have a smooth bar as well as relatively smooth and continuous arms. In some galaxies of this type, the arms start at or near the ends of the bar, with conspicuous dust lanes along the inside of the bar that can be traced right up to the nucleus. Others have arms that start tangent to a ring external to the bar. In SBc galaxies, both the arms and the bar are highly resolved into star clouds and stellar associations. The arms are open in form and can start either at the ends of the bar or tangent to a ring.
Most representatives of this class consist of grainy, highly irregular assemblages of luminous areas. They have neither noticeable symmetry nor an obvious central nucleus, and they are generally bluer in colour than are the arms and disks of spiral galaxies. An extremely small number of them, however, are red and have a smooth, though nonsymmetrical, shape.
Hubble recognized these two types of irregular galaxies, Irr I and Irr II. The Irr I type is the most common of the irregular systems, and it seems to fall naturally on an extension of the spiral classes, beyond Sc, into galaxies with no discernible spiral structure. They are blue, are highly resolved, and have little or no nucleus. The Irr II systems are red, rare objects. They include various kinds of chaotic galaxies for which there apparently are many different explanations, including most commonly the results of galaxy-galaxy interactions, both tidal distortions and cannibalism; therefore, this category is no longer seen as a useful way to classify galaxies.
Some irregular galaxies, like spirals, are barred. They have a nearly central bar structure dominating an otherwise chaotic arrangement of material. The Large Magellanic Cloud is a well-known example.
Other classification schemes and galaxy types
Other classification schemes similar to Hubble’s follow his pattern but subdivide the galaxies differently. A notable example of one such system is that of de Vaucouleurs. This scheme, which has evolved considerably since its inception in 1959, includes a large number of codes for indicating different kinds of morphological characteristics visible in the images of galaxies. The major Hubble galaxy classes form the framework of de Vaucouleurs’s scheme, and its subdivision includes different families, varieties, and stages. The de Vaucouleurs system is so detailed that it is more of a descriptive code for galaxies than a commonly used classification scheme.
The external galaxies
The extra-galactic distance scale
Before astronomers could establish the existence of galaxies, they had to develop a way to measure their distances. In an earlier section, it was explained how astronomers first accomplished this exceedingly difficult task for the nearby galaxies during the 1920s. Until the late decades of the 20th century, progress was discouragingly slow. Even though increased attention was being paid to the problem around the world, a consensus was not reached. In fact, the results of most workers fell into two separate camps, in which the distances found by one were about twice the size of the other’s. For this reason, shortly after its launch into Earth orbit in 1990, the Hubble Space Telescope (HST) was assigned the special task of reliably determining the extragalactic distance scale. Led by the Canadian-born astronomer Wendy Freedman and the American astronomer Robert Kennicutt, the team used a considerable amount of the HST’s time to measure the properties of the Cepheid variable stars in a carefully selected set of galaxies. Their results were intermediate between the two earlier distance scales. With subsequent refinements, the scale of distances between the galaxies is now on fairly secure footing.
The HST distance scale project established the scale of distances for the nearby universe. Establishing the distances to galaxies over the entire range of present observations (several billion light-years) is an even more difficult task. The process involved is one of many successive steps that are all closely tied to one another. Before even the nearby galaxy distances measured by the HST can be established, distances must first be determined for a number of galaxies even closer to the Milky Way Galaxy, specifically those in the Local Group. For this step, criteria are used that have been calibrated within the Milky Way Galaxy, where checks can be made between different methods and where the ultimate criterion is a geometric one, basically involving trigonometric parallaxes, especially those determined by the Hipparcos satellite. These distance criteria, acting as “standard candles,” are then compared with the HST observations of galaxies beyond the Local Group, where other methods are calibrated that allow even larger distances to be gauged. This general stepwise process continues to the edge of the observable universe.
The Local Group of galaxies is a concentration of approximately 50 galaxies dominated by two large spirals, the Milky Way Galaxy and the Andromeda Galaxy. For many of these galaxies, distances can be measured by using the Cepheid P-L law, which has been refined and made more precise since it was first used by American astronomer Edwin Hubble. For instance, the nearest external galaxy, the Large Magellanic Cloud, contains thousands of Cepheid variables, which can be compared with Cepheids of known distance in the Milky Way Galaxy to yield a distance determination of 160,000 light-years. This method has been employed for almost all galaxies of the Local Group that contain massive-enough stars to include Cepheids. Most of the rest of the members are elliptical galaxies, which do not have Cepheid variables; their distances are measured by using Population II stars, such as RR Lyrae variables or luminous red giants.
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).
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 network like 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.