The identification of other galaxies, or independent stellar systems, goes back to 1924, when Edwin Hubble found Cepheid variables in the nearby galaxies Messier 33 and Messier 31. Application of the Period‐Luminosity Relation for these variables in the Magellanic Clouds established that these were objects outside the confines of the Milky Way and of sizes comparable to it. More thorough studies show that galaxies exhibit a wide range of properties, from giant galaxies of 10 13 solar masses and sizes over 150,000 pc in diameter to dwarf galaxies of 10 6 solar masses and about 1,000 pc in size. Dwarf galaxies are by far the most numerous. It is estimated that about 100 billion galaxies in the universe are observable with existing telescopes.
Hubble quickly realized that the vast majority of galaxies have only a small number of shapes. A classification based on their optical appearance or morphology is limited to four fundamental types of galaxies—ellipticals, spirals, irregulars, and S0s. The Hubble classification has proven to be immensely valuable to the study of galaxies. Although originally based solely on optical appearance, appearance is also closely correlated with other physical properties of galaxies.
Elliptical (E) galaxies
Ellipticals (also sometimes called early‐type galaxies) were so named because they look like elliptical blobs of light. In general, they show no obvious structural features other than a smooth concentration of light to the center. The decrease in surface brightness with distance may be expressed in different ways, but one reasonable approximation is I(r) = I ⊙/(a + r) 2 where I ⊙ is a central brightness, r is the distance from the center, and a is a distance at which the brightness is one‐quarter of that in the center. In other words, the brightness roughly falls off as the inverse square of the distance from the center of the galaxy.
Many ellipticals are round, but others are noticeably elongated or flattened. If the long axis is measured to have a dimension of a and the perpendicular short axis is measured as b, then an ellipticity can be defined as ϵ = 10 (1 – b/ a); rounded to the nearest unit, ϵ is used as a subtype to distinguish between ellipticals (E) with different shapes. An E0 is a round galaxy, whereas an E6 is a rather flattened system (but not a disk in the sense of a flat spiral galaxy) (See Figure ). A serious problem with ellipticals, however, is determination of their real shape: A flat elliptical may look round if seen from above or below or face‐on in the same manner that a dinner plate can look very different depending upon the position of the viewer.
Statistical studies suggest that the typical elliptical is moderately flattened; but this argument rests on an implicit assumption that ellipticals have an equatorial or circular symmetry, like a pumpkin (the technical description is an oblate spheroid). Such would be the case if the flattening were related to rotation, in the same sense that the equatorial bulge of a planet like Jupiter is produced by its rapid rotation. But ellipticals show only a slow rotation; the balance against gravitation is primarily accomplished by random (in and out) motions of the stars, not by rotation. Theoretical studies suggest that the true spatial distribution of stars in an elliptical is more similar to a bar‐like structure (for example, like an eraser) known as a tri‐axial spheroid.
Of all classes of galaxies, elliptical galaxies show the widest range of properties between the dwarf examples and the giant systems, with mass ranging from 10 6 to 10 13 solar masses, sizes from 1 kpc to 150 kpc in diameter, and luminosities 10 6 to 10 12 solar luminosities. Perhaps 70 percent of all galaxies are ellipticals, but the vast majority are dwarfs.
In terms of stellar content, ellipticals appear to contain no bright, young stars and, in fact, most show no evidence of recent star formation at all. But some ellipticals, especially those in the center of clusters, do show blue stars and a UV excess indicating recent star formation. With overall reddish colors, ellipticals were long considered to contain a single population of old stars with the brightest stars being red giants. These old stars, however, are not standard Population II stars as in the Milky Way Galaxy, because spectroscopic analysis shows that many of them have a metallicity like the Sun, or even a greater abundance of heavy elements. The past star formation history of an elliptical thus must be very different than that which occurred in the Galaxy. Ellipticals appear to be pure star systems, with virtually no interstellar material (< 0.01% of the total mass), although there are a few exceptions to this rule. This lack of interstellar matter poses a problem, because stars evolve and lose mass. Because ellipticals do not appear to be forming new stars that would get rid of such gas over the lifetime of an elliptical, about 2 percent of the mass would have been returned to the interstellar medium (assuming that one had 100 percent conversion of material into stars at the time of formation of the galaxy).
Spiral (SA and SB) galaxies
About 15 percent of galaxies are spirals, flat galaxies with a central light concentration that show spiral arms in an outer disk. The central regions of spiral galaxies appear reddish and are composed of older Population II stars, such as those in the halo of the Milky Way Galaxy. These stars are distributed in an almost spherical region around the center of a galaxy and exhibit little rotation. Their concentration toward the center produces the appearance of a central bulge in the light distribution. The outer disks of spirals appear bluish because of the presence of young, blue stars that have formed relatively recently out of the interstellar material. Redder stars are present in the arms as well, though they are not as bright and therefore contribute less to the brightness of the arms. The star formation is concentrated into the spiral arms that look brighter because of the exceptionally luminous O and B stars. In reality, the mass distribution in the disk is very smooth, with the spiral arm regions representing only a small density excess over the mean density (this is true even though the density enhancement for interstellar gas, a minor part of the total mass distribution, may be large). Circular motions predominate in the disk, and all other characteristics of the stars are typical of Population I objects like those of the Milky Way. The outer mass distribution (as implied by the distribution of light) is clearly different than that of the elliptical galaxies. Surface brightness in the disk decreases radially outward as I(r) = I ⊙ exp (‐r/a) where the length a represents a scale factor, a distance over which the brightness drops by a given amount.
Spiral galaxies range from intermediate to large galaxies, with masses in the range of 10 9 to 10 12 solar masses, diameters 6 kpc to 100 kpc, and luminosities 10 8 to 10 11 solar luminosities. The observed appearance of a spiral depends on the observer's point of view: Seen from above or below, a spiral looks basically round, but if viewed from the side, a spiral appears very flat, typically with an axial ratio b/a ≈ 0.1. Making allowance for this, spirals still exhibit a far greater range of intrinsic shapes than do the ellipticals.
First, there is a fundamental distinction between spirals that show an axisymmetrical light distribution from center to edge (Hubble called these type S galaxies, but SA is probably preferred in a modern classification) and those whose centers are dominated by what appears to be a luminous bar across the center (barred spiral galaxies, type SB). The SA galaxies look like pinwheels with the spiral features curving symmetrically out of the nuclear region. The SB galaxies are typically two‐armed spirals with the arms originating at the ends of the luminous bar crossing the central region. In making this distinction, Hubble actually identified the two extreme forms of spiral galaxies. About one‐third of spirals show no evidence of a bar and are axisymmetric, about one‐third have light patterns dominated by a bar, but the remaining third are intermediate in morphology, hence they are considered type SAB. Our own Milky Way has a bar in the center.
Spirals also show a wide range in the characteristics of the disk and its size in comparison to the central or nuclear bulge. Some galaxies have a bulge that is large relative to the disk (or, equivalently, a disk that is barely more extended than the nuclear bulge). In such galaxies, the spiral arms are barely visible, showing only a small contrast to the brightness of the rest of the disk. These spiral features also look thin and appear tightly wound about the center of the galaxy. Hubble labeled this subtype with the letter a, as in SAa and SBa (also termed early‐type spirals for historical reasons). Other galaxies, labeled subtype b, show a less prominent bulge and a larger disk with more extensive spiral arms, more open and with a greater arm‐interarm brightness contrast. Hubble's third subtype, c (late‐type spirals), is represented by galaxies with hardly any bulge at all, with open, high‐contrast spiral arms going right into the center of the galaxy. These three characteristics, the bulge‐to‐disk ratio, the openness of the winding of the spiral arms, and their brightness contrast tend to change with each other, although there are exceptions. In some modern versions of the Hubble classification are added types Sd (galaxies with no bulge, and spiral arms in a disk with barely enough symmetry to be called a spiral at all) and Sm (representing Magellanic‐type irregular galaxies that have no particular symmetry; for example, a classification scheme considering the irregular galaxies to be an extension of the spiral types).
Although Hubble's classification again was based only on the optical appearance of galaxies, its utility lies in that the classification correlates with other galaxy properties. The Sa (the SAa and SBA galaxies together, making no distinction between the two) galaxies have little interstellar material, about 1 percent on average, and show a low rate of current stellar formation, correlating with the low brightness contrast of the spiral arms. Sb galaxies are more typically about 3 percent interstellar matter and have a greater rate of star formation, hence brighter spiral arms. Sc galaxies are even more gas rich, about 10 percent, and have even higher rates of star formation. That Sd galaxies are typically 20 percent interstellar material and Sm (=Im) galaxies are closer to 50 percent suggests a natural extension to the spiral types defined by Hubble.
Regardless of the type of spiral galaxy, in their disks it is the rotational motion of the stars in nearly circular orbits that produces the balance against gravity. The circular velocities are typically a few hundred kilometers per second.
Irregular (Ir) galaxies
Irregular galaxies ( Ir) show little, if any, symmetry in their luminosity structure; their appearance really does appear irregular, and therefore they were defined by Hubble as a separate class of galaxy. In modern modifications of Hubble's classification system, some astronomers consider them to be a morphological extension of the spiral types of galaxy. Irregulars represent about 15 percent of all galaxies. These are mostly relatively low‐mass systems, with 10 7 to 10 10 solar masses or so, and contain the greatest fraction of interstellar material of any of the galaxies, up to 50 percent in some cases. Structurally, these are flat galaxies whose mass distributions are actually more symmetric than their light distributions. The high gas content is responsible for the greater rate of star formation. Where star formation does take place, there is a greater contrast in the surface brightness between the star‐forming regions and the non–star‐forming areas. These are also small galaxies in which the inward pull of gravity can be balanced by relatively low rotational velocities. However, this in turn means little in the way of differential rotation, and therefore, star‐forming regions are not smeared into spiral arcs, unlike the more massive spirals. In other words, the basic difference between the spirals and the irregulars is mass; the spirals are the high‐mass, gassy disk galaxies, and the irregulars are the low‐mass disk galaxies. Differences in the history and present manner of conversion of interstellar mass into stars and the consequent optical appearance directly follow from differences in the circular motions that are needed to balance gravity.
A fourth type of galaxy, the S0 (“ess‐zero”) is recognized as being distinct in appearance from both the spirals and ellipticals, though this type shares some characteristics of each. The S0 galaxies have smooth light distributions, like the ellipticals. On the other hand, they are definitely flat systems that are more like spirals containing both a halo population of stars (S0 galaxies show nuclear bulges) as well as a disk population of stars. Their rotational characteristics are like those of the faster rotating spirals and the surface brightness fades away toward the edge in the same manner as the spirals. As for other properties, these galaxies have intermediate sizes, masses, and luminosities; that is, no truly giant or truly dwarf S0 types are found. In Hubble's interpretation, these galaxies are composed only of stars, with no interstellar gas, and consequently no star formation‐defining spiral arm regions. The S0 galaxy (and its barred counterpart, the SB0) were considered to be an “intermediate” or “transition” form of galaxy between the ellipticals and spirals. In the modern understanding of galaxies, this interpretation has been called into question, because it is now known that there exist apparently perfectly normal S0 galaxies that have significant fractions of their mass in the form of interstellar gas.
Hubble “tuning fork” diagram for classification
The purpose of any classification is not only to separate objects into distinct classes but also to seek an understanding of the relationships between the classes. Two aspects of the Hubble galaxy types are suggestive of a progressive relationship between the several types. The first is the distinction between pure stellar systems versus those with some content of interstellar material. Second, but related to the first, is a recognizable trend from “round” to “flat” galaxies. To visually portray the different types of galaxies in a simple manner, Hubble placed the round elliptical galaxies at the left and set the progressively flatter galaxies to the right, with the axisymmetric and barred spiral galaxies placed along two parallel paths. Arranged in this manner, the galaxies form what looks like a tuning fork on its side; that is, a “tuning fork” diagram (see Figure 2).
Hubble “tuning fork” diagram for galaxies.
As popularly portrayed, the Hubble diagram is misleading. Spiral galaxies are neither highly symmetric about the center (axisymmetric) or barred, but they show a full range of appearances from non‐barred to barred. In other words, the area between the two prongs of the fork is actually filled with galaxies.
Originally, Hubble believed that his diagram mapped a sequence of evolutionary stages of galaxies, from an initial structure as an elliptical (hence the term early‐type galaxy) followed by continued flattening into a final thin disk (late‐type spiral galaxy). This perspective is incorrect because all galaxies show evidence of the same large age; every galaxy has a population of old stars. Even the irregulars have old stars whose faintness makes them hard to see under the light contributed by the bright young stars. If there are young galaxies, they are exceptions and are the result of special circumstances, such as collisions or mergers. Observational studies show large differences between the galaxy types in rotational properties (random motions are dominant in ellipticals but inconsequent in spirals), total masses (the largest ellipticals are more massive than the largest spirals; even among spirals, the most massive Sa galaxies are larger than the most mass Sc galaxies), mass distributions (pure “halo” for ellipticals, halo plus disk in different proportions for the spirals), and content (“old” stars and no interstellar material in ellipticals; old and young stars and a range of interstellar material in the spirals). Theoretical studies confirm that galaxies cannot change form in any simple manner from one type to another. Unlike stars that do evolve from Main Sequence to giants to white dwarfs, galaxies cannot evolve from ellipticals to spirals or vice‐versa.
The galaxy types therefore appear to be fundamentally different for reasons other than dynamical evolution. A deciding factor appears to be the fraction of material in old stars versus old/young stars in disks (as indicated by the halo/bulge to disk size comparison), which would necessarily be related to the history of the conversion of interstellar material into stars in each galaxy type. These differences in the proportions of the halo and disk would suggest that galaxy types are due to differences at the time of formation, or slightly different primordial conditions leading to different final forms of galaxies. For example, it was suggested that an initial pregalactic cloud of gas with virtually no rotation would collapse into an elliptical galaxy, its gas going completely into stars during the collapse. If there were some rotation, then conservation of angular momentum would cause the gas to flatten into a rotating disk whose differential rotation would act to oppose star formation, hence a spiral galaxy with star formation continuing to the present time. The use of high speed computers to test these ideas numerically shows that it is impossible to form gas‐free elliptical galaxies from an initial gravitational collapse. Star formation from the interstellar material is very inefficient, the remaining gas flattens to a disk, and only disklike gassy galaxies (not necessarily identical to the spiral galaxies of today) are formed. As elliptical galaxies do exist, then they must form in some manner from these original disk galaxies, a process that has come to be understood only by study of those galaxies that cannot be fit into the four types defined by Hubble.