The Milky Way and Beyond (35 page)

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.

So 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 discussed below.

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 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.

Galaxies with unusual properties often have shorthand names that refer to their characteristic properties. Common examples are:

D: Galaxies with abnormally large, distended shapes, always found in the central areas of galaxy clusters and hypothesized to consist of merged galaxies.

S: Seyfert galaxies, originally recognized by the American astronomer Carl K. Seyfert from optical spectra. These objects have very bright nuclei with strong emission lines of hydrogen and other common elements, showing velocities of hundreds or thousands of kilometres per second. Most are radio sources.

N: Galaxies with small, very bright nuclei and strong radio emission. These are probably similar to Seyfert galaxies but more distant.

Q: Quasars, or QSOs, small, extremely luminous objects, many of which are strong radio sources. Quasars apparently are related to Seyfert and N galaxies but have such bright nuclei that the underlying galaxy can be detected only with great difficulty.

There are also different schemes used for extremely distant galaxies, which we see in their youth. When a very distant galaxy is examined with a very large telescope, we see its structure as it was when the light was emitted billions of years ago. In such cases, the distinctive Hubble types are not so obvious. Apparently, galaxies are much less well organized in their early years, and these very distant objects tend to be highly irregular and asymmetrical. Although special classification schemes are sometimes used for special purposes, the general scheme of Hubble in its updated form is the one most commonly used.

Beyond the Milky Way stretches the vast universe filled with billions of galaxies. Astronomers have measured their distances and determined their properties to a degree that would have seemed unbelievable only a few decades ago.

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.