14-2 Hubble devised a system for classifying galaxies according to their appearance

With large telescopes, millions of galaxies are visible across every unobscured part of the sky. Although all galaxies are made up of large numbers of stars, they come in a variety of shapes and sizes. Hubble classified them into four broad categories based on their appearance; this Hubble classification is a scheme that is still used today. The four classes of galaxies are the spirals, classified S; barred spirals, or SB; ellipticals, E; and irregulars, Irr. Table 14-1 summarizes some key properties of each class. These various types of galaxies differ not only in their shapes but also in the kinds of processes that take place within them.

Spiral Galaxies: Stellar Birthplaces

M51 (Figure 14-1), M31 (Figure 14-2), and M100 (Figure 14-3) are examples of spiral galaxies. The spiral arms contain young, hot, blue stars and glowing hydrogen gas regions, indicating ongoing star formation.

Thermonuclear reactions within stars create metals, that is, elements heavier than hydrogen or helium. These metals are dispersed into space as the stars evolve and die. So, if new stars are being formed from the interstellar matter in spiral galaxies, they will incorporate these metals and be metal-rich Population I stars, similar to our Sun. Indeed, when we measure the visible-light spectrum of the disk of a spiral galaxy, we can see obvious absorption lines from metals. Such a spectrum is a composite of the spectra of many stars and shows that the stars in the disk are principally of Population I. By contrast, there is relatively little star formation in the central bulges of spiral galaxies, and these regions are dominated by old Population II stars that have a low metal content. This also explains why the central bulges of spiral galaxies have a yellowish or reddish color; as a population of stars ages, the massive, luminous blue stars die off first, leaving only the longer-lived, low-mass red stars.

Edwin Hubble further classified spiral galaxies according to the size of their central bulges and the appearance of their arms. Spirals with the largest and most obvious central bulges and tightly wrapped, smooth, broad spiral arms are called Sa galaxies, for spiral type a (Figure 14-5a); those with moderately well defined spiral arms and a moderate-size central bulge, like M81, are Sb galaxies (Figure 14-5b); and galaxies with narrow, well-defined but loosely wrapped spiral arms and a tiny central bulge are Sc galaxies (Figure 14-5c).

Figure 14-5: RIVUXG Spiral Galaxies Edwin Hubble classified spiral galaxies according to the size of their central bulges and the appearance of their arms. At one end of the range, (a) Sa galaxies have the largest and most obvious central bulges and tightly wrapped, smooth, broad spiral arms, while (b) Sb galaxies are more moderate, and (c) Sc galaxies have the smallest central bulges with loosely wrapped arms. (a: Adam Block/Steve Mandel/Jim Rada and Students/NOAO/AURA/NSF; b: NASA/JPL-Caltech/ESA/Harvard-Smithsonian CfA; c: FORS Team, 8.2-meter VLT, ESO)

The differences between Sa, Sb, and Sc galaxies may be related to the relative amounts of gas and dust they contain. Observations with infrared telescopes—which detect the emission from interstellar dust—and radio telescopes—which detect radiation from interstellar gases such as hydrogen and carbon monoxide—show that about 4% of the mass of an Sa galaxy is in the form of gas and dust. This percentage is 8% for Sb galaxies and 25% for Sc galaxies.

Interstellar gas and dust is the material from which new stars are formed, so an Sc galaxy has a greater proportion of its mass involved in star formation than an Sb or Sa galaxy. Hence, an Sc galaxy has a large disk, where star formation occurs, and a small central bulge, where there is little or no star formation. By comparison, an Sa galaxy, which has relatively little gas and dust and thus less material from which to form stars, has a large central bulge and only a small star-forming disk.

Barred Spiral Galaxies: Spirals with an Extra Twist

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In barred spiral galaxies, such as those shown in Figure 14-6, the spiral arms originate at the ends of a bar-shaped region running through the galaxy’s nucleus rather than from the nucleus itself. As with ordinary spirals, Hubble subdivided barred spirals according to the relative size of their central bulge and the character of their spiral arms. An SBa galaxy has a large central bulge and thin, tightly wound spiral arms (Figure 14-6a). Likewise, an SBb galaxy is a barred spiral with a moderate central bulge and moderately wound spiral arms (Figure 14-6b), while an SBc galaxy has lumpy, loosely wound spiral arms and a tiny central bulge (Figure 14-6c). As for ordinary spiral galaxies, the difference between SBa, SBb, and SBc galaxies may be related to the amount of gas and dust in the galaxy.

Figure 14-6: RIVUXG Barred Spiral Galaxies As with spiral galaxies, Hubble classified barred spirals according to the texture of their spiral arms (which correlates to the sizes of their central bulges). SBa galaxies have the smoothest spiral arms and the largest central bulges, while SBc galaxies have narrow, well-defined arms and the smallest central bulges.

Bars appear to form naturally in many spiral galaxies. This conclusion comes from computer simulations of galaxies, which set hundreds of thousands of simulated “stars” into orbit around a common center. As the “stars” orbit and exert gravitational forces on one another, a bar structure forms in most cases. Indeed, barred spiral galaxies outnumber ordinary spirals by about two to one.

Elliptical and Irregular Galaxies

Elliptical galaxies, so named because of their distinctly elliptical shapes, have no spiral arms. Hubble subdivided these galaxies according to how round or flattened they look. The roundest elliptical galaxies are called E0 galaxies and the flattest, E7 galaxies. Elliptical galaxies with intermediate amounts of flattening are given designations between these extremes (Figure 14-7).

Figure 14-7: RIVUXG Elliptical Galaxies Hubble classified elliptical galaxies according to how round or flattened they look. A galaxy that appears round is labeled E0, and the flattest-appearing elliptical galaxies are designated E7.

Elliptical galaxies look far less dramatic than their spiral and barred spiral cousins. The reason is that ellipticals have virtually no interstellar gas and dust. Consequently, there is little material from which stars could have recently formed, and indeed there is no evidence of young stars in most elliptical galaxies. For the most part, star formation in elliptical galaxies ended long ago. Hence, these galaxies are composed of old, red, Population II stars with only trace amounts of any metals.

Elliptical galaxies come in a wide range of sizes and masses. Both the largest and the smallest galaxies in the known universe are elliptical. Figure 14-8 shows two giant elliptical galaxies that are about 20 times larger than an average galaxy. These giant ellipticals are located near the middle of a large cluster of galaxies in the constellation Virgo.

Figure 14-8: RIVUXG Giant Elliptical Galaxies The Virgo cluster is a rich, sprawling collection of more than 2000 galaxies about 56 million ly from Earth. Only the center of this huge cluster appears in this photograph. The two largest members of this cluster are the giant elliptical galaxies M84 and M86.

Giant ellipticals are rather rare, but dwarf elliptical galaxies are quite common. Dwarf ellipticals are only a fraction of the size of their normal counterparts and contain so few stars—only a few million, compared to more than 100 billion (10¹¹) stars in our Milky Way Galaxy—that these galaxies are completely transparent. You can actually see straight through the center of a dwarf galaxy and out the other side, as Figure 14-9 shows.

Figure 14-9: RIVUXG A Dwarf Elliptical Galaxy This diffuse cloud of stars is a nearby E4 dwarf elliptical called Leo I. It actually orbits the Milky Way at a distance of about 600,000 ly. Leo I is about 3000 ly in diameter but contains so few stars that you can see through the galaxy’s center.

Hubble also identified galaxies that are midway in appearance between ellipticals and the two kinds of spirals. These are denoted as S0 and SB0 galaxies, and are also called lenticular galaxies. Although they look somewhat elliptical, lenticular (“lens-shaped”) galaxies have both a central bulge and a disk like spiral galaxies, but no clear spiral arms. They are therefore sometimes referred to as “armless spirals,” mostly because we are seeing them edge-on. Figure 14-10 shows an example of an S0 (pronounced “esszero”) lenticular galaxy.

Figure 14-10: RIVUXG A Lenticular Galaxy NGC 5866 is tilted nearly edge-on to our line of sight. Close inspection reveals a crisp dust lane dividing the galaxy into two halves centered on a subtle, reddish bulge surrounding a bright nucleus, a blue disk of stars running parallel to the dust lane, and a transparent outer halo. Viewed face on, it would look like a smooth, flat disk with little spiral structure. At a distance of 44 million ly, it has a diameter of roughly 60,000 ly—only two-thirds the diameter of the Milky Way, although its mass is similar to our Galaxy.

Hubble summarized his classification scheme for spiral, barred spiral, and elliptical galaxies in a diagram, now called the tuning fork diagram for its shape (Figure 14-11).

Figure 14-11: Hubble’s Tuning Fork Diagram Edwin Hubble’s classification of regular galaxies is shown in his tuning fork diagram. An elliptical galaxy is classified by how flattened it appears. A spiral or barred spiral galaxy is classified by the texture of its spiral arms and the size of its central bulge. A lenticular galaxy is intermediate between ellipticals and spirals. Irregular galaxies do not fit into this simple classification scheme.

Finally, galaxies that do not fit into the scheme of spirals, barred spirals, and ellipticals are conveniently referred to as irregular galaxies. They are generally rich in interstellar gas and dust, and have both young and old stars. For lack of any better scheme, the irregular galaxies are sometimes placed between the ends of the tines of the Hubble tuning fork diagram, as in Figure 14-11.