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NGC 2997, a spiral galaxy (Type Sc) |
NGC 1365, a barred spiral galaxy (Type SBc) |
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From Images of Galaxies, by David Malin |
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Galaxies are classified into three basic different types: Spirals (S), Ellipticals (E), and Irregulars (Irr). Examples can be found here: Galaxy types. The largest galaxies tend to be either spirals (about 80%) or ellipticals (about 20%).
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NGC 2997, a spiral galaxy (Type Sc) |
NGC 1365, a barred spiral galaxy (Type SBc) |
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From Images of Galaxies, by David Malin |
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Like the Milky Way, spiral galaxies are composed of a central bulge and halo of Population II stars and globular clusters and also a disk of gas and young Population I stars. (You can review Populations I and II in Lesson 8.) Some spiral galaxies have a greater fraction of their mass in the bulge/halo population and some are mostly disk population. Just as in the Milky Way, the spiral arms contain dense clouds of gas where new stars are forming.
Many spiral galaxies have a "bar" of stars in their centers from which the spiral arms emerge. Recently, astronomers realized that the Milky Way is actually a barred spiral galaxy. -- see Passing the Bar Exam, from Astronomy Magazine.
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M87, a giant elliptical galaxy (Type E1) |
The Large Cloud of Magellan (LMC), an Irregular galaxy (Type Irr) |
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From Images of Galaxies, by David Malin |
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Elliptical galaxies have no Population I stars, no spiral arms, no cool interstellar gas, and no star formation. They are dead galaxies. If spiral galaxies are like rain forests, with cool life-giving interstellar clouds, elliptical galaxies are like deserts, with hot dry winds and little life.
Edwin Hubble developed the scheme illustrated above (source) to classify galaxies according to their shapes. He classified elliptical galaxies according to their shapes, ranging from spherical (denoted E0) to very flattened (denoted E7). S0 denotes a "lenticular" (lens-shaped) galaxy that is nearly elliptical but shows some hint of a spiral disk. Hubble divided spiral galaxies into normal spirals (denoted S) and barred spirals (denoted SB). A third letter (a, b, c) denotes how tightly wound the spiral arms are, ranging from so tight that the arms can hardly be discerned (Sa and SBa) to a relatively open pattern (Sc and SBc). You can see plenty of examples of galaxies classified according to Hubble's scheme here: Hubble Types. But it's an imperfect scheme. Two galaxies of the same Hubble Type can look very different.
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Superposition of two images of the spiral galaxy M74. The blue image actually represents the galaxy as seen in ultraviolet light, where we see the hot massive stars in the spiral arms. The red image is dominated by the light from red giant stars, many of which have lived long enough to migrate out of the spiral arms where they were formed. Source: Beyond the Blue from the Ultraviolet Imaging Telescope. |
Galaxies can appear very different depending on the wavelength band of the electromagnetic spectrum in which they are observed. For example, at red wavelengths the galaxy image is dominated by the old stars in the elliptical bulge, while at ultraviolet wavelengths the hot young stars in spiral arms are much more prominent, as illustrated above. See also this movie, from Beyond the Blue, showing how the appearance of the spiral galaxy M81 changes as the image morphs from ultraviolet to red light.
Irregular is a catch-all definition for all sorts of galaxies that cannot easily be classified by Hubble's scheme of spirals and ellipticals. Many irregular galaxies are the result of interactions between galaxies (see COLLISIONS). Many of the smaller "dwarf galaxies" are irregulars.
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Last modified March 20, 2002
Copyright by Richard McCray
