1. Chapter 24 Galaxies
Chapter 24 opener. Active galaxies are much more energetic than the normal galaxy—the Milky Way—in which we live. The “central engines” powering them are thought to be supermassive black holes. This is a double image, superposing optical light (acquired by the Hubble Telescope in Earth orbit) on top of radio radiation (captured by the Very Large Array in New Mexico). At center (in white) is a giant, visible elliptical galaxy catalogued as NGC 1316, which is probably devouring its small northern neighbor. The result is the complex radio emission (in orange), called Fornax A, that extends more than a million light-years across. (NRAO/STSCI)

2. 24.1 Hubble’s Galaxy Classification
This pair of images shows the Coma cluster of galaxies. Almost every object visible is a galaxy.
Figure Coma Cluster (a) A collection of many galaxies, each consisting of hundreds of billions of stars. Called the Coma Cluster, this group of galaxies lies more than 100 million pc from Earth. (The blue spiked object at top right is a nearby star; virtually every other object in this image is a galaxy.) (b) A recent Hubble Space Telescope image of part of the cluster. (AURA; NASA)

3. 24.1 Hubble’s Galaxy Classification
Spiral galaxies are classified according to the size of their central bulge:
Figure Spiral Galaxy Shapes Variation in shape among spiral galaxies. As we progress from type Sa to Sb to Sc, the bulges become smaller and the spiral arms tend to become less tightly wound. (NASA/ESA)

4. 24.1 Hubble’s Galaxy Classification
Type Sa has the largest central bulge, Type Sb is smaller, and Type Sc is the smallest.
Type Sa tends to have the most tightly bound spiral arms with Types Sb and Sc progressively less tight, although the correlation is not perfect.
The components of spiral galaxies are the same as in our own galaxy: disk, core, halo, bulge, and spiral arms.

5. 24.1 Hubble’s Galaxy Classification
The Sombrero galaxy, with its large central bulge, is a type Sa. We cannot see the spiral arms, as they are edge-on.
Figure Sombrero Galaxy The Sombrero Galaxy, a spiral system seen edge-on. Officially cataloged as M104, this galaxy has a dark band composed of interstellar gas and dust. The large size of this galaxy’s central bulge marks it as type Sa, even though its spiral arms cannot be seen from our perspective. The inset shows this galaxy in the infrared part of the spectrum, highlighting its dust content in false-colored pink. (NASA)

6. 24.1 Hubble’s Galaxy Classification
Similar to the spiral galaxies are the barred spirals:
Figure Barred-Spiral Galaxy Shapes Variation in shape among barred-spiral galaxies. The variation from SBa to SBc is similar to that for the spirals in Figure 24.2, except that now the spiral arms begin at either end of a bar through the galactic center. In frame (c), the bright star is a foreground object in our own Galaxy; the object at top center is another galaxy that is probably interacting with NGC (NASA; AAT; ESO)

7. 24.1 Hubble’s Galaxy Classification
Elliptical galaxies have no spiral arms and no disk. They come in many sizes, from giant ellipticals of trillions of stars, down to dwarf ellipticals of less than a million stars.
Ellipticals also contain very little, if any, cool gas and dust, and they show no evidence of ongoing star formation.
Many do, however, have large clouds of hot gas, extending far beyond the visible boundaries of the galaxy.

8. 24.1 Hubble’s Galaxy Classification
Ellipticals are classified according to their shape from E0 (almost spherical) to E7 (the most elongated):
Figure Elliptical Galaxy Shapes Variation in shape among elliptical galaxies. (a) The E1 galaxy M49 is nearly circular in appearance. (b) M84 is a slightly more elongated elliptical galaxy, classified as E3. Both galaxies lack spiral structure, and neither shows evidence of cool interstellar dust or gas, although each has an extensive X-ray halo of hot gas that extends far beyond the visible portion of the galaxy. (c) M110 is a dwarf elliptical companion to the much larger Andromeda Galaxy. (AURA; SAO; R. Gendler)

9. 24.1 Hubble’s Galaxy Classification
S0 (lenticular) and SB0 galaxies have a disk and bulge, but no spiral arms and no interstellar gas:
Figure S0 Galaxies (a) S0 (or lenticular) galaxies contain a disk and a bulge, but no interstellar gas and no spiral arms. They are in many respects intermediate between E7 ellipticals and Sa spirals in their properties. (b) SB0 galaxies are similar to S0 galaxies, except for a bar of stellar material extending beyond the central bulge. (Palomar/Caltech)

10. 24.1 Hubble’s Galaxy Classification
The irregular galaxies have a wide variety of shapes. The small and large Magellanic Clouds are close neighbors to our own Milky Way:
Figure Magellanic Clouds The Magellanic Clouds are prominent features of the night sky in the Southern Hemisphere. Named for the 16th-century Portuguese explorer Ferdinand Magellan, whose around-the-world expedition first brought word of these fuzzy patches of light to Europe, they are dwarf irregular (Irr I) galaxies, gravitationally bound to our own Milky Way Galaxy. They orbit our Galaxy and accompany it on its trek through the cosmos. (a) The Clouds’ relationship to one another in the southern sky. Both the Small (b) and the Large (c) Magellanic Cloud have distorted, irregular shapes, although some observers claim they can discern a single spiral arm in the Large Cloud. (F. Espenak; Harvard Observatory)

11. 24.1 Hubble’s Galaxy Classification
Here are three other irregular galaxies: NGC 4485 and NGC 4490 on the left, and M82 on the right.
Figure Irregular Galaxy Shapes Some irregular (Irr II) galaxies. (a) The strangely shaped galaxy NGC 1427A is probably plunging headlong into a group of several other galaxies (not shown), causing huge rearrangements of its stars, gas, and dust. (b) The galaxy M82 seems to show an explosive appearance, probably the result of a recent galaxywide burst of star formation. (AURA; Subaru)

12. 24.1 Hubble’s Galaxy Classification
A summary of galaxy properties by type:

13. Summary of Chapter 24
Hubble classification organizes galaxies according to shape.
Galaxy types: spiral, barred spiral, elliptical, irregular
The Milky Way is a spiral galaxy lying within a small cluster of galaxies called the Local Group.
Other galaxy clusters may contain thousands of galaxies.