Ellipticals Stellar Footballs

Elliptical galaxies present a strikingly different appearance from the spirals. When viewed with a telescope, they look a bit dull compared to their flashy spiral cousins.

Star Words

An elliptical galaxy is one that has no discernible disk or bulge, and looks like an oval or circle of stars in the sky. The true shapes of ellipticals vary from elongated ("footballs") to spherical ("baseballs") to flattened ("hamburger buns"). Elliptical galaxies consist of old stars, and appear to have little or no gas left.

Star Words

An elliptical galaxy is one that has no discernible disk or bulge, and looks like an oval or circle of stars in the sky. The true shapes of ellipticals vary from elongated ("footballs") to spherical ("baseballs") to flattened ("hamburger buns"). Elliptical galaxies consist of old stars, and appear to have little or no gas left.

There are no spiral arms, nor any discernable bulge or disk structure. Typically, these galaxies appear as nothing more than round or football-shaped collections of stars, with the most intense light concentrated toward the center and becoming fainter and more wispy toward the edges.

Of course, the orientation of an elliptical galaxy will influence its shape on the sky—that is, its apparent shape may not be its true shape. A football viewed from the side has a sort of oval shape, but, viewed on end, looks like a circle. Regardless, Hubble differentiated within this classification by apparent shape. E0 ("E-zero") galaxies are almost circular, and E7 galaxies are very elongated (or elliptical). The rest—E1 through E6—range between these two extremes.

Typically, elliptical galaxies are made up of only old stars in random orbits—much like the galactic halo of a spiral galaxy (see Chapter 21). Radio astronomers have detected no ionized hydrogen in these galaxies, which means that there is little or/no ongoing star formation.

Hickson Compact Group (HCG) 87 consists of four galaxies. The edge-on spiral at the bottom of the image and the elliptical to the right are both known to have "active nuclei," most likely related to the presence of a black hole.

(Image from AURA/STScl/NASA)

Hickson Compact Croup 87

Hickson Compact Croup 87

■ Hubble Heritage

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Close Encounter

Those of us confined to the earth's northern hemisphere don't get to see the most spectacular of the irregular galaxies, the Small and Large Magellanic Clouds. Named for the sixteenth-century explorer Ferdinand Magellan, whose men brought word of them to Europe at the conclusion of their global voyage, the Magellanic Clouds are rich in hydrogen gas. The clouds, like moons, are believed to orbit the much larger Milky Way. Like any object with mass in the universe, galaxies feel the irresistible tug of gravity and are pulled into groupings by its effects.

There is no such thing as a typical size for an elliptical galaxy. Their diameters range from a thousand parsecs (these dwarf ellipticals are much smaller than the Milky Way) to a few hundred thousand parsecs (giant ellipticals). The giant ellipticals are many times larger than our Galaxy.

Like any classification scheme, Hubble's has its share of duck-billed platypuses, objects that don't quite fit in. Some elliptical-shaped galaxies exhibit more structure than others, showing evidence of a disk and a galactic bulge. They still lack spiral arms and, like the other ellipticals, they do not contain star-forming gas. This type of galaxy (since it does have a disk and a bulge) is designated SO (pronounced "S-zero"). Some of these galaxies even contain a bar, and are designated SBO.

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