National Aeronautics and Space Administration
Galaxies come in three major classes distinguished by their appearance: spirals, like the Milky Way, are shaped like pinwheels; irregulars have no discernible shape at all; and ellipticals are round- or oval-shaped objects.
Spirals and irregulars are typically sites of ongoing star-formation and therefore contain young stars. Ellipticals, having finished their supply of fresh gas, cannot form stars any more and contain mostly very old stars.
Spiral galaxies are a composite of stars and gas in a disk surrounding a central bulge, which is rather similar to an elliptical galaxy, just smaller. Waves in the disk form the spiral arms and cause the gas to collapse and form new stars. Therefore, the disk is rich in young stars. Older stars are typically found in the bulge.
Elliptical galaxies and the bulges of spirals have been the subject of several decades of observational and theoretical work. For decades, astronomers thought that the rotation rate of these spherical star systems determined whether they would be round or oval shaped, with the more rapidly rotating ellipticals being the flattest.
Detailed studies of thousands of ellipticals over the years now suggest an entirely different picture. Ellipticals and bulges are supported against their self-gravity, which would cause them to shrink, by the random velocities of the stars, pretty much like the motion of molecules in a hot gas. The distribution of stellar motion determines the final shape of the galaxy, that is, whether it is spherical, oblate, or very flattened.
In recent years, astronomers also have discovered that apparently simple galaxy shapes hide the complex, violent events that occurred in these galaxies long ago.
Some contain dense cores in which millions of stars move in orbits completely different than stars farther out from the galaxy's center.
In many ways, the cores of some resemble isolated populations transplanted from outside the galaxy. Astronomers are beginning to believe that these cores are the remains of companion galaxies that were consumed when they wandered too close to these elliptical galaxies many millions of years ago. When galaxies collide, the rapidly changing gravitational fields also can synchronize the stellar orbits, creating great rings of stars that surround some ellipticals like haloes.
Elliptical galaxies also contain some of the oldest stars in the universe. While spirals and irregulars continue to produce new stars even to the present day, most ellipticals stopped forming stars more than 10 billion years ago in what must have been one great star-forming epoch.
Ellipticals contain little or no gas and dust of their own, apparently having consumed what they had when their stars were born long ago.
Those ellipticals that contain higher concentrations of gas and dust apparently accumulated the material because they cannibalized their companion galaxies.
The material accumulated from these cannibalizations collides as it sinks farther and farther into the galaxy's core,
and in many instances, creates new generations of massive, luminous stars.
Eventually over the course of millions of years, the gas reaches the center of the galaxy where super-massive black holes may lie in wait for a new supply of fuel.
For more information and pictures, link to Hubble Identifies Primeval Galaxies, Uncovers New Clues to the Universe's Evolution (HubbleSite - NewsCenter - Background Info, December 6, 1994)
Updated: January 11 '98
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