What Are Elliptical Galaxies?

Identification

• Elliptical galaxies are a collection of stars in an elliptical shape. The galaxies do not have much interstellar matter. The size of the galaxies varies from a dwarf elliptical of 10 million solar masses in the form of stars (galaxy M32) to more than 10 trillion solar masses (galaxy M87). They are one of three classes of galaxy originally described by Edwin Hubble in a 1936 work. The other two were spiral and lenticular galaxies.

Origin

• Elliptical galaxies were once considered to be some of the oldest galaxies, but that opinion has changed. Computer simulations suggest that elliptical galaxies were formed by the collision of spiral galaxies. Starburst galaxies are believed to be an intermediate step between the collision and the formation of elliptical galaxies.

Insterstellar Matter

• The belief that elliptical galaxies have little interstellar matter is also being challenged. About one-third of the galaxies appear to have enough dust to create rings. They also contain both warm and cool gases, though not in enough volume to form stars. Some of the solar masses have temperatures of 10 million K, according to X-ray observations. This leads to the assumption of a pressure-supported atmosphere around the galaxy.

Star Orbits

• Stars within elliptical galaxies maintain orbits in random directions. The orbits themselves may be elliptical, speeding far away from the center only to be pulled back close to it. The galaxy barely rotates and has different dimensions in all three planes. The shape of the galaxy is maintained because of the orbits of the stars.

Hubble Classification

• Elliptical galaxies are identified by the letter E followed by a number from 0 to 7. The number refers to how flattened the galaxy appears. E0 would be an elliptical galaxy with no flattening while an E7 galaxy would be very elongated.