Expert: Philip Stahl - 5/13/2007

Question
How did galaxies end up shaped differently?  What are the differences, besides the shape, in differently shaped galaxies?

Answer
Hello,

Right now it isn't possible to say with any certainty how the differing shapes of galaxies arose. It is largely speculation, in other words.

For example, a "density wave" model has been explored to try to account for the spiral-shape of spiral galaxies like the Milky Way and Andromeda. The model basically assumes that a two- or four-armed "spiral wave" sweeps through the galactic plane and order the gas into overall spiral shape.

While the model succeeds in explaining aspects of the spiral structure, it fails on a number of grounds, including:

- inability to account for the origin of these "density waves"

- No idea of what sustains the waves long enough to form the spirals

Similar problems apply to the other types of galaxies.

The differences - besides shape- that apply to the different galaxies include:

"Irregular" galaxies, lacking any spiral structure, generally have no conspicuous dust clouds and are dominated by OB -type stars and neutral hydrogen or HII regions.

Spiral galaxies, including disk spirals, contain rotating disks of material, in the form of neutral hydrogen gas. This is confirmed by radio observations (e.g. at the 21 cm line for neutral hydrogen)

Elliptical galaxies (such as M 87 in Virgo) have very little dust or gas, and OB stars are also absent. They also tend to have a reddish overall color, suggesting older stars.

Hope this info helps!