General Properties of Galaxies

Property Spirals - Barred Spirals Irregulars Dwarf Ellipticals - Giant Ellipticals
Diameter (kpc) 30 6 10 - 50
Mass (Sun=1) 1011 to 1012 106 106 - 1013
Luminosity (Sun=1) 1010 109 108 - 1011
Shape and structural properties Highly flattened disk of stars and gas, containing spiral arms and thickening to central bulge. Sa and Sba galaxies have largest bulges, the least obvious spiral structure, and roughly spherical stellar halos. SB galaxies have an elongated central "bar" of stars and gas. No obvious structure. Some irregulars have "explosive" appearance. No disk. Stars smoothly distributed through an ellipsoidal volume ranging from nearly spherical (E0) to very flattened (E7) in shape. No obvious substructure other than a dense central nucleus.
Stellar content Disks contain both young and old stars; halos consist of old stars only. Contain both young and old stars. Contain old stars only.
Color Bluish (disk), reddish (halo and nucleus) Bluish Reddish
Neutral gas (fraction of mass) 5 % 15 % Less than 1 %
Gas and Dust Disks contain substantial amounts of gas and dust; halos contain little of either. Very abundant in gas and dust. Contain little or no gas and dust.
Star formation Ongoing star formation in spiral arms. Vigorous ongoing star formation. No significant star formation during the last 10 billion years.
Stellar motion Gas and stars in disk move in circular orbits around the galactic center; halo stars have random orbits in three dimensions. Stars and gas have very irregular orbits. Stars have random orbits in three dimensions.