||Spirals - Barred Spirals
||Dwarf Ellipticals - Giant Ellipticals|
||10 - 50|
||1011 to 1012
||106 - 1013|
||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
||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.
||Disks contain both young and old stars; halos consist of
old stars only.
||Contain both young and old stars.
||Contain old stars only.
||Bluish (disk), reddish (halo and
|Neutral gas (fraction of mass)
||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.
||Ongoing star formation in spiral arms.
||Vigorous ongoing star formation.
||No significant star formation during the last 10 billion
||Gas and stars in disk move in circular orbits around the
galactic center; halo stars have random orbits in three
||Stars and gas have very irregular orbits.
||Stars have random orbits in three dimensions.