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SPIRAL/BARRED SPIRAL
(S, SB) |
ELLIPTICAL(E) |
IRREGULAR (IRR) |
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Shape and structural properties
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Highly flattened disk of stars and gas, containing spiral arms and thickening to a 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
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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
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No obvious structure; Irr II galaxies often have "explosive" appearance
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Stellar content
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Disks contain both young and old stars; halos consist of old stars only
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Contain old stars only
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Contain both young and old stars
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Gas and dust
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Disks contain substantial amounts of gas and dust; halos contain little of either
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Contain little or no gas and dust
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Very abundant in gas and dust
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Star formation
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Ongoing star formation in spiral arms
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No significant star formation during the last 10 billion years
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Vigorous ongoing star formation
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Stellar motion
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Gas and stars in disk move in circular orbits around the galactic center; halo stars have random orbits in three dimensions
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Star have random orbits in three dimensions
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Stars and gas have very irregular orbits
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