Chapter 17: Galaxies. Galaxy Types Cosmic Distances Cosmic Ages Galaxy Evolution. Islands of Stars. Hubble Deep Field: Tiny slice of the sky imaged during 10 days with HST. Galaxies of many sizes, colors and shapes. The observable universe contains > 8x10 10 galaxies. Galaxy Types.
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Barred spiral galaxies have a straight bar of stars cutting across the center.
Lenticular (lens-shaped) galaxies have disks but do not appear to have spiral arms.
The galaxy classification system proposed by Edwin Hubble remains widely used. Letter E for elliptical with a number for the elongation. S for spirals. SB for barred spirals. SO for lenticulars. Lower case letters indicate the size of the relative sizes of bulge and disk.