1. This page was copied from Nick Strobel's Astronomy Notes. Nick Strobel's Astronomy Notes.Nick Strobel's Astronomy NotesNick Strobel's Astronomy Notes Go to his site at Go to his site at www.astronomynotes.comwww.astronomynotes.com www.astronomynotes.com www.astronomynotes.com for the updated and corrected version.

2. History Edwin Hubble divided the galaxies into three basic groups: ellipticals, spirals, and irregulars Edwin Hubble divided the galaxies into three basic groups: ellipticals, spirals, and irregulars In 1936 Hubble put these groups onto a two-pronged sequence that looks like a tuning fork In 1936 Hubble put these groups onto a two-pronged sequence that looks like a tuning fork

4. Elliptical galaxies they have much more random star motion than orderly rotational motion they have much more random star motion than orderly rotational motion they have very little dust and gas left between the stars they have very little dust and gas left between the stars they have no spiral structure they have no spiral structure no new star formation occuring now and no hot, bright, massive stars no new star formation occuring now and no hot, bright, massive stars

5. Messier 32: a dwarf elliptical (E2) satellite galaxy of the Andromeda Galaxy. Courtesy of NOAO/AURA/NSFNOAO/AURA/NSF Messier 110: dwarf elliptical (E6) satellite galaxy of the Andromeda Galaxy. Courtesy of Bill Schoening, Vanessa Harvy/REU program/NOAO/AURA/NSF --- cropped from their M 31 image))NOAO/AURA/NSF

6. Messier 87: giant elliptical (E1) at the Virgo Cluster's core. It has grown very large by ``eating'' other galaxies Leo I: dwarf elliptical (E3) in the Local Group.

7. Spirals they have more orderly, rotational motion than random motion they have more orderly, rotational motion than random motion they have some or a lot of gas and dust between the stars they have some or a lot of gas and dust between the stars new star formation occuring in the disk, particularly in the spiral arms new star formation occuring in the disk, particularly in the spiral arms they have a spiral structure they have a spiral structure

8. NGC 2997: a large face-on spiral galaxy (Sc). NGC 1365: a barred spiral galaxy (SBbc).

9. Irregulars have no definite structure have no definite structure Some irregulars have a lot of dust and gas so star formation is possible Some irregulars have a lot of dust and gas so star formation is possible The stars are bunched up but the patches are randomly distributed throughout the galaxy The stars are bunched up but the patches are randomly distributed throughout the galaxy Most irregulars are small and faint Most irregulars are small and faint dwarf irregulars may be the most common type of galaxy dwarf irregulars may be the most common type of galaxy

10. Messier 82: a starburst galaxy. Courtesy of Sven Kohle and Till CrednerSven KohleTill Credner Large Magellanic Cloud: a dwarf irregular satellite galaxy of the Milky Way