1. TYPES OF GALAXIES

2. Edwin Hubble Early 1900’s: astronomers determined that galaxies appear in a low number of shapes Hubble (1920s): developed a classification scheme for galaxies that is still used today

3. Hubble Classification of Galaxies 4 main shapes with subclasses Spirals Barred spirals Ellipticals Irregulars

4. Properties of Galaxies Spiral and barred spiral EllipticalIrregular Mass (x M sun )10 9 to 4 x 10 11 10 7 to 10 13 10 8 to 3 x 10 10 Luminosity (x L sun )10 8 to 2 x 10 10 3 x 10 5 to 10 11 10 7 to 10 9 Diameter (ly)1.6 x 10 4 to 8 x 10 5 3 x 10 3 to 6.5 x 10 5 3 x 10 3 to 3 x 10 4 Percentage of Observed Galaxies 77%20%3%

5. Spiral Galaxies Characterized by a central bulge and curved lanes of stars, which appear as arms that spiral outward The more tightly wound the spiral, the larger the bulge Three types of spiral galaxies: Sa (spiral type a): tightly wound spiral arms Sb: moderately wound spiral arms Sc: loosely wound spirals Typical spiral galaxies contain 100 billion stars and measure 10 5 light years in diameter Closest spiral galaxy to us: Andromeda Galaxy

6. Spiral Galaxies Aside from amount of winding, the spiral arms can have one of two structures Some galaxies have spiral arms that are broad, fuzzy, chaotic, and poorly defined: flocculent spirals Others have spiral arms that are thin, delicate, graceful and well defined: grand-design spirals

7. Barred Spiral Galaxies A spiral galaxy with a bar of stars crossing through the central bulge Include our own Milky Way Spiral: no line of stars across central bulge: Barred spiral: line of stars is present

8. Barred Spiral Galaxies Have less mass than normal spiral galaxies Account for about 1/3 of all spirals Two main arms typically extend from the ends of the bar rather than from the central bulge itself

9. Elliptical Galaxies Have no spiral arms Tremendous range in size and mass Dwarf elliptical galaxies are the most common ellipticals Contain only a few million stars Can see straight through the center of the galaxy Hard to detect Giant elliptical galaxies are more rare

10. Elliptical Galaxies Are subdivided based on how round or oval they look Numbered from E0 to E7 Roundest elliptical galaxies are E0 galaxies Most elongated elliptical galaxies are E7 galaxies

11. Irregular Galaxies All other observed galaxies that cannot be classified as spiral, barred spiral, or elliptical

12. Hubble’s Summary S0: intermediate between ellipticals and spirals Most galaxies do not change shape once formed