1. Galaxy Types

2. Distant Nebulae
Charles Messier’s objects included some that seemed to be star clusters.
Thought to be in Milky Way
With telescopes these are much richer than star clusters.
20 to 200 billion stars each

3. How Far? Cepheid variables were used to locate globular clusters.
Apply to these objects
Multiple readings
These were other galaxies millions of parsecs (Mpc) away.
M100: 17 Mpc = 56 Mly

4. How Fast? Spectral lines tell us the velocity of galaxies.
Doppler shift
Longer wavelengths (l) at greater speed; red shift
Astronomers list velocities of galaxies in terms of red shift (z).

5. Hubble’s Law Edwin Hubble measured the velocities of many galaxies.
Most moving away
Distant galaxies move faster
The relationship between distance and velocity is called Hubble’s Law.
v = H0 d Hubble’s Law
v = recessional velocity
d = distance
H0 = 1.5 to 3.1  per sec
Hubble “constant”

6. Spirals Spiral galaxies have a dense nucleus and multiple arms.
Spiral arms can be tight or wide apart.
Spirals are usually large and have active star formation.

7. Barred Spirals
Some spiral galaxies show a distinct bar-shaped nucleus.
The central bar is anchored to two large arms.

8. Elliptical Galaxies Elliptical galaxies are round or ovoid.
They are often small compared to spiral galaxies.
Ellipticals have little dust and new star formation is rare.

9. Irregular Galaxies Irregular galaxies have ill-defined shapes.
Small like ellipticals
Star formation like spirals
Irregular galaxies are usually seen near larger galaxies.

10. Hubble’s System Edwin Hubble used the shapes to label galaxy types.
E: elliptical galaxy
Number for amount of stretch
S (or SA): spiral galaxy
SB: barred spiral galaxy
Small letter for amount of arm spread