Galaxy Types

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

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

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).

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  10-18 per sec Hubble “constant”

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.

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

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.

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

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