1. Galactic Structure

2. Motion of the Stars
The gravity of stars in the galaxy causes stars to move.
This motion can be separated into two parts:
Radial motion is motion towards or away from the Earth.
Transverse motion moves across the path from the Earth.
Sun & Earth
observed star
radial motion
transverse motion
actual motion

3. Proper Motion
Proper motion measures a star’s position after adjusting for the Earth’s motion around the Sun.
Combine with distance
Used to find transverse motion
Doppler shift is used to find the radial motion.
Barnard’s Star is close to Earth

4. Runaway Stars Some stars appear to move away from a common point.
Past explosion
53 Arietis, AE Aurigae, and Mu Columbae would all be in the area of the Orion Nebula about 3 million years ago.
AE Aurigae
Mu Columbae
53 Arietis

5. Populations Stars are segregated by color in other galaxies.
disk stars more blue – population I
halo stars more red – population II
Similar segregation is present in clusters in the Milky Way.
sun
side view

6. Cluster Types Type I Clusters Open clusters Hot young stars
Lots of gas and dust
Abundant in heavy elements
Active star formation
Type II Clusters
Globular clusters
Old red stars
No gas and dust
No heavy elements, just hydrogen and helium
No star formation, just old stars

7. Galactic Motion Gravity applies to stars in the galaxy.
The motion of the globular clusters indicates the center.
The stars in the disk also orbit the center.
Sun orbits in 230 million years

8. Fountains
Inside the Milky Way there are areas moving into and out of the disk.
Clusters pass through the disk
Stars and black holes

9. Galaxy Formation Like stars, galaxies start with gas.
First formation about 13 billion years ago
Gravity pulled small gas clouds together—with enough density, stars form.
Galaxies merged to form the Milky Way