Galactic Structure

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 stars position after
  adjusting for the Earths motion around the Sun.
• Combine with distance
• Used to find transverse motion
• Doppler shift is used to find the radial motion.

Barnards 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.

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 togetherwith
  enough density, stars form.
• Galaxies merged to form the Milky Way