Galactic Structure

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

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

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

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

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

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

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

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