The Milky Way Galaxy
What does the Milky Way look like?
William & Caroline Herschel
1750-1848 1738-1822
Jacobus Kapteyn (1851-1922)
First to study the extent of the Milky Way Galaxy
Grindstone Model
Sun at center of irregularly shaped cloud of stars.
Harlow Shapely (1885-1972)
Studied globular clusters of stars and used ………
Henrietta Leavitt’s earlier work on Cepheid Variable Stars to determine the true shape of the Milky Way galaxy
Cepheid Variable Stars
They are stars which change their luminosity (reliably) over time.
WHY?
The instability of stellar evolution produces a reliable fluctuation of a star’s absolute brightness.
Instability Strip
HR Diagram & Stellar Evolution
Instability Strip
Luminosity
O
B
A
F
G
K
M
Stars in this phase of their life are susceptible to pulsations (size & luminosity)
This behavior can help us determine distances!
If we know the intrinsic luminosity of a star we can compare it to the apparent luminosity of a star and determine distance.
Cepheids give us distance to objects – and we can then determine where things are.
Central Bulge
Galactic Disk
The Galactic Disk:
• Most stars are here. Nearly all the interstellar gas. • Old Stars (1010yrs) to Younger Stars (106yrs) • Star Formation is occurring now. • Composition: Old Metal Poor to Young Metal Rich stars. • Motions – coplanar, direct, elliptical orbits. • Spiral Arms (?)
Globular Clusters
Halo
Bulge
Corona
Disk
Venus
Mercury
Earth
The Galactic Bulge & Halo
The Galactic Halo
• Thin scattering of stars & clusters • Stars (OLD), globular clusters (OLD), no interstellar material.
• Metal Poor material (mostly H, He, very little else)
• Random eccentric orbits
The Galactic Bulge
• Like the halo, only more crowded