AST101
Lecture 19
Discovery of the Galaxy
Northern Milky Way
Southern
Milky Way
My God, it’s full of stars…
Star Counts
Star Counts
The Panchromatic Milky Way
• Different objects emit at different wavelengths
• For a blackbody, peak brightness is at
λ ~ 1/T
• Other processes emit in different ways
Gamma-rays
X-rays - Hot Gas
Optical light - Stars
Near Infrared - old stars
Infrared light - Dust
21 cm (radio) - Hydrogen
Radio - Electrons
Shape of the Galaxy
Shape of the Galaxy
You are here
You are here
• About 28,000 light years from the
Center of the Galaxy.
• Our orbital velocity is about 220 km/s.
• The Galactic Year is about 220 million
years long.
• The Sun is about 21 galactic years old.
The Mass of the Galaxy
• A star orbiting the center of the galaxy is the
same as a planet orbiting the Sun.
• Use Newton’s laws
M=v2r/G
• But, the mass depends on radius
Rotation-Velocity Curve
Mass of the Galaxy
The mass of the Galaxy is 2x1044gm, or
1011 solar masses.
If the typical star is 1/4 solar masses,
there are 4 x 1011 stars in the Galaxy
The Center of the Galaxy
Radio:
minispiral
VLA,
6 cm
X-rays
Chandra
Infrared
IR
K band
K band
Keck AO
A. Ghez
UCLA
Orbits at the Center
The central object
• Orbits  Mass ~ 2.5 x 106 M
• Orbits  radius < 1 AU
• Density > 0.4 gm/cm3
• Unseen at any wavelength
A black hole