ast101 lecture 19 discovery of the galaxy. northern milky way
TRANSCRIPT
AST101Lecture 19
Discovery of the Galaxy
Northern Milky Way
SouthernMilky 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: mini-spiral
VLA,6 cm
X-rays
Chandra
Infrared
IR K band
K bandKeck AOA. GhezUCLA
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