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