AST101 Lecture 19 Discovery of the Galaxy. Northern Milky Way

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