For any star in the sky, we KNOW:
Apparent Magnitude (m)
Spectral Type (O, B, A, F, G, K, M)
Luminosity Class (Main Sequence, Giant, etc…). These are denoted by a roman numeral (V, III, I,…).
Combine spectral type and luminosity class to get absolute magnitude (M).
From Lecture 7B: m – M give you distance.
Deneb is A2Ia star
m = 1.25
A2 Blue star
M = -8.8
Distance = 1000 pc
If we know how bright something should be, and we know how bright it looks Distance
RR Lyra stars
For RR Lyrae stars:
Average luminosity is a standard candle
Always ~ 100 x Sun
For Cepheid variables:
Pulsation period is proportional to average luminosity
Observe the period find the luminosity
Good to 15 Mpc!
Objects in the disk, rotate in the disk.
Objects in the halo, swarm in a halo.
… and Formation
Picture the formation of the Sun:
Condenses to disk
Planets in a plane
Oort cloud sphere.
Perhaps the same with the galaxy?
From variable stars we know distances.
From Doppler shift we know rotation velocity.
Use Kepler’s Third Law (again) to get mass of the Milky Way.
M = 1011 x Msun
What causes the mass to keep on increasing?
Don’t see anything there. Thus “dark” matter.
Use gravitational lensing (last lecture) to look for these “dark” objects.
Because of all the dust in the Galaxy, we can’t see its center in visible light.
Can use IR and radio to pierce the dust.