文档介绍：Expansion of the Universe
Lab 10
Some Facts of Our Milky Way Galaxy
Distance from Sun to galactic center = 8 kpc ± 1 kpc
Disk of our galaxy = 50 kpc diameter, kpc thick, with a central bar-shaped bulge
Central bulge is 2 kpc in diameter(has both Pop I and Pop II stars)
Sun orbits center of galaxy at 790,000 km/hr, takes 220 million years plete 1 orbit
Facts about H
Hydrogen most abundant element in the universe but cannot emit visible light in the depths of cold interstellar space
However, DOES emit radio waves
This H is neutral not ionized, so called H I
Detection of Spin-flips
Protons and electrons have mass, charge
Also have angular momentum (spin) which creates tiny ic field
So energy of H atom is different depending on orientation of spins (if same ↓, if opposite directions ↑, but energy difference = 10-6 x electron orbits)
photon emitted in a spin-flip transition has λ= 21 cm (long, radio)
Dark Matter!!!!
Most of mass of galaxy (90%) does not emit anything that we can detect, so it is called dark matter
90 million solar masses of matter exists inside the solar orbit
But orbital speeds do NOT decrease with distance from galactic center
So a large amount of mass must exist OUTSIDE solar orbit
Therefore, total mass of our galaxy = 1012 Mסּ
MACHOs and WIMPs
Half of dark matter halo posed of MACHOs
pact Halo Objects are ~ Mסּ
Some part of dark matter are neutrinos
Also, some other subatomic particles called WIMPs (weakly interacting massive particles) which have a mass of 10-10,000> proton
Winding Dilemma
Stars, dust, gas all orbit the galactic center with ~ same speed
But this creates a winding dilemma
So density waves must sweep around galaxy, which move more slowly around the galaxy than the matter inside
This crowding promotes stellar birth and recycling of ISM
Just so you know……
At center of our galaxy is a black hole millions of times more massive than Sun
Starlight warms dust grains to 10-90˚K, which then emits radiation at λ= 30-300 μm