Milky Way
Structure
Spiral Arms
Dark Matter
What is a Galaxy?
Collection of stars, gas, dust and assorted bits (black holes, pulsars, etc.) all held together by its own gravity.
Our solar system is one of billions floating around in a galaxy called the Milky Way, sometimes called just The Galaxy.
Milky Way Galaxy
No Good Picture
Since we live inside of the Milky Way, it is hard to get a good picture of it.
Best we can see is a bright streak across the sky.
However, we can look at our closest companion, the Andromeda Galaxy, which we believe to be very similar to the Milky Way.
Andromeda Galaxy - M31
Basic Structure
Galactic Disk - Flat, frisbie-shaped part of the galaxy containing the majority of luminous stars and gas
100,000 light years across
Galactic Bulge - Round ball of stars at galaxy’s center
Galactic Center - Core of the galaxy, containing million solar mass black hole.
Possibly different than Andromeda, which instead has strange double core...
Galactic Structure
Sun’s Location
1st discovered by Howard Shapley at the beginning of this century.
He measured distances and locations of big star cluster balls known as globular clusters
He found they centered not on our Sun, but around a distant location in the direction of Sagitarrius
Our solar system lies in the outer part of the disk, roughly 25,000 light years from the Galactic Center
Our Galaxy’s Halo
Globular clusters are spread around galaxy in gigantic spherical Halo around the Milky Way, going right down into the spherical bulge.
Globular clusters contain some of the oldest known stars
Sometimes referred to as Population II stars (older, redder).
Probably formed first, followed by disk, where Population I stars (younger,bluer stars) live.
Halo vs. Disk
Orbits in the Disk
Halo stars tend to fly around randomly, but Disk stars (like our Sun) travel around in well-defined orbits
Our Sun is orbiting the Milky Way once every 225 million year
Stars closer in rotate around faster.
A more complicated Kepler’s 3rd Law
Means galaxy rotates differentially
I.e., different "rings" rotate at different speeds, not like a solid disk
Spiral Arms
Looking down on the disk of our Galaxy would should beautiful, pinwheel-like Spiral arms.
But if the disk spins differentially (different speeds, different distances), how does the galaxy keep from winding up its arms?
Spiral Density Waves
The trick is that the stars are not moving around in spiral arms at all.
The arms are a density wave traveling through the disk
Imagine traffic on a busy freeway
Traffic will seem to start and stop for no reason. These are waves of density traveling through the (barely) moving cars.
The cars themselves are not moving along in a wave, but a wave of denser packed cars IS moving.
Star Formation
Density Waves are just a pattern moving through the disk, not tied to the actual stars
This means arms can move both forward and backwards (leading or trailing) from what we would expect.
As density increases, it squeezes clouds, triggering star formation
That is why the arms are so bright and distince: They are where Stars are being born.
Flat Rotation Curve
As one goes away from our Sun (where all the mass is) the speed of the planets drops
Kepler’s 3rd law
As one goes away from the Center of the Galaxy, speed of stars drops as first, But then flattens off.
This means there is a lot of mass on the outside part of our galaxy
But we don’t see it.
Dark Matter
The Halo of our galaxy is mostly a non-luminous and very mysterious Dark Matter
Might be cold objects like black holes, or brown dwards
Might be strange subatomic particles that we haven’t identified yet (Axions?)
Whatever it is, there is more Dark Matter (by at least a factor of 10!) than light matter in the Universe.
Identifying it is key to determining Universe’s density and fate...
Galactic Center
Galactic Center in Radio