Modern Astronomy Stars & Galaxies Lecture 7. The Milky Way Galaxy: Putting it all together Geraint F. Lewis University of Sydney 2007. Outline. Galactic Structure Where are the stars? Rotation and spiral structure Into the middle Formation of the Galaxy Milky Way and friends.
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
The Milky Way Galaxy:
Putting it all together
Geraint F. Lewis
University of Sydney 2007
Our galaxy itself contains 100 billion stars.
It’s a 100 thousand light years side to side.
It bulges in the middle, 16 thousand light years thick,
But out by us, it’s just 3000 light years wide.
We’re 30,000 light years from galactic central point.
We go ‘round every 200 million years,
And our galaxy is only one of millions of billions
In this amazing and expanding Universe
From the Meaning of Life
Star on circular orbits
Many young stars (active star formation)
Metal rich stars (Pop I)
Pronounced spiral arms
Thin & thick disksThe Galactic Disk: Stars
Unlike the Earth orbiting the Sun, the stars in the Bulge are not orbiting a single, massive object.
It is the gravitational attraction of all the stars together that keeps the bulge as a single object.
In effect, they are orbiting nothing!
Stars must have formed a long time ago out of almost pristine material. Stars must have small masses to have lived this long!
The Sun orbits the Galaxy at 220km/s
We can tell by looking at the Rotation Curve
Given the distribution of stars and gas in the Milky Way, we would not expect a flat rotation curve.
A lot (10-100x) more than seen!!!
The identification of a massive dark matter halo enveloping the Milky Way has some important implications
Where does spiral structure come from?
We have seen the disk doesn’t rotate like a solid wheel and so it would appear the spirals cannot be “fixed”!
The spiral structure is seem in hot, young blue stars and gas, but not in old red stars.
Do the spirals just rotate with the stars?
So spirals should rapidly become over wound!
Density waves are enhancements in the surface density of the galaxy
Unlike stars, clouds of gas do not pass through density waves unscathed
Both cause cloud collapse & forming new stars, especially hot, luminous (OB) stars. These light up the spiral arms.
What causes density waves?
So, what is occurring in the Galactic centre?
The region is clearly very energetic!
What is this nothing which the gas and stars are rapidly orbiting?
Only real possibility is a Black Hole!
Must have a mass of 2.7£106M¯.
Not a black hole from a single stellar collapse, but must be built up over time.
We shall meet these again when we come to look at quasars, but we have to ask “Just how active was the young Milky Way”?
Nice picture, but it does not explain everything!!!
The Magellanic Clouds