1. Galaxies

2. Galaxies
A galaxy is a huge collection of billions of stars, gas, and dust held together by gravity
There are three main types of galaxies.
Spiral, Elliptical, and Irregular.
There are billion galaxies in the universe

3. The “Discovery” of Galaxies
At the beginning of the 20th century, what we now call spiral galaxies were referred to as “spiral nebulae” and most astronomers believed them to be clouds of gas and stars associated with our own Milky Way. The breakthrough came in 1924 when Edwin Hubble was able to measure the distance to the “Great Nebula in Andromeda” (M 31, at right) and found its distance to be much larger than the diameter of the Milky Way. This meant that M 31, and by extension other spiral nebulae, were galaxies in their own right, comparable to or even larger than the Milky Way.
Edwin P. Hubble ( )

4. Spiral Galaxies Pinwheel shape
Huge, about 32,000 to 160,000 light years across
Large amounts of gas and dust
Young and old stars present
New stars forming
Revolve quickly around a central bulge, looks like a thin disk from the side with a halo present

6. The Nuclear Bulge of M31
Young stars have formed along the foreground spiral arm. M31’s two satellite galaxies M32 and NGC 205, both dwarf elliptical galaxies, are in the bottom center and upper right.
(NOAO/AURA Photos)

7. Examples of Spirals

8. Elliptical Galaxies Round to oval shape
Small amounts of cool gas and dust
Massive, 6.5 million light years across
Almost no new stars
Rotate as a whole with each star having a unique orbit
Mainly old stars present
Bulge but no disk, halo present

10. Irregular Galaxies No regular shape Very large amounts of gas and dust
Many new stars present with mostly young blue stars
Huge, 3,200 to 32,500 light years across
Halo is present, may show signs of a bulge
Unsure of movement

12. Clusters of Galaxies
Rather than occurring individually in space, galaxies are grouped in clusters ranging in size from a few dozens to thousands of galaxies. The Coma Cluster, shown at right, is 300 million light years from the Milky Way and contains more than 1,000 (and possibly as many as 10,000) galaxies.
The Milky Way is a member of a small cluster called the Local Group which contains about 40 galaxies. The largest member of the Local Group is M 31, with the Milky Way coming in second in size.
(NOAO/AURA Photo)

13. Milky Way Galaxy
Our galaxy
Spiral in shape
100,000 light years across

17. Units of distance – Light Year
Light year is a unit of distance
Light moves at a velocity of 300,000 kilometers per second or 186,000 thousand miles per second.
So, in one year it can travel about 10 trillion km.
The Andromeda galaxy is 2.3 million light years away and the nearest star Proxima Centauri is 4.24 light years away.

18. Units of distance- Astronomical Unit
A little closer to home to home we use an AU or astronomical unit which is the average distance between the Earth and the sun to measure distances in our solar system.
1 AU=93 million miles
Mercury =.39
Earth=1.0
Jupiter=5.2