1. Galaxies
Cities of Stars

2. Characteristics of Galaxies
Most stars reside in dense groups known as galaxies.
These 'island universes' come in many shapes and sizes, and contain anywhere from a few million stars to a trillion or more.

3. astronomers have found that galaxies come in many shapes, sizes, and colors.
The universe may contain 100 billion galaxies or more.

4. Galaxies also contain vast clouds of gas and dust, which are the raw materials for new stars.
Galaxies also contain vast quantities of "dark matter" -- matter that produces no detectable light or other form of energy, but that reveals its presence through its gravitational pull on the visible stars and gas.

5. In theory, if the universe lasts long enough, the galaxies will die
In theory, if the universe lasts long enough, the galaxies will die. Their stars will burn out.
Some of the stars will drift away, but some will fall into giant "black holes" that lurk in the hearts of most galaxies.
Eventually, all galaxies will disappear from sight.

6. Types of Galaxies
Galaxies can be classified in several ways. The most common is a system developed by Edwin Hubble, which is based on the shapes of galaxies

7. Spirals
Spirals are flat disks of stars with bright bulges in their centers. Spiral arms wrap around these bulges.
In some spirals, a wave organizes the stars in the center into a bar. The arms of barred galaxies spiral outward from the ends of the bar.

9. Elliptical
they look like fat, fuzzy footballs. Instead of spreading out into a thin disk, as they do in spirals, the stars in ellipticals wrap completely around the galaxy's heart in all directions.
most of them appear to contain "supermassive" black holes at their hearts
The largest galaxies in the universe are giant ellipticals

11. Irregulars
The final class of galaxies contains a hodge-podge of shapes -- anything that looks neither spiral nor elliptical.
These are the irregulars.
These galaxies have no identifiable form.
Their stars, gas, and dust spread randomly.
These are the smallest galaxies

13. Our Galaxy: The Milky Way
The Milky Way consists of a bulge of stars in the core, a thick bar of stars flanking the core, and bright spiral arms around the core
The Milky Way is a spiral galaxy
Our solar system is about 27,000 light-years from its center.
100,000 light years in total diameter

14. The Milky way…
Milky Way is part of a cluster of galaxies called the local group.
A group of 22 galaxies make up this cluster
Includes the Andromeda Galaxy

15. Components…. Our galaxy is full of dust.
Elements like silicon, carbon, and iron are forged in the cores of stars and released into space late in the stars' lives.
The dust is concentrated in the Milky Way's disk.

16. Structure… Disk contains between 200 billion and 400 billion stars.
A black hole perhaps four million times as massive as the Sun sits in the middle of the Milky Way.

18. Where are we?
Our solar system is located in the outer reaches of one of the spiral arms.
It takes approximately million years for our solar system to orbit once around the Milky Way.

20. There are no photos of the Milky Way galaxy taken from outside
There are no photos of the Milky Way galaxy taken from outside. That would require a space probe to be able to get beyond the galaxy, but that hasn't yet been done.
What do you think is the most common type of galaxy?
What do you think is the brightest type of galaxy?