1. Galaxies

2. What is a galaxy?
A galaxy is a large grouping of stars, gas, and dust in space that are held together by gravity.
The largest galaxies contain more than a trillion stars. Smaller galaxies may have only a few million.
Scientists estimate the number of stars from the size and brightness of the galaxy.

3. How many Galaxies are there?
100 billion stars in each galaxy
Over 300 billion galaxies in the universe…
How many universes are there?
Only one is currently observed at present…

4. Why is it called a Galaxy?
Greek galaxias (γαλαξίας), literally means "milky", a reference to the Milky Way.
Galaxies have a milky appearance in the night sky.

5. Main Types of Galaxies
Spiral
Irregular
Elliptical

6. Properties of Spiral Galaxies
Shaped like flattened disks with one or more spiral arms.
Our galaxy, Milky Way, is a spiral galaxy.
Our solar system is located in one of the spiral arms.

7. Properties of Elliptical Galaxies
About 1/3 of all galaxies.
Simply massive blobs of stars.
Round or elliptical in shape.
Contain very little gas or dust.
Because of little gas, no new stars are forming.
The largest and smallest galaxies are elliptical galaxies.

8. Properties of Irregular Galaxies
Do not fit into any other category.
Chaotic mix of stars, gas and dust
Usually found near large spiral galaxies who may be distorting their shape.

9. Contents of Galaxies
Galaxies are made of stars, planetary systems, gas clouds, and star clusters. Nebulas are giant clouds of gas and dust where stars may be forming.
Nebula are found in spiral galaxies but not elliptical galaxies.

10. Galaxy Quiz

11. Galaxy Quiz

12. Galaxy Quiz

13. What is the name of our galaxy?+

14. Describe the Milky Way Galaxy

15. Describe the Milky Way Galaxy
Flattened Disk
Bulge in Middle
Fried Egg
Cinnamon Roll – Halle
Hurricane
Not solid from edge to edge
Huge
Light Years Thick
100,000 Light Years across (Diameter)

17. Where is our solar system in our galaxy?
The Sun and our solar system is located in one of the partial outer arms (Orion’s Arm or Orion’s Spur) of the galaxy. The distance from the Sun to the center of the galaxy is about 30,000 light years.
All objects in the galaxy revolve around its center.
The sun and our solar system take about 240 million years to make one trip around the center of our galaxy.
This is sometimes called a “cosmic year.”

19. Which way does the milky way spin?
The direction of the spin of any galaxy depends on your perspective when you look at it. Scientists believe that on large scales the Universe is isotropic (evenly in all directions). Thus, from our perspective, half of all spiral galaxies should spin clockwise, and half counter-clockwise. the Universe is indeed isotropic - we see the same number of clockwise as counter-clockwise spirals.

20. Thus the Milky Way…
Depends on your point of view and we are in the middle.
Prograde motion - revolving and/or rotating the same direction with other celestial objects.
Retrograde motion – revolving and/or rotating in the opposite direction of other celestial bodies.