1. Galaxies and the Universe
4-3 Notes
Galaxies and the Universe

2. Think About…
Why do galaxies look hazy?

3. The Milky Way
Galaxy: a huge group of stars, gas, and dust held together by gravity.
The Sun is part of the Milky Way galaxy.

4. The Milky Way Without a telescope, you can only see nearby stars.
Nearby stars are a tiny fraction of the several hundred billion in the galaxy.

5. The Milky Way
The Milky Way is shaped like a disk with a bulge in the center.
Because our view is from inside the galaxy, all we can see of the Milky Way is one band or arm of stars across the sky.

6. The Milky Way The Milky Way is more than 100,000 light years across.
The Sun is about 28,000 light-years away from the center, which is a clump of very densely packed stars.

7. The Milky Way
In addition to stars, the Milky Way contains clouds of gas and dust called nebulae (plural for nebula).
Heart nebula
Orion nebula
Carina nebula
NGC 3603

8. The Milky Way
The stars and nebulae orbit the center of the galaxy at very high speeds.
The Sun takes about 250 million years to orbit the Milky Way galaxy once.

9. Galaxies
Galaxies range in size from a few hundred million stars to over a trillion stars.
Galaxies also have different shapes.

10. Arms of stars, gas, and dust curve away from the center in a spiral
Disk-shaped with a central bulge
Composed of a mix of old and young stars
Examples: Milky Way, Andromeda
Spiral Galaxies

11. Andromeda Galaxy

12. Grand Design Galaxy

13. Southern Pinwheel Galaxy

14. Elliptical Galaxies Shaped like spheres or eggs.
Very little dust and gas.
Made entirely of old stars.
Examples: IC 1101, M89

15. NGC 1132

16. M87

17. Irregular Galaxies Faint galaxies without a definite shape.
Smaller than other types.
A lot of gas and dust and many new stars.
Examples: the large and small Magellanic Clouds

18. Large Magellanic Cloud

19. Small Magellanic Cloud

20. Centers of Galaxies
Most large galaxies likely have supermassive black holes at the center.
These black holes have masses millions or billions of times greater than the Sun.

21. Centers of Galaxies
Supermassive black holes are so large that they often draw in huge whirlpools of gas from surrounding stars.

22. Centers of Galaxies
If the center of a galaxy is very bright, it may look like a star from very far away.
Quasar: the bright center of a very distant galaxy.
The galaxy surrounding a quasar is often hard to see because the quasar is so bright.

23. The Universe The Milky Way is just one of about 100 billion galaxies.
Galaxies occur in groups called superclusters.
Between the superclusters are massive spaces of near emptiness.

24. The Universe
The same elements found in our solar system and galaxy are found throughout the universe.

25. The Universe
The same physical laws that govern our solar system and galaxy also hold true throughout the universe.

26. When we look at distant objects, we are looking back in time.
For example, when we look at the Andromeda galaxy, we are seeing it as it was about 2 million years ago, because the galaxy is about 2 million light- years away
Looking Back in Time

27. Looking Back in Time
Suppose a distant intelligent life form 65 million light-years away has a telescope that can see Earth.
They wouldn’t be looking at us-they’d be looking at the dinosaurs!

28. Motion of Galaxies
Have you ever noticed that the sound of a siren changes as it travels toward and then away from you?
The pitch seems to get higher as it approaches you, then drops as it moves away.

29. Motion of Galaxies
This is called the Doppler effect: a change in the observed wavelength or frequency of a wave that occurs when the source of the wave or the observer is moving.
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30. Motion of Galaxies
The Doppler effect occurs with light as well as sound.
Objects moving toward us seem to have a “blue shift” because the light is being compressed into shorter wavelengths.

31. Motion of Galaxies
Objects moving away from us seem to have a “red shift” because the light is seemingly stretched into longer wavelengths.

32. Motion of Galaxies
Most galaxies we observe from Earth have a red shift- meaning they are all moving away from one another.

33. Motion of Galaxies
This supports the idea that the universe is expanding.

34. Development of the Universe
Because the galaxies are moving apart, scientists have concluded that there was a moment in time when the universe began to expand, which we refer to as the big bang.
This occurred around billion years ago.
Development of the Universe

35. Development of the Universe
After the big bang, the universe was extremely dense and hot.
We still have a lot to learn about what the early universe was like.
Stars began forming after about 1 billion years.

36. Review The Doppler effect is used to determine
A. The number of stars in a galaxy.
B. The number of galaxies in the universe.
C. The size of the universe.
D. Whether a galaxy is moving toward or away from Earth.

37. Review 2. What is the big bang? The collision of galaxies.
The formation of the solar system.
The beginning of the universe’s expansion.
The time when stars began to form.