2. UNIT NINE: Matter and Motion in the Universe
Chapter 26 The Solar System
Chapter 27 Stars
Chapter 28 Exploring the Universe

4. Chapter Twenty-Eight: Exploring the Universe
28.1 Tools of Astronomers
Galaxies
Theories about the Universe

5. Section 28.2 Learning Goals
Classify galaxies according to shape.
Examine the features of the Milky Way galaxy.
Tell how astronomers determine the distances between objects in space.

6. 28.2 Galaxies
A galaxy is a huge group of stars, dust, gas, and other objects bound together by gravitational forces.
The sun, along with an estimated 200 billion other stars, belongs to the Milky Way galaxy.

7. 28.2 Types of galaxies
The Milky Way is a flattened, rotating system that contains young to middle-aged stars, along with gas and dust.
Astronomers identify it as a spiral galaxy.

9. 28.2 Types of Galaxies
Astronomers classify galaxies according to their shape.
Spiral galaxies consist of a central, dense area surrounded by spiraling arms.
Barred spiral galaxies have a bar-shaped structure in the center.
Elliptical galaxies look like the central portion of a spiral galaxy without the arms.
Lenticular galaxies are lens-shaped.

10. Can you classify these galaxies?

11. 28.2 The central black hole theory
Recent studies have suggested that a black hole, with a mass of more than a million Suns, exists at the very center of our galaxy.
The evidence for a huge black hole comes from measurements of the orbital velocities of stars and gas at the center.
One of the strangest predictions of Einstein’s theory of relativity is the existence of black holes.

12. 28.2 The central black hole theory
The minimum speed an unpowered projectile must have to escape the planet’s gravity is called the escape velocity.
A black hole is an object with such strong gravity that its escape velocity equals or exceeds the speed of light.

13. 28.2 Distances between galaxies
The distances between stars are 10,000 times greater than the distances between planets.
The distances between galaxies are a million times greater than the distances between stars.

14. 28.2 Distances between galaxies
Figuring out the distance between galaxies is one of the more difficult tasks in astronomy.
A faint object in the night sky could be a dim object that is relatively nearby or a bright object that is far, far away.

15. 28.2 Distances between galaxies
The most reliable method for estimating the distance to a galaxy is to find a star whose luminosity is known.
If the luminosity is known, the inverse square law can be used to find the distance from the observed brightness.

16. 28.2 Distances between galaxies
The inverse square law shows how the brightness of an object decreases as you move away from it.

17. 28.2 Distances between galaxies
The inverse square law is important to astronomers because if they know the brightness and luminosity of an object, they can determine its distance by rearranging equation variables.

18. 28.2 Distances between galaxies
Astronomers have found a way to infer values for luminosity (L) using a source of light called a standard candle.
A standard candle is an object, such as a star, whose luminosity is known.
By determining the unknown star’s temperature (using a spectrometer), they can infer its luminosity by choosing a similar main sequence star on the H-R diagram.

19. 28.2 Distances between galaxies
A second type of standard candle is called a Cepheid star.
Cepheid stars “pulsate” in regular periods ranging from a few days to a few weeks.
By measuring the period of a Cepheid star, astronomers determine its luminosity and then calculate its distance.

20. Astronomy and Light Key Question: Investigation 28B
How do we measure the distance to stars and galaxies?