1. Characteristics of Stars
Chapter 21 Section 2

2. Constellations
Ancient observers around the world imagined that stars are group in shapes of people or animals.

3. Classifying Stars
Characteristics used to classify stars include color, temperature, size, composition, and brightness.
The color of stars can tell you the temperature. (Think of light bulb or toaster oven)

4. Star Size

5. Brightness of Stars
The brightness of a star depends upon both its size and temperature.
Apparent Brightness: a stars brightness as seen from Earth
Absolute Brightness: the brightness the star would have if it were a standard distance from Earth

6. Measuring Distance to Stars
Astronomers use a unit called the light-year to measure distances between the stars.
A light-year is the distance that light travels in one year, about 9.5 million kilometers.
*A light year is a measure of distance, not time.

7. Measuring Distance to Stars
Astronomers have found ways to measure the distance between us and the stars!
They often use parallax to measure distances to nearby stars.
Parallax is the apparent change in position of an object when you look at it from different places.

8. Parallax

9. The Hertzsprung-Russell Diagram (H-R Diagram)
This diagram shows the relationship between the surface temperature and absolute brightness of stars.
Astronomers use H-R diagrams to classify stars and to understand how stars change over time.
Most of the stars in the H-R diagram form a diagonal area called the main sequence.

10. Which star has a hotter surface: Rigel or Aldebaran?

11. Questions??? *What are three characters we use to classify stars?
*What is the difference between apparent and absolute brightness?
*Star A and B have about the same apparent brightness, but star A is about twice as far away from Earth as Star B. Which star has greater absolute brightness?
*What two characteristics of stars are shown in an H-R diagram?