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-Seven: Stars  27.1 The Sun  27.2 Stars  27.3 The Life Cycle of Stars

5. Chapter 27.2 Learning Goals  Compare the Sun to other stars.  Interpret an H-R diagram.  Explain how stars are classified.

6. Investigation 27A  Key Question: What are stars made of? Stars and Spectroscopy

7. 27.2 Classifying Stars  Astronomers classify stars according to:  size/mass,  temperature,  color, and  brightness.

8. 27.2 The size of stars  Stars come in a range of sizes and masses.  Our Sun is a medium- sized star.  The largest stars, giant stars have a mass of about 60 times the mass of the Sun.

9. 27.2 The size of stars  There are two types of giant stars.  Red giants are cooler than white stars.  Blue giant stars are hot and much more massive than our sun.

10. 27.2 The size of stars  Stars that are smaller than the sun come in two main categories, dwarfs and neutron stars.  Sirius, the Dog Star, is the largest known white dwarf.

11. 27.2 Temperature and color  If you look closely at the stars on a clear night, you might see a slight reddish or bluish tint to some stars.  This is because their surface temperatures are different.

12. 27.2 Temperature and color  The color of light is related to its energy.  White light is a mixture of all colors at equal brightness.

14. 27.2 Brightness and luminosity  Brightness, also called intensity, describes the amount of light energy per second falling on a surface.

15. 27.2 Brightness and luminosity  For a distant source of light like a star, the brightness decreases as the inverse square of the distance.

16. 27.2 Brightness and luminosity  Luminosity is the total amount of light given off by a star in all directions.  Luminosity is a fundamental property of a star whereas brightness depends on both luminosity and distance.

17. 27.2 Temperature and luminosity  In the early 1900s, Danish astronomer Ejnar Hertzsprung and American astronomer Henry Russell developed an important tool for studying stars.  Their graph showed luminosity on the y axis…  …and surface temperature on the x axis

18. 27.2 Temperature and luminosity  H-R diagrams are useful because they help astronomers categorize stars into groups:  Main sequence stars, like the Sun, are in a very stable part of their life cycle.  White dwarfs are hot and dim and cannot be seen without a telescope.  Red giants are cool and bright and some can be seen without a telescope. Can you locate blue giants on the H-R diagram?