2. Astronomy

3. Chapter Nineteen: Galaxies and the Universe 19.1 Tools of Astronomers 19.2 Stars 19.3 Galaxies and the Universe

4. Investigation 19B What are stars made of? Stars

5. 19.2 Stars On a clear night, about 6,000 stars can be seen without a telescope. A constellation is a group of stars that form a pattern when seen from Earth.

6. 19.2 Stars Stars generate light and heat through nuclear fusion. Fusion reactions in the sun combine hydrogen to make helium.

7. 19.2 The size of stars Stars come in a range of sizes and temperatures. Our Sun is a medium-sized star.

8. 19.2 The size of stars Red stars are cooler than white stars. Blue giant stars are hot and much more massive than the sun.

9. 19.2 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.

10. 19.2 Temperature and color of stars 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.

11. 19.2 Temperature and color of stars The color of light is related to its energy. White light is a mixture of all colors at equal brightness.

13. 19.2 Brightness and luminosity From experience, you know that as you move away from a source of light, the brightness decreases.

14. 19.2 Brightness and luminosity Brightness, also called intensity, describes the amount of light energy per second falling on a surface.

15. 19.2 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.

16. 19.2 Analyzing light from stars Spectroscopy is a tool of astronomy in which the light produced by a star or other object (called its spectrum) is analyzed.

17. 19.2 Analyzing light from stars A spectrometer splits light into a spectrum of colors and displays lines of different colors along a scale.

18. 19.2 Analyzing light from stars In 1861, Sir William Huggins used spectroscopy to determine that the Sun and the stars are made mostly of hydrogen.

20. 19.2 The life cycle of stars A star, regardless of its size, begins its life inside a huge cloud of gas (mostly hydrogen) and dust called a nebula. A protostar is the earliest stage in the life cycle of a star. The Orion Nebula was the birthplace of these protostars.

21. 19.2 The life cycle of stars As a star grows old, its core begins to run out of hydrogen fuel. When hydrogen fusion stops, the core glows brightly and is called a white dwarf. When a white dwarf stops glowing, it is called a brown dwarf, the final stage in the life cycle of Sun-like stars.

22. 19.2 How the solar system formed Scientists think that the solar system was formed out of the same nebula that created the Sun.