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UNIT NINE: Matter and Motion in the Universe Chapter 26 The Solar System Chapter 27 Stars Chapter 28 Exploring the Universe Chapter Twenty-Seven: Stars 27.1 The Sun 27.2 Stars 27.3 The Life Cycle of Stars Chapter 27.2 Learning Goals Compare the Sun to other stars. Interpret an H-R diagram. Explain how stars are classified. Investigation 27A Stars and Spectroscopy Key Question: What are stars made of? 27.2 Classifying Stars Astronomers classify stars according to: size/mass, temperature, color, and brightness. 27.2 The size of stars Stars come in a range of sizes and masses. Our Sun is a mediumsized star. The largest stars, giant stars have a mass of about 60 times the mass of the Sun. 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. 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. 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. 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. 27.2 Brightness and luminosity Brightness, also called intensity, describes the amount of light energy per second falling on a surface. 27.2 Brightness and luminosity For a distant source of light like a star, the brightness decreases as the inverse square of the distance. 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. 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 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?