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Astronomy Chapter Nineteen: Galaxies and the Universe • 19.1 Tools of Astronomers • 19.2 Stars • 19.3 Galaxies and the Universe Investigation 19B Stars • What are stars made of? 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. 19.2 Stars • Stars generate light and heat through nuclear fusion. • Fusion reactions in the sun combine hydrogen to make helium. 19.2 The size of stars • • Stars come in a range of sizes and temperatures. Our Sun is a medium-sized star. 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. 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. 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. 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. 19.2 Brightness and luminosity • From experience, you know that as you move away from a source of light, the brightness decreases. 19.2 Brightness and luminosity • Brightness, also called intensity, describes the amount of light energy per second falling on a surface. 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. 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. 19.2 Analyzing light from stars • A spectrometer splits light into a spectrum of colors and displays lines of different colors along a scale. 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. 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. 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. 19.2 How the solar system formed • Scientists think that the solar system was formed out of the same nebula that created the Sun.