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Laws of Motion and Energy Chapter Seven: Gravity and Space • 7.1 Gravity • 7.2 The Solar System • 7.3 The Sun and the Stars Investigation 7B The Sizes of the Solar System • How big is the solar system? 7.3 Stars • A star is essentially an enormous, hot ball of gas held together by gravity. • The density at the sun’s core is about 158.0 g/cm3. This is about 18 times the density of solid copper! 7.3 Why stars shine • At high density and pressure, nuclear fusion occurs, releasing tremendous amounts of energy. • Fusion reactions in the sun combine hydrogen to make helium. 7.3 Anatomy of the sun • • The corona is the outermost layer of the sun’s atmosphere, extending millions of kilometers beyond the sun. Sunspots are areas of gas that are cooler than the gases around them. 7.3 Types of stars • • • Stars come in a range of sizes and temperatures. Blue giant stars are hot and much more massive than the sun. Stars that are smaller than the sun come in two main categories, dwarfs and neutron stars. 7.3 Distances to the nearest stars • One light year is the distance that light travels through space in one year. • A light year is a unit of distance, not time. 7.3 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 stars’ surface temperatures are different. 7.3 Temperature and color • Red stars are cooler than white stars, and blue stars are the hottest. • White light is a mixture of all colors at equal brightness. 7.3 Galaxies • A galaxy is a huge group of stars, dust, gas, and other objects bound together by gravitational forces. • The sun, along with an estimated 200 billion other stars, belongs to the Milky Way galaxy. 7.3 Galaxies • The disk of the Milky Way is a flattened, rotating system that contains young to middle-aged stars, along with gas and dust. 7.3 Distances between galaxies • • The distances between stars are 10,000 times greater than the distances between planets. The distances between galaxies are a million times greater than the distances between stars. 7.3 Distances between galaxies • • Figuring out the distance between galaxies is one of the more difficult tasks in astronomy. A faint object in the night sky could be a dim object that is relatively nearby or a bright object that is far, far away. 7.3 Distances between galaxies • • The most reliable method for estimating the distance to a galaxy is to find a star whose luminosity is known. If the luminosity is known, the inverse square law can be used to find the distance from the observed brightness. Astronomy Connection Extraterrestrial Volcanoes • There are active volcanoes on Earth, but did you know that a moon of Jupiter is considered the most volcanically active place in the solar system? Activity Sunspots • Sunspots are large, dark regions that appear on the surface of the Sun. • In this activity, you will determine the diameter of the Sun and the number of sunspots on the Sun.