Unit 1 Lesson 2 Stars Copyright © Houghton Mifflin Harcourt Publishing Company

Reach for the Stars! What is a star? Unit 1 Lesson 2 Stars Reach for the Stars! What is a star? A star is a large celestial body that is composed of gas and emits light. Stars are made mostly of hydrogen and helium. They also contain other elements in small amounts. Stars emit light and vary in brightness. Copyright © Houghton Mifflin Harcourt Publishing Company 2

Unit 1 Lesson 2 Stars What is a star? The temperatures of stars vary, resulting in differences in color. Stars range in color from red, which indicates a cool star, to blue, which indicates a very hot star. The sun is a relatively cool yellow star. Copyright © Houghton Mifflin Harcourt Publishing Company 3

Unit 1 Lesson 2 Stars What is a star? Stars have different sizes, ranging from 1/100 the size of the sun to 1,000 times the size of the sun. Two or more stars may be bound together by gravity, which causes them to orbit each other. Three or more stars that are bound by gravity are called multiple stars or multiple star systems. Copyright © Houghton Mifflin Harcourt Publishing Company 4

Unit 1 Lesson 2 Stars What is a star? The sun is a star and is composed mostly of hydrogen and helium. It also contains oxygen, carbon, neon, and iron. At the center of the sun lies the core, where gases are compressed and heated and temperatures reach 15 million degrees Celsius. The sun’s core is where matter is converted into energy. Copyright © Houghton Mifflin Harcourt Publishing Company 5

Unit 1 Lesson 2 Stars What is a star? The sun’s atmosphere has several layers and extends millions of kilometers into space. The photosphere is the layer of the sun’s atmosphere we see from Earth. It has an average temperature of 5,527 °C. Energy is transferred from the sun’s core to the photosphere and escapes into space as visible light, other forms of radiation, heat, and wind. Copyright © Houghton Mifflin Harcourt Publishing Company 6

Unit 1 Lesson 2 Stars What is a star? The sun’s middle atmosphere is called the chromosphere. Its temperatures range from 4,225 °C to 6,000 °C. In the sun’s outer atmosphere, or corona, temperatures may reach 2 million degrees Celsius. Copyright © Houghton Mifflin Harcourt Publishing Company 7

You’re a Shining Star How is star brightness measured? Unit 1 Lesson 2 Stars You’re a Shining Star How is star brightness measured? Apparent magnitude is the measure of a star’s brightness as seen from Earth. Ancient astronomers, using only their eyes, described star brightness by magnitude. They called the brightest stars they could see first magnitude and the faintest stars they could see sixth magnitude. Copyright © Houghton Mifflin Harcourt Publishing Company 8

How is star brightness measured? Unit 1 Lesson 2 Stars How is star brightness measured? Using telescopes, astronomers see many stars that are too dim to see with the unaided eye. They added to the magnitude system. Today, the brightest stars have a magnitude of about –2, and the faintest stars that we can see with a telescope have a magnitude of +30. Dim stars have positive (larger) numbers, and bright stars have negative (smaller) numbers. Copyright © Houghton Mifflin Harcourt Publishing Company 9

How is star luminosity measured? Unit 1 Lesson 2 Stars How is star luminosity measured? When astronomers use the word luminosity, they mean the actual brightness of a star. They measure it on a scale called absolute magnitude. Absolute magnitude is a measure of how bright a star would be if the star were located at a standard distance. Absolute magnitude is a measure of the brightness of a star whose distance from Earth is known. Copyright © Houghton Mifflin Harcourt Publishing Company 10

How is star luminosity measured? Unit 1 Lesson 2 Stars How is star luminosity measured? Stars with the same absolute magnitude may have different apparent magnitudes. Copyright © Houghton Mifflin Harcourt Publishing Company 11

Too Hot to Handle How are the surface temperatures of stars measured? Unit 1 Lesson 2 Stars Too Hot to Handle How are the surface temperatures of stars measured? Stars have different colors. The differences in the colors of stars are due to differences in their surface temperatures. The same is true of all objects that glow. Copyright © Houghton Mifflin Harcourt Publishing Company 12

How are the surface temperatures of stars measured? Unit 1 Lesson 2 Stars How are the surface temperatures of stars measured? If an object’s color depends only on temperature, the object is called a blackbody. As the temperature of a blackbody rises, it glows brighter and brighter red. As it gets hotter, its color changes to orange, yellow, white, and blue-white. Copyright © Houghton Mifflin Harcourt Publishing Company 13

How are the surface temperatures of stars measured? Unit 1 Lesson 2 Stars How are the surface temperatures of stars measured? Stars that have the lowest surface temperatures (below 3,500 °C) are red. Stars that have the highest surface temperatures (above 25,000 °C) are blue. Copyright © Houghton Mifflin Harcourt Publishing Company 14

How are the sizes of stars measured? Unit 1 Lesson 2 Stars How are the sizes of stars measured? Stars differ greatly in size. Some are about the same size as Earth, and others are larger than the size of Earth’s orbit around the sun. Astronomers use the size of the sun to describe the size of other stars. Copyright © Houghton Mifflin Harcourt Publishing Company 15

How are the sizes of stars measured? Unit 1 Lesson 2 Stars How are the sizes of stars measured? Very small stars, called white dwarfs, have about the same radius as Earth, which is approximately 0.01 solar radius. Very large stars, called giant stars, typically have sizes between 10 and 100 times the sun’s radius. Some rare, extremely large stars have sizes of up to 1,000 solar radii. They are called supergiants. Copyright © Houghton Mifflin Harcourt Publishing Company 16

How are the sizes of stars measured? Unit 1 Lesson 2 Stars How are the sizes of stars measured? Compare the sizes and temperatures of the red, blue, and yellow stars. Copyright © Houghton Mifflin Harcourt Publishing Company 17