Unit 1 Lesson 2 Stars Power Notes Outline

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. • 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. • 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. • 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. • 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. • 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.

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. • 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.

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. • Stars with the same absolute magnitude may have different apparent magnitudes.

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. • 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. • 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. 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. • 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. • Compare the sizes and temperatures of the red, blue, and yellow stars.