Download Lesson 2 Power Notes Outline

Unit 1 Lesson 2 Stars Power Notes Outline
Reach for the Stars!
What is a star?
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A star is a large celestial body that is composed of gas and emits light.
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Stars are made mostly of hydrogen and helium. They also contain other elements in small
amounts.
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Stars emit light and vary in brightness.
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The temperatures of stars vary, resulting in differences in color.
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Stars range in color from red, which indicates a cool star, to blue, which indicates a very hot star.
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The sun is a relatively cool yellow star.
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Stars have different sizes, ranging from 1/100 the size of the sun to 1,000 times the size of the
sun.
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Two or more stars may be bound together by gravity, which causes them to orbit each other.
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Three or more stars that are bound by gravity are called multiple stars or multiple star systems.
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The sun is a star and is composed mostly of hydrogen and helium. It also contains oxygen,
carbon, neon, and iron.
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At the center of the sun lies the core, where gases are compressed and heated and temperatures
reach 15 million degrees Celsius.
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The sun’s core is where matter is converted into energy.
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The sun’s atmosphere has several layers and extends millions of kilometers into space.
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The photosphere is the layer of the sun’s atmosphere we see from Earth. It has an average
temperature of 5,527 °C.
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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.
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The sun’s middle atmosphere is called the chromosphere. Its temperatures range from
4,225 °C to 6,000 °C.
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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?
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Apparent magnitude is the measure of a star’s brightness as seen from Earth.
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Ancient astronomers, using only their eyes, described star brightness by magnitude.
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They called the brightest stars they could see first magnitude and the faintest stars they could
see sixth magnitude.
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Using telescopes, astronomers see many stars that are too dim to see with the unaided eye.
They added to the magnitude system.
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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.
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Dim stars have positive (larger) numbers, and bright stars have negative (smaller) numbers.
How is star luminosity measured?
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When astronomers use the word luminosity, they mean the actual brightness of a star. They
measure it on a scale called absolute magnitude.
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Absolute magnitude is a measure of how bright a star would be if the star were located at a
standard distance.
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Absolute magnitude is a measure of the brightness of a star whose distance from Earth is known.
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Stars with the same absolute magnitude may have different apparent magnitudes.
Too Hot to Handle
How are the surface temperatures of stars measured?
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Stars have different colors.
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The differences in the colors of stars are due to differences in their surface temperatures.
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The same is true of all objects that glow.
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If an object’s color depends only on temperature, the object is called a blackbody.
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As the temperature of a blackbody rises, it glows brighter and brighter red.
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As it gets hotter, its color changes to orange, yellow, white, and blue-white.
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Stars that have the lowest surface temperatures (below 3,500 °C) are red.
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Stars that have the highest surface temperatures (above 25,000 °C) are blue.
How are the sizes of stars measured?
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Stars differ greatly in size.
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Some are about the same size as Earth, and others are larger than the size of Earth’s orbit
around the sun.
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Astronomers use the size of the sun to describe the size of other stars.
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Very small stars, called white dwarfs, have about the same radius as Earth, which is
approximately 0.01 solar radius.
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Very large stars, called giant stars, typically have sizes between 10 and 100 times the sun’s
radius.
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Some rare, extremely large stars have sizes of up to 1,000 solar radii. They are called
supergiants.
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Compare the sizes and temperatures of the red, blue, and yellow stars.