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Unit 1 Lesson 2 Stars
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Unit 1 Lesson 2 Stars
How is star luminosity measured?
• Stars with the same absolute magnitude may
have different apparent magnitudes.
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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.
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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.
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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.
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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.
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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.
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Unit 1 Lesson 2 Stars
How are the sizes of stars measured?
• Compare the sizes and temperatures of the red,
blue, and yellow stars.
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