Download Size Color and Temperature

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Page 462
Size
It is hard to get a sense of how large stars are from viewing them in
the sky. Even the Sun, which is much closer than any other star, is far
larger than its appearance suggests. The diameter of the Sun is about
100 times greater than that of Earth. A jet plane flying 800 kilometers per
hour (500 mi/h) would travel around Earth’s equator in about two days.
If you could travel around the Sun’s equator at the same speed, the
trip would take more than seven months.
A star the size
of the Sun
Diameter = 1.4
million kilometers
(900,000 mi)
Some stars are much larger than the Sun. Giant and supergiant
stars range from ten to hundreds of times larger. A supergiant called
Betelgeuse (BEET-uhl-JOOZ) is more than 600 times greater in diameter
than the Sun. If Betelgeuse replaced the Sun, it would fill space in our
solar system well beyond Earth’s orbit. Because giant and supergiant
stars have such huge surface areas to give off light, they are very bright.
Betelgeuse is one of the brightest stars in the sky, even though it is
522 light-years away.
There are also stars much smaller than the Sun. Stars called white
dwarfs are about 100 times smaller in diameter than the Sun, or roughly
the size of Earth. White dwarfs cannot be seen without a telescope.
Color and Temperature
If you observe stars closely, you may notice that they vary slightly in
color. Most stars look white. However, a few appear slightly blue or
red. The differences in color are due to differences in temperature.
White dwarf
1/100 the Sun’s
diameter
Giant star
10–100 times the
Sun’s diameter
Supergiant star
100–1000 times
the Sun’s diameter
You can see how temperature affects color by heating up metal.
For example, if you turn on a toaster, the metal coils inside will start to
glow a dull red. As they get hotter, the coils will turn a brighter orange.
The illustration on page 463 shows changes in the color of a metal bar
as it heats up.
Like the color of heated metal, the color of a star indicates its
temperature. Astronomers group stars into classes by color and surface
temperature. The chart on page 463 lists the color and temperature
range of each class of star. The coolest stars are red. The hottest stars are
blue-white. Our Sun—a yellow, G-class star—has a surface temperature
of about 6000°C.
Stars of every class give off light that is made up of a range of colors.
Astronomers can spread a star’s light into a spectrum to learn about the
star’s composition. The colors and lines in a spectrum reveal which
gases are present in the star’s outer layers.
How does a star’s temperature affect its appearance?
462 Unit 4: Space Science
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