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Stars Part 2
Composition of Stars
• To learn what stars are made of,
astronomers study the light from stars.
• When you look at white light through a
glass prism, you see a rainbow of colors
called a spectrum.
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• Astronomers use a spectroscope to
separate a star’s light into a spectrum.
• The spectrum can be read to determine
the elements in a star’s atmosphere.
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• Stars are made of mostly hydrogen
and helium gases. They also contain
C, N, O, Ca, Fe, and Na.
Brightness
• Are all stars the same brightness? No. How
bright a star looks depends on how bright the
star really is, and secondly it depends on how
far away the star is.
• Positive numbers represent dim stars and
negative numbers represent bright stars. The
brightest star in the night sky, Sirius, has a
magnitude of -1.4.
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• How bright a star looks in the sky from
Earth is called apparent magnitude.
• A dim star might look bright to us on Earth
if it is close. At the same time, we may
barely be able to see a very bright star if it
is too far away.
• Betelgeuse which is one of the brightest stars
in the sky is 310 light years away. There are
thousands of closer stars in the sky that we
do not see because they are too dim.
• So, how can you tell how bright a star really
is?
• Absolute magnitude is a measure of how
bright a star would be if the star were 32.6
light-years away from Earth.
• The absolute magnitude of the sun is +4.8.
But because the sun is so close to Earth, its
apparent magnitude is -26.8, which makes it
the brightest object in the sky.
Distance to the Stars
• Because stars are so far away, we use
light-year to measure the distance
from Earth to the stars.
• A light-year is the distance that light
travels in one year.
• The distance to the North Star is 431
light years or 4,080,000,000,000,000
km.
Motion of Stars
• Because of Earth’s rotation, all of the
stars in the sky appear to make one
complete circle around Polaris every 24
hours.
• The actual motion of stars is hard to see
because they are so distant.