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Introduction
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Most stars look like
faint dots of light in
the night sky.
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But stars are
actually huge, hot,
bright spheres of
gas that are trillions
of kilometers away
from the Earth.
Color of Stars
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Although red and yellow may be thought of as
“warm” colors and blue may be thought of as a
“cool” color, scientists consider the opposite.
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BLUE = HOT
RED (and Yellow) = COOL
Betelgeuse, which is red, and Rigel, which is
blue, are the stars that form two corners of the
constellation Orion.
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Because the stars are two different colors, we can
conclude that they have different temperatures.
Composition of Stars
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When you look at white light through a
glass prism, you see a rainbow of colors
called a spectrum – including red, orange,
yellow, green, blue, indigo, and violet.
Composition of Stars
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Astronomers use an instrument called a
spectrograph to break a star’s light into a
spectrum (discontinuous).
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Because some light has been absorbed
by the star, the spectrum provides
information about the star’s composition
AND temperature.
Temperature of Stars
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In the 1800’s, astronomers started to collect and
classify the spectra of many stars.
At first, letters were assigned to each type of spectra.
Stars were classified according to the elements of
which they were made.
Brightness of Stars
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With only their eyes to aid them, astronomers
created a system to classify stars based upon
their brightness.
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They called the brightest stars in the sky firstmagnitude stars and the dimmest stars sixthmagnitude stars.
When they began to use telescopes,
astronomers were able to see many stars that
had been too dim to see before.
Rather than replace the old system, of
magnitudes, they added to it.
Brightness of Stars
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Positive numbers
represent dimmer
stars and negative
numbers represent
brighter stars.
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The brightest star in
the night sky, Sirius
(the “dog star”), has
a magnitude of -1.4.
Brightness of Stars
Brightness of Stars
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The brightness of a light or star as
observed from Earth is called apparent
magnitude.
Absolute magnitude is the actual
brightness of a star.
Brightness of Stars
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Factors that determine a star’s
brightness are:
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Temperature
Size
Distance to Stars
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A light-year is the distance that light
travels in one year.
The closest star to Earth is the SUN
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The next closest star is the Alpha
Centauri triple star system.
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93 million miles or 150 million km away
4.3 light-years away (Proximi Centauri 4.2?)
The North Star (Polaris), which is part of
Ursa Minor is 431 light-years away.
Distance to Stars
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Astronomers use
parallax and simple
trigonometry to find
the actual distance
to stars that are
close to Earth.
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A star’s apparent shift in
position is called
parallax.
This can only be seen
through telescopes.
Motions of Stars
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Daytime and nighttime are caused by the
Earth’s rotation.
The Earth’s tilt and revolution around the sun
cause the seasons.
During each season, the Earth faces a
different part of the sky at night.
This is why you see a different set of
constellations at different times of the year.
Because stars are so distant, their actual
motion is hard to see.
Motions of Stars
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Because of Earth’s rotation, the sun appears to
move across the sky.
Likewise, if you look at the night sky long
enough, the stars also appear to move.
All of the stars appear to rotate around Polaris,
the North Star, which is almost directly above
the Earth’s North Pole.
Because of Earth’s rotation, all of the stars
appear to make one complete circle around
Polaris every 24 hours.
How are stars classified?
 Brightness
(magnitude)
 Distance (light years)
 Temperature
 Color
LESSON
7.3 Evolution of Stars