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Chapter 15
Section 1 Stars
Bellringer
List ways that stars differ from one another.
How is the sun like other stars? How is it different?
Write your answers in your Science Journal.
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Chapter 15
Section 1 Stars
What You Will Learn
• Stars differ in size, temperature, composition,
brightness, and color.
• Distances between stars are very large and are
measured in light-years.
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Chapter 15
Section 1 Stars
Color of Stars
• The color of a star indicates the star’s temperature.
• Red stars are the coolest, and blue stars are the
hottest.
• If two stars differ in color, you can conclude that they
differ in temperature too.
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Chapter 15
Section 1 Stars
Composition of Stars
• Astronomers use an instrument called a
spectroscope to separate a star’s light into a
spectrum (plural, spectra).
• A spectrum is the band of colors produced when
white light passes through a prism.
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Chapter 15
Section 1 Stars
Composition of Stars, continued
• A continuous spectrum shows all of the colors, while
an absorption spectrum shows which wavelengths of
light are absorbed.
• The spectrum of a star is an absorption spectrum
because the atmosphere of the star absorbs certain
portions of the light produced by the star.
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Chapter 15
Section 1 Stars
Composition of Stars, continued
• When a chemical element emits light, only some
colors in the spectrum show up. The colors that
appear are called emission lines.
• Every element has a unique set of emission lines that
act like a fingerprint for that element.
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Chapter 15
Section 1 Stars
Composition of Stars, continued
• The pattern of lines in a star’s absorption spectrum is
unique to that star.
• A star’s absorption spectrum can be used to
determine the elements in that star’s atmosphere and
the stage the star occupies in its life cycle.
• Stars are made of mostly hydrogen and helium
gases.
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Chapter 15
Stars, Galaxies, and the Universe
Composition of Stars, continued
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Chapter 15
Section 1 Stars
Classifying Stars
• Stars are now classified by how hot they are.
• Temperature differences between stars result in color
differences that can be seen. For example, class O
stars are blue—the hottest stars.
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Chapter 15
Section 1 Stars
Classifying Stars, continued
• Magnitude is used to compare the brightness of one
object with the brightness of another object.
• To express the brightness of stars, astronomers use
a system of magnitudes.
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Chapter 15
Section 1 Stars
Classifying Stars, continued
• Positive magnitude numbers represent dim stars.
Negative magnitude numbers represent bright stars.
• The brightest star in the night sky, Sirius, has a
magnitude of -1.4.
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Chapter 15
Stars, Galaxies, and the Universe
Classifying Stars
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Chapter 15
Section 1 Stars
How Bright Is That Star?
• The apparent magnitude is the brightness of a star
as seen from Earth. The absolute magnitude is the
brightness that a star would have at a distance of
32.6 light-years from Earth.
• If all stars were the same distance away, their
absolute magnitudes would be the same as their
apparent magnitudes.
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Chapter 15
Stars, Galaxies, and the Universe
Absolute and Apparent Magnitude
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Chapter 15
Section 1 Stars
Distance to the Stars
• Because stars are so far away, astronomers use a
unit called a light-year to measure the distance from
Earth to the stars.
• A light-year is the distance that light travels in one
year, about 9.46 trillion kilometers.
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Chapter 15
Section 1 Stars
Distance to the Stars, continued
• Parallax is an apparent shift in the position of an
object when viewed from different locations.
• Astronomers use parallax and trigonometry to find
the actual distance to stars that are close to Earth.
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Chapter 15
Section 1 Stars
Motions of Stars
• All of the stars in the sky appear to make one
complete circle around Polaris every 24 h.
• This apparent motion of the stars is due to Earth’s
rotation and its revolution around the sun.
• Each star is actually moving in space. But because
stars are so distant, their actual motion is hard to see.
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Chapter 15
Stars, Galaxies, and the Universe
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