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Astronomy Study Guide
Chapter 4 Section 2 pp.126-133—Study workbook pages 55-59 also
Know Main Ideas
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Be able to interpret the Hertzsprung-Russell Diagram on page 132. The diagram shows the
relationship between surface temperature and absolute brightness of the stars.
Students will be asked to answer questions based on a sample of the diagram.
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Apparent brightness is based on the distance of the star from the Earth. If two stars have the same
absolute brightness then the only way to tell the apparent brightness is to know the distance from Earth

Astronomers measure the distance between the stars in units called light years (the distance light
travels in 1 year: about 9.5 million million kilometers)

Astronomers use parallax to measure the distance to nearby stars by looking at the placement of the
star in the sky in January and then again in July. They look to see how much the star has moved in that
time period to calculate the distance to the star. The more it moves the closer to Earth it is. (see figure 9
on page 131 in the textbook)
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The main characteristics used to classify stars
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Color
Temperature
Size
Composition
Brightness
Classifying the sun
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Color—yellow
Temperature—5,800o Celsius
Size—medium-sized
Composition—73% hydrogen, 25% helium, 2% other
Brightness—
 apparent brightness—very bright
 absolute brightness—average brightness
Know your vocabulary
Apparent brightness—the brightness of a star as seen from Earth
Absolute brightness—a star’s brightness as if it were a standard distance from Earth
Constellation—an imaginary pattern of stars (example—Orion)
Hertzsprung - Russell diagram (H-R diagram)—a graph of stars showing surface temperature on the x-axis
and absolute brightness on the y-axis
Light year—a unit that is often used to measure distances between stars
Main sequence—the region of the Hertzsprung-Russell diagram that most stars fall within
Parallax—is often used to determine the distance to nearby stars
Spectrograph—a device that breaks light into colors and produces an image