1. Section 2: Measuring the Stars
Stellar classification is based on measurement of light spectra, temperature, and composition. K
What I Know W
What I Want to Find Out L
What I Learned

2. Essential Questions How are distances between stars measured?
What is the difference between brightness and luminosity?
What are the properties used to classify stars?
Measuring the Stars
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3. Vocabulary Review New wavelength constellation binary star parsec
parallax
apparent magnitude
absolute magnitude
luminosity
Hertzsprung-Russell diagram
main sequence
Measuring the Stars
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4. Patterns of Stars
Long ago, many civilizations looked at the brightest stars and named groups of them after animals, mythological characters, or everyday objects. These groups of stars are called constellations.
Today, astronomers group stars by the 88 constellations named by ancient peoples.
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5. Patterns of Stars
Some constellations are visible throughout the year, depending on the observer’s location. Constellations that appear to rotate around one of the poles are called circumpolar constellations. Ursa Major, which contains the Big Dipper, is a circumpolar constellation for most of the northern hemisphere.
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6. Patterns of Stars
Unlike circumpolar constellations, the other constellations can be seen only at certain times of the year because of Earth’s changing position in its orbit around the Sun.
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7. Patterns of Stars
The most familiar constellations are the ones that are part of the zodiac. These twelve constellations can be seen in both the northern and southern hemispheres.
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Measuring the Stars

8. Patterns of Stars Star clusters
By measuring distances to stars and observing how their gravities interact with each other, scientists can determine which stars are gravitationally bound to each other. A group of stars that are gravitationally bound to each other is called a cluster.
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9. Patterns of Stars Star clusters
An open cluster is a group of stars that are not densely packed.
A globular cluster is a group of stars that are densely packed into a spherical shape.
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10. Patterns of Stars Binaries
When only two stars are gravitationally bound together and orbit a common center of mass, they are called binary stars.
More than half of the stars in the sky are either binary stars or members of multiple-star systems.
Most binary stars appear to be single stars to the human eye, even with a telescope. The two stars are usually too close together to appear separately, and one of the two is often much brighter than the other.
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Measuring the Stars

11. Visualizing Star Groupings
When you look into the night sky, the stars seem to be randomly spaced from horizon to horizon. Upon closer inspection, you begin to see groups of stars that seem to cluster in one area.
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12. Patterns of Stars Doppler shifts
The most common way to tell that a star is one of a binary pair is to find subtle wavelength shifts, called Doppler shifts.
Scientists use Doppler shifts to determine the speed and direction of a star’s motion.
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Measuring the Stars

13. Visualizing Stars

14. Patterns of Stars Doppler shifts
When a star moves toward the observer, the light emitted by the star shifts toward the blue end of the electromagnetic spectrum. When a star moves away from the observer, its light shifts toward the red.
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Measuring the Stars

15. The Doppler Effect FPO Concepts In Motion
link to Animation from p. 840 here.
Measuring the Stars
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16. Stellar Positions and Distances
Astronomers use two units of measure for long distances. One is the light-year (ly). A light-year is the distance that light travels in one year, equal to × 1012 km. Astronomers often use a unit larger than a light-year—a parsec (pc), which is equal to 3.26 ly, or × 1013 km.
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Measuring the Stars

17. Stellar Positions and Distances
Parallax
When estimating the distance of stars from Earth, astronomers must account for the fact that nearby stars shift in position as observed from Earth. This apparent shift in position caused by the motion of the observer is called parallax.
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Measuring the Stars

18. Stellar Positions and Distances
Parallax
The distance to a star can be estimated from its parallax shift by measuring the angle of the change. With advancements in technology, such as the Hipparcos satellite, astronomers can find accurate distances up to 100 pc by using the parallax technique.
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Measuring the Stars

19. Basic Properties of Stars
The basic properties of a star are mass, diameter, and luminosity, which are all related to each other. Temperature is another property and is estimated by finding the spectral type of a star.
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Measuring the Stars

20. Basic Properties of Stars
Temperature controls the nuclear reaction rate and governs the luminosity, or absolute magnitude. Apparent magnitude is how bright the stars and planets appear in the sky from Earth.
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Measuring the Stars

21. Basic Properties of Stars
Magnitude
Absolute magnitude is how bright a star would appear if it were placed at a distance of 10 pc. The absolute magnitude compared to the apparent magnitude is used to find the distance to a star.
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Measuring the Stars

22. Basic Properties of Stars
Magnitude
The classification of stars by absolute magnitude allows comparisons that are based on how bright the stars would appear at equal distances from an observer. The disadvantage of absolute magnitude is that it can be difficult to determine unless the actual distance to a star is known.
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23. Basic Properties of Stars
Magnitude
Apparent magnitudes do not give an actual measure of energy output. To measure the energy output from the surface of a star per second, called its power or luminosity, an astronomer must know both the star’s apparent magnitude and how far away it is.
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Measuring the Stars

24. Basic Properties of Stars
Magnitude
Luminosity is measured in units of energy emitted per second, or watts. The Sun’s luminosity is about 3.85 × 1026 W. The values for other stars vary widely, from about to more than 1 million times the Sun’s luminosity. No other stellar property varies as much.
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25. Classification of Stars
Temperature
Stars are assigned spectral types in the following order: O, B, A, F, G, K, and M. Each class is subdivided into more specific divisions with numbers from 0 to 9.
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26. Classification of Stars
Temperature
The classes were originally based only on the pattern of spectral lines, but astronomers later discovered that the classes also correspond to stellar temperatures, with the O stars being the hottest and the M stars being the coolest. Thus, by examination of a star’s spectrum, it is possible to estimate its temperature.
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27. Classification of Stars
Temperature
Temperature is also related to luminosity and absolute magnitude. Hotter stars put out more light than stars with lower temperatures.
Distance can be determined by calculating a star’s luminosity based on its temperature.
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28. Classification of Stars
Composition
All stars, including the Sun, have nearly identical compositions, despite differences in their spectra. The differences in the appearance of their spectra are almost entirely a result of temperature differences.
Typically, about 73 percent of a star’s mass is hydrogen, about 25 percent is helium, and the remaining 2 percent is composed of all the other elements.
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Measuring the Stars

29. Classification of Stars
H-R diagrams
A Hertzsprung-Russell diagram (H-R diagram) is a graph that relates stellar characteristics— class, mass, temperature, diameter, and luminosity.
Absolute magnitude is plotted on the vertical axis and temperature or spectral type is plotted on the horizontal axis.
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Measuring the Stars

30. Classification of Stars
H-R diagrams
Most stars occupy the region in the diagram called the main sequence, which runs diagonally from the upper-left corner, where hot, luminous stars are represented, to the lower-right corner, where cool, dim stars are represented.
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Measuring the Stars

31. Classification of Stars
Main sequence
While stars are in the main sequence, they are fusing hydrogen in their cores. As stars evolve off the main sequence, they begin to fuse helium in their cores and burn hydrogen around the core edges.
A star’s mass determines almost all its other properties, including its main-sequence lifetime. The more massive a star is, the higher its central temperature and the more rapidly it burns its hydrogen fuel.
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Measuring the Stars

32. Classification of Stars
Main sequence
Red giants are large, cool, luminous stars. They are so large—more than 100 times the size of the Sun in some cases—that Earth would be swallowed up if the Sun were to become a red giant.
Small, dim, hot stars are called white dwarfs. A white dwarf is about the size of Earth but has a mass about as large as the Sun’s.
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Measuring the Stars