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Reading Review Chapter 11 Basic Skills/ Definitions Quiz What is the approximate chemical composition (by mass) with which all stars are born? - Three quarters hydrogen, one quarter helium, no more than 2% heavier elements. The total amount of power (in watts, for example) that a star radiates into space is called its Luminosity. For example, the luminosity of the Sun is 3.7 x 10 to the 26 th watts. According to the universe square law of light, how will the apparent brightness of an object change if its distance to us doubles? Its apparent brightness will decrease by a factor of 4. Assuming that we can always measure the apparent brightness of a star, what does the inverse square law for light allow us to do? It allows us to calculate a star’s luminosity if we know its distance, or calculate its distance if we know its luminosity. If star A is closer to us than star B, then Star A’s parallax angle is: Larger than that of star B. A larger parallax angle means the star is closer. Ten parsecs is about: 32.6 light-years If star A has an apparent magnitude of 3 and star B has an apparent magnitude of 5, which star is brighter in our sky? Star A. The magnitude scale is “backward” so that smaller numbers mean greater apparent brightness. From hottest to coolest, the order of the spectral types of stars is: OBAFGKM Our Sun is a star of spectral type: G Astronomers can measure a star’s mass in only certain cases. Which one of the following cases might allow astronomers to measure a star’s mass? The star is a member of a binary star system. If we can measure the orbital properties of a star around its companion, then we can measure the mass with Newton’s version of Kepler’s third law. Which of the following terms is given to a pair of stars that we can determine are orbiting each other only by measuring their periodic Doppler shifts? Spectroscopic binary The axes on a Hertzsprug- Russel diagram represent: Luminosity and surface temperature What can we infer, at least roughly, from a star’s luminosity class? It’s radius.

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