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Roger A. Freedman • William J. Kaufmann III. Universe Eighth Edition. CHAPTER 17 The Nature of Stars. Chapter 16 and 17 online quizzes due Monday 11/8 by midnight. HW. Parallax measurements are best made using a telescope in orbit. This is because.
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Roger A. Freedman • William J. Kaufmann III Universe Eighth Edition CHAPTER 17 The Nature of Stars
Chapter 16 and 17 online quizzes due Monday 11/8 by midnight HW
Parallax measurements are best made using a telescope in orbit. This is because • a telescope in orbit is closer to the stars. • larger telescopes can be placed in orbit and so the resolution is significantly improved. • the baseline is longer and so the parallax angle is larger. • chromatic aberration from the telescope lens is eliminated. • an observatory in space is unhampered by the Earth’s atmosphere. Q17.1
Parallax measurements are best made using a telescope in orbit. This is because • a telescope in orbit is closer to the stars. • larger telescopes can be placed in orbit and so the resolution is significantly improved. • the baseline is longer and so the parallax angle is larger. • chromatic aberration from the telescope lens is eliminated. • an observatory in space is unhampered by the Earth’s atmosphere. A17.1
At the distance of the Earth from the Sun (1 AU) the intensity of sunlight is 1370 watts/m2. What is the intensity at the distance of Saturn from the Sun (10 AU)? • 13,700 watts/m2 • 1370 watts/m2 • 137 watts/m2 • 13.7 watts/m2 • 1.37 watts/m2 Q17.4
At the distance of the Earth from the Sun (1 AU) the intensity of sunlight is 1370 watts/m2. What is the intensity at the distance of Saturn from the Sun (10 AU)? • 13,700 watts/m2 • 1370 watts/m2 • 137 watts/m2 • 13.7 watts/m2 • 1.37 watts/m2 A17.4
Where is the Sun located on this H-R diagram? • A • B • C • D • E Q17.10
Where is the Sun located on this H-R diagram? • A • B • C • D • E A17.10
Key Ideas • Measuring Distances to Nearby Stars: Distances to the nearer stars can be determined by parallax, the apparent shift of a star against the background stars observed as the Earth moves along its orbit. • Parallax measurements made from orbit, above the blurring effects of the atmosphere, are much more accurate than those made with Earth-based telescopes. • Stellar parallaxes can only be measured for stars within a few hundred parsecs. • The Inverse-Square Law: A star’s luminosity (total light output), apparent brightness, and distance from the Earth are related by the inverse-square law. If any two of these quantities are known, the third can be calculated.
Key Ideas • The Population of Stars: Stars of relatively low luminosity are more common than more luminous stars. Our own Sun is a rather average star of intermediate luminosity. • The Magnitude Scale: The apparent magnitude scale is an alternative way to measure a star’s apparent brightness. • The absolute magnitude of a star is the apparent magnitude it would have if viewed from a distance of 10 parsecs. A version of the inverse-square law relates a star’s absolute magnitude, apparent magnitude, and distance.
Key Ideas • Photometry and Color Ratios: Photometry measures the apparent brightness of a star. The color ratios of a star are the ratios of brightness values obtained through different standard filters, such as the U, B, and V filters. These ratios are a measure of the star’s surface temperature. • Spectral Types: Stars are classified into spectral types (subdivisions of the spectral classes O, B, A, F, G, K, and M), based on the major patterns of spectral lines in their spectra. The spectral class and type of a star is directly related to its surface temperature: O stars are the hottest and M stars are the coolest. • Most brown dwarfs are in even cooler spectral classes called L and T. Unlike true stars, brown dwarfs are too small to sustain thermonuclear fusion.
Key Ideas • Hertzsprung-Russell Diagram: The Hertzsprung-Russell (H-R) diagram is a graph plotting the absolute magnitudes of stars against their spectral types—or, equivalently, their luminosities against surface temperatures. • The positions on the H-R diagram of most stars are along the main sequence, a band that extends from high luminosity and high surface temperature to low luminosity and low surface temperature.
