Astro 130, Fall 2011, Homework, Chapter 17, Due Sep 29, 2011 Name: Date:

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1 Astro 130, Fall 2011, Homework, Chapter 17, Due Sep 29, 2011 Name: Date: 1. If stellar parallax can be measured to a precision of about 0.01 arcsec using telescopes on Earth to observe stars, to what distance does this correspond in space? A) 500 pc B) 200 pc C) 100 pc D) 0.01 pc 2. The star Procyon has a parallax of arcsec and a proper motion of 1.25 arcsec per year. What is the tangential velocity of Procyon? A) 68 km/s B) 21 km/s C) 1.7 km/s D) 16 km/s 3. Suppose that two identical stars (having the same total light output or luminosity) are located such that star A is at a distance of 5 pc and star B is at a distance of 25 pc. How will star B appear, compared to star A? A) 1/2.2 as bright B) 1/25 as bright C) 1/20 as bright D) 1/5 as bright 4. The luminosity of a star is A) its apparent magnitude. B) the total energy emitted at all wavelengths toward Earth. C) the total energy emitted at all wavelengths into all space from its whole surface. D) the total energy emitted by the star within the sensitive range of the eye, in the socalled V filter band. 5. If Star A has an apparent magnitude of +5, and Star B has an apparent magnitude of +10, then A) Star A is twice as bright as Star B. B) Star B is twice as bright as Star A. C) Star A is 100 times as bright as Star B. D) Star B is 100 times as bright as Star A. Page 1

2 6. The star Bellatrix in Orion looks bluish to the naked eye. Why is this? A) The spectrum of light emitted from Bellatrix peaks in the blue region of the spectrum, and in fact, almost all of its light is concentrated in the blue region of the spectrum. B) The spectrum of light emitted from Bellatrix peaks in the ultraviolet region of the spectrum. Within the visible part of the spectrum, there is more emission in the blue than in any other color. C) Bellatrix is moving toward us rapidly enough that its light is appreciably blueshifted. D) Bellatrix is made of blue material. 7. The ratio of the brightness of a star at two different colors, blue and visual b V /b B is a direct measure of what property of the star? A) distance from Earth B) surface temperature C) luminosity D) radius 8. The color ratio b B /b V has the following values for four stars. Which value indicates the coolest surface temperature? A) 0.5 B) 1.0 C) 2.5 D) The spectral type of a star is most directly related to (and determines uniquely) its A) size or radius. B) absolute magnitude. C) luminosity. D) surface temperature. 10. Spectral classification determines a nearby star's surface temperature by examining A) the peak wavelength of the star spectrum. B) the relative intensities of light measured through different wavelength band filters. C) the pattern of spectral absorption lines from various atoms. D) the overall shape of the star's spectrum compared to a black body. Page 2

3 11. The spectral class of the Sun is G2 and the star Enif is K2. From this information, we know that Enif is A) intrinsically fainter than the Sun. B) cooler than the Sun. C) intrinsically brighter than the Sun. D) hotter than the Sun. 12. Why is it that stars with low surface temperature, much cooler than 10,000 K, show very weak or no hydrogen Balmer lines in absorption? A) To absorb at Balmer wavelengths, atoms need to have electrons in the n = 2 level, and electrons will not be excited to this level by collisions at these low temperatures. B) The stellar gas is so cool that there is no radiation to be absorbed at the wavelength of Balmer lines. C) Hydrogen atoms have to be hot enough to be ionized to show Balmer absorption. D) Hydrogen atoms will have no electrons at any energy level at these low temperatures. 13. In the spectral classification of stars, strong absorption lines of which of the following atomic or molecular constituents would indicate very low surface temperature? A) He II B) Fe II C) Mg II D) TiO 14. The Hertzsprung-Russell diagram is a plot of A) apparent brightness vs. intrinsic brightness or luminosity of a group of stars. B) luminosity vs. period of variation for variable stars. C) apparent brightness vs. distance for stars near the Sun. D) intrinsic brightness or luminosity vs. temperature of a group of stars. 15. Where on the Hertzsprung-Russell diagram do most local stars in our universe congregate? A) in the supergiant area, where the most massive stars spend a significant time B) in the giants area, where most stars spend the longest time of their lives C) in the white dwarf area, the graveyard of stars D) on the main sequence where stars are generating energy by fusion reactions Page 3

4 16. What fraction of the stars in the night sky are main-sequence stars? A) almost none of them, less than 1% B) relatively few of them, about 20% C) almost all of them, about 90% D) roughly half of them, about 55% 17. Compared to a star in the middle of the diagram, a star in the lower left part of the Hertzsprung-Russell diagram is A) brighter. B) larger. C) cooler. D) smaller. 18. A white dwarf is A) an object intermediate between planets and stars, that will never become a star. B) a star at the end of its life, with a size close to that of Earth. C) any main sequence star with a surface temperature between about 9000 K and 15,000 K. D) a star at the beginning of its life, with a size two to ten times that of the Sun. 19. Betelgeuse (the bright star in the constellation Orion) has a surface temperature of 3500 K and a luminosity 100,000 times that of the Sun. What is its approximate radius in terms of that of the Sun? (Hint: Use Fig a of Freedman and Kaufmann, Universe, 8th ed.) A) the same B) 1000 times larger C) 100 times larger D) 10 times larger 20. One star has a spectral-luminosity class of B5 V (main sequence) and the other has a spectral-luminosity class of B5 Ia (supergiant). What difference do we see in the spectra of these two stars and why? (Note: Both the difference and the reason must be correct.) A) The main sequence star has broader hydrogen absorption lines, because its atmosphere is hotter than that of the supergiant. B) The main sequence star has narrower hydrogen absorption lines, because its atmosphere is cooler than that of the supergiant. C) The main sequence star has broader hydrogen absorption lines, because its atmosphere is denser and at higher pressure than that of the supergiant. D) The main sequence star has narrower hydrogen absorption lines, because its atmosphere is more compact (less physically extended) than that of the supergiant. Page 4

5 21. The star Spica is classified as B1 V, which means that it is a A) cool main sequence star. B) cool giant. C) hot supergiant. D) hot main sequence star. 22. Which of the following four spectral-luminosity classes would correspond to a red supergiant? A) B7 I B) G2 III C) M3 V D) M2 I 23. What is the difference between an optical double star and a visual binary star? A) There is no difference they are two names for the same thing. B) Optical double stars can be seen as separate stars only through a telescope, whereas visual binaries can be seen with the unaided eye (e.g., the star Mizar in the Big Dipper's handle). C) The stars in an optical double star are actually orbiting each other, whereas a visual binary is an illusion the stars are at vast distances from each other and are not actually orbiting each other. D) An optical double is an illusion. The stars are at vast distances from each other and are not actually orbiting each other, whereas in a visual binary, the stars are actually orbiting each other. 24. How do two unequal mass stars move around each other in a binary system? A) in a common elliptical orbit, always remaining diametrically opposite to each other through one of the foci of the ellipse B) in straight lines, back and forth past each other C) in elliptical orbits, about a common center of mass D) The low-mass star moves in a circular orbit around the stationary high-mass star. 25. Which important stellar parameter can be determined by the study of binary stars? A) the age of the stars B) stellar masses C) surface temperatures of the stars D) the distance of the stars from Earth Page 5

6 26. A simple relationship exists between mass and luminosity for A) all stars. B) main sequence stars only. C) giant stars and main sequence stars. D) main sequence stars and brown dwarfs. 27. Absorption lines in the spectra of some binary stars are seen to change periodically from single to double lines and back again. Why is this? A) Oscillations on the surfaces of the stars leads to Doppler-shifted lines. B) Periodically, the magnetic field of one star produces Zeeman splitting of spectral lines in atoms of the second star. C) The effect of the gravitational field of one star on the atoms of the second star produces spectral line shifts periodically. D) Motion toward and away from Earth during their orbital motion results in Doppler shift of light from these stars at times and no shift when the stars are moving perpendicular to the line of sight. 28. An eclipsing binary system is A) two stars whose spectral lines move back and forth, indicating relative motion. B) a star that is periodically eclipsed by the Moon. C) two stars whose combined light output when measured from Earth appears to vary periodically as the two stars move in front of one another. D) two stars that are clearly seen as separate but associated in the sky. 29. What condition is necessary for us to see eclipses of stars in binary star systems? A) One of the stars must be much bigger than the other so that it can hide its smaller companion when the orbital plane is at a large angle to the line of sight. B) The stars must have very similar surface temperatures whatever the inclination of their orbital plane to the line of sight, for us to see a significant eclipse. C) The line of sight from Earth to the star system must be in or very close to the orbital plane of the stars. D) The line of sight from Earth to the star system must be very close to the perpendicular to the orbital plane of the stars. Page 6