普通天文學期中考

1. How did astronomers first detect the high temperatures in the corona of the Sun?

A) By observing emission lines of highly ionized elements, like iron.

B) By direct measurements using space probes.

C) By observing the effects the high temperature has on Mercury and Venus.

D) By measuring the brightness of the corona in visible (white) light.

2. The star Alderamin has an apparent magnitude of 2.4 and an absolute magnitude of 1.4. The star Merak has an apparent magnitude of 2.4 and absolute magnitude of 0.5. Assuming that neither star has been dimmed by interstellar clouds, we can say for sure that

A) Merak is closer to us than is Alderamin.

B) Merak and Alderamin are the same distance from us.

C) Merak is an intrinsically fainter star than is Alderamin.

D) Merak is farther away from us than is Alderamin.

3. What is the name of a sudden eruptive surge on the surface of the Sun?

A) A sunspot. B) A prominence.

C) A plage. D) A flare.

4. How bright (in terms of total energy output per second) are the brightest stars in the universe, compared to the Sun?

A) About 1,000,000 times brighter.

B) About 1000 times brighter.

C) About 1,000,000,000 times brighter.

D) About 100 times brighter.

5. In a particular binary star system, only one star is visible because the other star is too faint to see at that distance. An astronomer measures the size (semimajor axis) and period of the orbit of the visible star. From this information the astronomer

A) can calculate the sum of the masses of the two stars, but not the mass of each star separately.

B) can calculate the mass of the visible star, but not that of the unseen star.

C) cannot calculate anything about the mass; both stars have to be visible to do this.

D) can calculate the mass of each star.

6. Sunspots are seen to be cooler than the rest of the Sun's surface, sometimes by as much as 1500 K. What would be the peak wavelength of the radiation from the sunspot when compared to that from the rest of the Sun?

A) It would be at a shorter or longer wavelength, depending on the position of the spot.

B) It would be at a longer wavelength.

C) It would be the same, since the light still originates at the Sun.

D) It would be at a shorter wavelength.

7. Which important stellar parameter can be derived from the study of binary stars mutually bound to each other by gravitational forces?

A) Surface temperatures of the stars.

B) The distance of the stars from Earth.

C) Stellar masses.

D) The age of the stars.

8. How can we tell that some stars are relatively close to us in the sky?

A) Because they appear to move periodically back and forth against the background stars because of the Earth's movement around the Sun.

B) Because they appear to be extremely bright and must therefore be very close to us.

C) Because they are occasionally occulted or eclipsed by our Moon, hence they must be close.

D) Because the light from these stars shows only a very small redshift caused by the universal expansion of the universe, so they must be close.

9. Of the following astronomical objects or systems, which is likely to be the oldest?

A) A globular cluster B) The Pleiades

C) A T Tauri star D) The Sun

10. An eclipsing binary system consists of

A) two mutually orbiting and gravitationally bound stars which are close enough to be resolved when viewed from Earth.

B) two stars which periodically eclipse each other, as seen from Earth.

C) two stars in which spectral lines move back and forth periodically because of Doppler shift, indicating mutually orbiting stars.

D) a star which is periodically eclipsed by the Moon.

11. Who, to our knowledge, first measured the rotation period of the Sun?

A) Galileo B) Roemer

C) Aristotle D) Ptolemy

12. Nuclear fusion is

A) the splitting of heavier nuclei to produce lighter nuclei and energy.

B) the combining together of hydrogen atoms to produce hydrogen molecules, H2, and energy.

C) the combining of electrons with nuclei to produce atoms and release energy.

D) the combining together of light nuclei (e.g., hydrogen) to produce heavier nuclei (e.g., helium) and energy.

13. The intensity of sunlight per square meter reaching Jupiter is approximately what fraction of that at the Earth's orbital distance? (See Fig. III-3, Kaufmann & Comins, Discovering the Universe, 5th Ed.)

A) 1/5. B) 25 times.

C) About the same. D) 1/25.

14. What particular and very important phenomenon frequently occurs in binary star systems where the stars are very close together?

A) The less massive star spirals slowly into its more massive companion because of tidal interactions.

B) The radiation from the hotter star will slowly heat and evaporate away the cooler star.

C) The less massive star, in its elliptical orbit, will repeatedly pass through the thin, extended atmosphere of the second star, producing periodic rises and falls in light output from the star system.

D) Mass lost from one star is deposited upon its companion.

15. One particular feature of the solar corona is

A) its very cold temperature.

B) its very uniform density and structure.

C) its variation with time over periods of a few minutes.

D) its very high temperature.

16. A star of apparent magnitude +2.1 appears

A) either brighter or fainter than a star of apparent magnitude-1.2, depending on the distance to the stars.

B) farther away than a star of apparent magnitude-1.2.

C) fainter than a star of apparent magnitude-1.2.

D) brighter than a star of apparent magnitude-1.2.

17. During helium burning, some 4He combines with 16O, much like the way it combines with 12C to form 16O. What is produced by the 16O + 4He reaction?

A) 18O (heavy isotope of oxygen).

B) 20Ne (regular isotope of neon).

C) 22Na (light isotope of sodium).

D) 20F (heavy isotope of fluorine).

18. The granulation pattern seen on the surface of the Sun results from

A) strong magnetic fields cooling the gas in certain regions.

B) convection of gas in the region under the photosphere.

C) heating of the photosphere by solar flares.

D) prominences and filaments above the solar surface.

19. What process makes an emission nebula glow?

A) Electrons descending toward the ground state in hydrogen atoms.

B) Free electrons emitting light as they pass close to positively charged ions.

C) Electric currents in ionized neon gas.

D) High-energy electrons spiraling along magnetic field lines.

20. Which of the following observations would NOT be an indication of a binary star system?

A) The "star" appears to move in a straight line against a background field of stars.

B) A "star" appears to become periodically dimmer for a few hours at a time.

C) A "star" image separates into two distinct images periodically and then blends again, periodically.

D) The "star" appears to wiggle in its path across our sky against the background stars.

21. How does the temperature of an interstellar cloud affect its ability to form stars?

A) Higher temperatures help star formation.

B) Star formation is independent of the temperature of the cloud.

C) Higher temperatures inhibit star formation.

D) Star formation is too complicated to be able to say how one quantity, such as temperature, affects it.

22. Protostars are

A) very young objects, still contracting before becoming true stars.

B) stars made almost entirely out of protons.

C) objects with masses less than about 0.08 solar masses, which do not have enough mass to become true stars.

D) old stars, contracting after using up all of their available hydrogen fuel.

23. What is a white dwarf star?

A) A main sequence star with a surface temperature near 12,000 K.

B) A star of about the same size (diameter) as the Earth.

C) Any star which is significantly smaller than a giant or supergiant star.

D) A large, planetary object, such as Jupiter.

24. If the surface temperatures of white dwarf stars are 4 times that of the Sun and energy output per unit area of a star depends upon the 4th power of the temperature by the Stefan-Boltzmann relation, why then are white dwarfs intrinsically so faint?

A) Because they are shrouded in very thick atmospheres.

B) Because they are moving rapidly away from the Sun and their spectra are extremely red-shifted, hence they appear faint at visible wavelengths.

C) Because they have very thin atmospheres which do not emit continuum radiation but only line emissions, like a low density gas.

D) Because they are very small.

25. Where on the Hertzsprung-Russell diagram do most local stars in our Universe congregate?

A) On the main sequence, where stars are generating energy by fusion reactions.

B) In the supergiant area, where the most massive stars spend a significant time.

C) In the giants area, where most stars spend the longest time of their lives.

D) In the white dwarf area, the "graveyard" of stars.

26. If granulation on the Sun's surface is a result of convective motion below it, and the centers of granular cells are where material is upwelling from below and returning in the regions between, what is the expected temperature distribution across a granular cell?

A) The center of the cell will be hotter than the edges.

B) The center of the cell will be cooler than the edges.

C) Alternate cell centers will be hot and cold, with the edges at an intermediate temperature.

D) The temperature will be uniform across the cell.

27. Spectral classification of stars into the lettered categories, O,B,A,F,G,K,M, is carried out by

A) finding the wavelength of peak emission in the continuum spectrum of the star.

B) determining their relative masses by the study of binary star motions, in order to place them into their proper mass classification.

C) examining the relative depths of absorption lines from various neutral and ionized atoms in a stellar spectrum.

D) determining the total energy emitted at all wavelengths by stars, taking account of the full spread of wavelengths and their distances, in order to place the star into its luminosity class.

28. Cepheid stars are

A) members of binary systems, in which one star periodically eclipses the other.

B) stars at an early stage in stellar evolution, pre-main-sequence.

C) white dwarf stars, late in their evolutionary life.

D) giant stars that pulsate in brightness, size, and temperature.

29. The main general features which make clusters of stars useful to astronomers are that

A) the stars are all at the same distance from Earth, have the same surface temperature, and joined the cluster at various times.

B) the stars all have the same apparent magnitude, the same surface temperatures, and the same sizes.

C) the stars are at the same distance from Earth, were formed at approximately the same time, and were made from same chemical mix.

D) the stars all have the same intrinsic brightness, but differ only in size and surface temperature.

30. What condition is considered sufficient for an interstellar cloud to collapse and form a star or stars (i.e., if this condition holds then the cloud has to collapse)?

A) The cloud must be cooler than 100 K.

B) Gravity must be strong enough to reach all parts of the cloud.

C) Gravity must dominate gas pressure inside the cloud.

D) The cloud must be alone in space (far from stars and other interstellar clouds).

31. What proportion of visible stars in our night-time sky are multiple-star systems, such as binary stars?

A) Less than 1%

B) Nearly 100%

C) Only about 1/4 or 25%

D) 1/2 or about 50%

32. In terms of nuclear reactions, what is the next stage of a star's life after the end of hydrogen burning in the core?

A) Hydrogen burning in a thin shell around the core.

B) Helium burning in the core.

C) Carbon burning.

D) Death (it becomes either a supernova or a white dwarf).

33. What is a protostar called in the stage after it has finished accreting mass?

A) A main sequence star.

B) A pre-main sequence star.

C) A white dwarf.

D) A red giant (or supergiant).

34. Which of the following stars are metal poor?

A) Population I stars. B) Population II stars.

C) Very young stars. D) Open cluster stars.

35. How is absolute magnitude defined?

A) It is the apparent magnitude a star would have if all of the energy from the star were concentrated in the visual region.

B) It is the apparent magnitude a star would have if the star were located at exactly 10 pc from the Earth.

C) It is the apparent magnitude a star would have if the star were located at exactly 10 AU from the Earth.

D) It is the apparent magnitude a star would have if the star were located at exactly 10 ly from the Earth.

36. What is the Zeeman effect?

A) When the temperature of a light source is increased, the wavelength of maximum emission decreases.

B) When light is shone onto a metal surface, electrons are ejected from the metal only if the wavelength of the light is shorter than some critical wavelength.

C) When a light source is located in a magnetic field, the spectral lines it emits are split into two or more components.

D) When a light source is moving relative to an observer, the wavelengths of its spectral lines are shifted to longer or shorter wavelengths.

37. The helium flash results from the

A) high temperature in the helium core of a blue (spectral class O or B) supergiant star.

B) electron degeneracy or quantum crowding in the core of a low-mass red giant star.

C) sudden onset of nuclear reactions at the end of the protostar.

D) sudden release of energy in strong magnetic fields near a sunspot.

38. How bright (in absolute magnitude) are the intrinsically brightest stars in the universe?

A) -10 B) 0

C) +1 D) +17

39. What are the two physical parameters of stars which are plotted in the Hertzsprung-Russell diagram?

A) luminosity and mass.

B) radius and mass.

C) mass and surface temperature.

D) luminosity and surface temperature.

40. What is the lifetime of a typical sunspot?

A) From a few hours to a few months.

B) From a few years to a few decades.

C) 11 years.

D) Here today, gone tomorrow!

41. What is the average length of a complete solar cycle of sunspots and magnetic fields?

A) 4.5 years. B) 22 years.

C) 7 years. D) 11 years.

42. How many neutrinos from the Sun pass through each square inch of your body every second?

A) 100 million. B) 100 billion.

C) a few hundreds. D) 100 trillion.

43. Where is the chromosphere on the Sun?

A) It is the layer above the visible surface of the Sun.

B) It is the visible surface of the Sun.

C) It is the layer below the visible surface of the Sun.

D) It is the outermost part of the Sun's atmosphere.

44. What happens to the positrons produced by the nuclear reactions in the core of the Sun?

A) They combine with neutrons to form protons.

B) They collide and stick together to form helium.

C) They collide with electrons, producing energy.

D) They escape from the Sun into space.

45. What is the physical reason why astronomers can find the luminosity class (I, II, III, IV, or V) of a star using the star's spectrum?

A) The absorption lines in the spectrum are affected by the star's surface temperature.

B) The relative amounts of hydrogen, helium, and other elements are different for stars of different luminosity classes.

C) The absorption lines in the spectrum are affected by the density and pressure of the star's atmosphere.

D) The wavelength of maximum emission (given by Wien's law) is affected by the size of the star.

46. What happens to a star after the start of helium nuclear reactions in its core, compared to what it was like before these reactions began?

A) The star is smaller and hotter.

B) The star is smaller and cooler.

C) The star is larger and hotter.

D) The star is larger and cooler.

47. A particular interstellar giant molecular cloud has a mass of 2,000,000 solar masses. What is the mass of hydrogen in this cloud?

A) 1,960,000 solar masses.

B) 1,500,000 solar masses.

C) 1,000,000 solar masses.

D) 40,000 solar masses.

48. Where would you expect to find spicules?

A) In binary star systems in which one star is a neutron star attracting and collecting mass from the other star.

B) In the atmosphere of the Sun.

C) In supernova remnants.

D) In interstellar clouds heated by hot, massive stars.

49. Suppose that an astronomical observatory announces the discovery of an object with about 50 times the mass of Jupiter (this is not enough mass to be a star). What name would the observatory apply to this object?

A) A red dwarf.

B) A brown dwarf.

C) A white dwarf.

D) An infrared dwarf.

50. Which of the following four spectral classifications represents the coolest stellar surface temperature?

A) G.

B) B.

C) K.

D) A.