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Review Reading 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. On an H-R diagram, stellar radii: Increase diagonally from the lower left to the upper right On an H-R diagram, stellar masses: Can be determined from a star's position in the diagram only for main sequence stars, and decrease from upper left to lower right. How is the lifetime of a star related to its mass? The more massive stars live considerably shorter lives than less massive stars. Each choice below lists a spectral type of luminosity class for a star. Which one is a red super giant? Spectral type M2, luminosity class I. What is the common trait of all main sequence stars? They generate energy through hydrogen fusion in their core. Suppose our Sun were suddenly replaced by a super giant star. Which of the following would be true? Earth would be inside the super giant, a super giant's radius is typically much more than 1 AU. What is a white dwarf? It is the remains of a star, composed mostly of carbon, that no longer produces energy by nuclear fusion. Which of the following statements comparing open and globular star clusters is NOT true? Open and globular clusters each typically contain a few hundred stars. -Globular clusters contain many more stars than open clusters. What do we mean by the main-sequence turnoff point of a star cluster, and what does it tell us? It is the spectral type of the hottest main sequence star in a star cluster, and it tells us the cluster's age. Chapter 11: Visual Quiz This figure shows stars as we see them in our sky from Earth, centered around the constellation Canis Major. Larger dots represent brighter stars, and a few of the brightest stars are identified. From this view alone what can you conclude about Sirius? It has the greatest apparent brightness of any star in this region of the sky. Why do some of the bright stars in this figure have cross-shaped spikes over them? The spikes are an artifact of photography through a telescope. All the stars in this photo are at about the same distance from Earth (some 26,000 light-years away). Which stars in this picture are the largest in size (radius)? The bright red stars. This graph shows how the apparent brightness of an eclipsing binary system changes with time. Which of the four labeled regions represents the system at a time when one star is eclipsing the other? Both II IV and What is the spectral type of the star Sirius? A Which of the following stars in the most massive? Beta Centauri Which of the following stars is the largest in size (radius)? Antares Which of the following stars has (or had) the longest hydrogen burning lifetime? Bernard's star This photo shows an object located in the halo of our Milky Way galaxy. What kind of object is it? A globular cluster of stars This photo shows an object located in the halo of our Milky Way galaxy. Notice the many bright red dots in the photograph. What are they? Red giant stars. Conceptual Quiz All stars are born with the same basic composition, yet stars can look quite different from one another. Which two factors primarily determine the innate characteristics of a star? Its mass and its stage of life. Based on the definition of apparent brightness, which units are appropriate for its measurement? Watts per square meter. It is defined as the power of starlight reaching us per unit area. Suppose two stars are identical except that one is twice as far away from us as the other. Which statement is true? Both stars have the same luminosity, but the apparent brightness of the closer star is four times as great as that of the more distance star. Suppose that a star has a parallax angle of 1/20 arc second. If so, it is 20 parsecs away. Distance in parsecs = 1/ (parallax angel in arc second) The star Vega has an absolute magnitude of about 4 and an apparent magnitude of about 0. Based on the definitions of absolute and apparent magnitude we can conclude that? Vega is nearer than 10 parsecs from Earth. Absolute magnitude is the magnitude it would have IF it were a distance of 10 parsecs (32.6 light years) away. Since its absolute magnitude is larger (meaning dimmer) than its apparent magnitude, it would look dimmer if it were 10 parsecs away -- which means it can't be that far. Which of following statements about spectral types of stars is NOT generally true? FALSE STATEMENT--The spectral type of a star can be used to determine its distance The spectral type of a star can be used to determine its surface temperature. The spectral type of a star can be used to determine its color. The spectral type of a star can be determined by identifying lines in its spectrum. Sirius is a spectral type A star whereas Rigel is a spectral type B star. What can we conclude? Rigel has a higher surface temperature than Sirius. Astronomers generally can measure a star's mass only if it it is a member of a binary star system. What characteristics of the stars must we know to measure the masses of the stars in a binary system? Their orbital period and average orbital distance. Careful measurements reveal that a star maintains a steady apparent brightness at most times, except that at precise intervals of 73 hours the star becomes dimmer for about 2 hours The most likely explanation is that The star is a member of an eclipsing binary star system. You observe a star in the disk of the Milky Way, and you want to plot the star on an H-R diagram. You will need to determine all of the following except: Mass of the star. The approximate main-sequence lifetime of a star of spectral type O is: 3 million years How did astronomers discover the relationship between spectral type and mass for main sequence stars? Astronomers discovered the relationship by measuring the masses of main-sequence stars in binary systems, and they assumed that the same relationship holds for single stars as well. The relationship between a main sequence star's type ad mass is based on the relationship calibrated by observing binary star systems. The choices below each describe the appearance of an H-R diagram for a different star cluster. Which cluster is the youngest? The diagram shows main-sequence stars of every spectral type except O, along with a few giants and super giants. Only O stars have died, so the cluster is no more that a few million years old. The choices below each describe the appearance of an H-R diagram for a different star cluster. Which cluster is more likely to be located in the halo of our galaxy. The diagram shows main-sequence stars of every spectral types G, K, and M, along with numerous giants and white dwarfs. This is an old cluster so it must be a globular cluster, and many globular clusters are in the halo of the galaxy. ... View Full Document

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