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1309

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Astronomy

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Dec 6, 2023

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Most of the really bright stars in our sky are NOT among the stars that are very close to us. Why then do they look so bright to us? these stars are intrinsically so luminous, that they can easily be seen even across great distances The most common kinds of stars in the Galaxy have _____. low luminosity compared to the Sun Which of the following characteristics of a single star (one that moves through space alone) is it difficult to measure directly? its mass Two stars that are physically associated (move together through space) are called _____. binary stars I am measuring the spectrum of the stars in a spectroscopic binary system. When one of the stars is moving toward the Earth in its orbit, we observe _____. that the lines in its spectrum show a blue-shift Which law do astronomers use to determine the masses of the stars in a spectroscopic binary system? Kepler's Third Law Why can astronomers not measure the diameters of stars directly? stars are so far away, we cannot resolve (distinguish) their diameters For what type of star can astronomers measure the diameter with relative ease? eclipsing binary stars An H-R Diagram plots the luminosity of stars against their _____. surface temperature In an H-R diagram, where can you see the spectral type of a star (whether it is an O type star or a G type star, for example)? along the bottom (the horizontal axis) Ninety percent of all stars (if plotted on an H-R diagram) would fall into a region astronomers call _____. the main sequence Where on the H-R Diagram would we find stars that look red when seen through a telescope? only on the right side of the diagram and never on the left Measurements show a certain star has a very high luminosity (100,000 times the Sun's) while its temperature is quite cool (3500° K). How can this be? it must be quite large in size A white dwarf, compared to a main sequence star with the same mass, would always be _____. smaller in diameter Imagine that powerful telescopes in the future give us a truly representative sampling of all the stars in the Sun's cosmic neighborhood. Where on the H-R diagram would most of the stars in our immediate vicinity lie? in the lower right, among the least luminous main sequence stars A team of astronomers discovers one of the most massive stars ever found. If this star is just settling down in that stage of its life where it will be peacefully converting hydrogen to helium in its core, where will we find it on the H-R diagram? near the very top of the main sequence, in the upper left Astronomers identify the main sequence on the H-R diagram with what activity in the course of a star's life? fusing hydrogen into helium in their cores Stars that lie in different places on the main sequence of the H-R diagram differ from each other mainly by having different _____. masses One of your good friends asks you to point out the stars with the smallest mass on an H-R diagram that you are studying. Where are you sure to find the stars with the lowest mass on any H-R diagram? among the stars at the bottom right of the main sequence A star that is quite hot and has a very small radius compared to most stars is called _____. a white dwarf When an astronomer talks about the luminosity of a star she is studying, she is talking about _____. How much energy the star gives off each second Two stars have the same luminosity, but star B is three times farther away from us than star A. Compared to star A, star B will look _____. Nine times fainter Two stars have the exact same luminosity, but star Y is four times dimmer looking that star X. This means that _____. Star Y is twice as far away as star X Using a good pair of binoculars, you observe a section of the sky where there are stars of many different apparent brightnesses. You find one star that appears especially dim. This star looks dim because it is _____. -very far away -partly obscured by a cloud -radiating most of its energy in the infrared region of the spectrum -very low luminosity It could be more than one of the above; there is no way to tell which answer is right by just looking at the star Which of the following looks the brightest in the sky? A star with magnitude -1 Why are astronomers much more interested in the luminosity of a star than its apparent brightness? because the luminosity tells us how bright a star really is, while apparent brightness only tells us how bright it happens to look from Earth
Which color star is likely to be the hottest? blue-violet Which of the following types of star is the coolest (has the lowest surface temperature)? M A team of astronomers takes spectra of thousands of different stars in different parts of the sky. The spectra show significant differences. The main reason the spectra of the stars do not all look alike is that the stars _____. Have different temperatures If hydrogen is the most common element in the universe, why do we not see the lines of hydrogen in the spectra of the hottest stars? In the hottest stars, hydrogen atoms are ionized, and so there are no elections to produce lines in the spectrum Astronomers arrange the stars into groups called spectral classes (or types) according to the kinds of lines they find in their spectra. These spectral classes are arranged in order of _____. Decreasing surface temperature Some objects in space just don't have what it takes to be a star. Which of the following is a "failed star", an object with too little mass to qualify as a star? A brown dwarf Studies of the spectra of stars have revealed that the element that makes up the majority of the stars (75% by mass) is _____. hydrogen At an astronomical conference, an astronomer gives a report on a star that interests astronomers because of hints that it may have a planet around it. In his report the astronomer gives the average speed with which this star is moving away from the Sun. How did the astronomer measure this speed? by looking at the Doppler shift in the lines of the star's spectrum A star moving toward the Sun will show _____. A shift in the special lines toward the blue end (as compared to the laboratory positions of these lines) Astronomers call the motion of a star across the sky (perpendicular to our line of sight) its _____. Proper motion An astronomer whose secret hobby is riding merry-go-rounds has dedicated his career to finding the stars that rotate the most rapidly. But the stars are all very far away, so none of them can be seen to spin even when he looks through the largest telescopes. How then can he identify the stars that rotate rapidly? stars that rotate have much wider lines in their spectra than stars that do not Which of the following can astronomers NOT learn from studying the spectrum of a star? All of these can be learned fro studying the spectrum: whether it is a star the size of the Sun or a giant star whether it is rotating slow or fast its surface temperature its motion toward or away from us The apparent brightness of stars in general tells us nothing about their distances; we cannot assume that the dimmer stars are farther away. In order for the apparent brightness of a star to be a good indicator of its distance, all the stars would have to be _____. the same luminosity Why did it take astronomers until 1838 to measure the parallax of the stars? because the stars are so far away that their annual shift of position in the sky is too small to see without a good telescope As astronomers use the term, the parallax of a star is _____. one half the angle that a star shifts when seen from opposite sides of the Earth's orbit What is the baseline that astronomers use to measure the parallax (the distance) of the nearest stars? 1/2 the diameter of the Earth's orbit around the Sun
How far away would a star with a parallax of 0.2 arcsec be from us? 5 parsecs If a star is 20 parsecs away, its parallax must be _____. 1/20th of an arcsecond Which of the following will show the smallest parallax shift? the star 51 Pegasi, about 50 lightyears away An astronomer is observing a single star (and one which does not vary) which she knows is located about 30 light-years away. What was the most likely method she or her colleagues used to obtain that distance? measuring the star's parallax A type of star that has turned out to be extremely useful for measuring distances is _____. the Cepheid variables A light curve for a star measures how its brightness changes with _____. time The period-luminosity relationship for Cepheid variables was discovered by _____. Henrietta Leavitt The measurement of cosmic distances was helped tremendously by the discovery, in the early part of the 20th century, that in Cepheid variable stars, the average luminosity was related to _____. The length of time they took to vary The higher the luminosity (intrinsic brightness) a Cepheid variable is, _____. the longer the period of its variations To get the distance to a Cepheid variable star, astronomers must take several steps. Which of the following is NOT one of these steps? measure the star's Doppler shift from its spectrum An astronomer is interested in a galaxy called M31, the nearest galaxy that resembles our Milky Way. It is about 2 million lightyears away. Which technique would be able to give us a distance to this galaxy? period-luminosity relation for Cepheid variables Which type of star has the least amount of pressure in its atmosphere? supergiants The luminosity class of a star tells an astronomer _____. whether the star is a supergiant, a giant, or a main-sequence star Astronomers must often know the distance to a star before they can fully understand its characteristics. Which of the following properties of a star typically requires a knowledge of distance before it can be determined? Its luminosity Astronomers use the term interstellar matter to refer to _____.
gas and dust that lies between stars When astronomers discuss a nebula, what are they talking about? a giant cloud of gas and dust between or among the stars Supposing we launched a very fast dart from the Space Shuttle, pointed in some direction away from any planet, so that it could travel beyond the solar system. What would it be most likely to hit first after traveling outward for a while? an atom of interstellar gas The red color we see on a lot of photographs of nebulae comes from which element? hydrogen You are observing a binary star system and obtain a series of spectra of the light from the two stars. In this spectrum, most of the absorption lines shift back and forth as expected from the Doppler Effect. A few lines, however, do not shift at all, but remain at the same wavelength. How can we explain the behavior of the non-shifting lines? the lines come from interstellar matter between us and the star, not from the stars themselves An astronomer wants to observe a cloud of cold neutral (not ionized) hydrogen, far away from any stars. What would be an instrument that could help in this task? a radio telescope, tuned to a wavelength of about 21 centimeters In order for a cold atom of hydrogen to emit a 21-cm wave, it must first be in a slightly higher energy state. What event usually "kicks" the hydrogen atom up to this higher state? gas atoms within the cloud collide Astronomers have found large quantities of cold, neutral hydrogen gas in our galaxy. How is this gas distributed? it is found mostly in a flat layer extending throughout the disk of our galaxy Some of the interstellar gas in our Galaxy has been heated to millions of degrees, a temperature that surprised astronomers when it was first discovered. How do we now think that gas between stars gets that hot? very powerful shock waves from exploding stars heat the gas they come into contact with If an astronomer wanted to find some relatively complex molecules in space, what technique should she use? point a radio telescope into regions in the galaxy where there is a lot of dust Astronomers now understand that the dark regions or rifts visible in parts of our Galaxy that are otherwise crowded with stars are caused by _____. clouds with a considerable amount of dust which blocks the light of the stars behind them Which of the following is NOT a way astronomers discover clouds of interstellar matter that have a large amount of dust in them? by giving off x-rays from hot gas surrounding the dust cloud An astronomer wants to observe a cloud of dust in a relatively close part of the Galaxy. Unfortunately, this dust cloud is not located in the direction of a crowded region of stars. What instrument would be the most help in finding this cloud. a sensitive infrared telescope in orbit around the Earth
A friend of yours who has not taken an astronomy class looks at your textbook and really likes the picture of the Pleiades, a cluster of stars surrounded by a bluish reflection nebula. She wants to know what causes that beautiful blue glow. To explain it to her, you want to compare the process that causes the blue glow to something that is in your friend's everyday experience. Which of the following terrestrial phenomena is the result of the same type of process that makes a reflection nebula in space? the blue color of the Earth's sky An astronomer is observing a star which puzzles her. The lines in the star's spectrum indicates that the star is very hot and should therefore be blue. But the star looks reddish in photographs and in measurements of the continuous spectrum. What is one possible explanation of this puzzle? we are seeing the light of the star through layers of interstellar dust The dust in the dust clouds in interstellar space consists of _____. tiny solid grains Among interstellar clouds, the hotter the cloud, _____. the lower the density of particles in it If you want to find stars that are just being born, where are the best places to search? in giant molecular clouds Which of the following are the small regions that are the embryos of stars (where individual stars are most likely to be born)? the cores within the clumps of molecular clouds The Orion Nebula is _____. a large cloud of gas and dust illuminated by the light of newly formed stars within it You are an astronomy graduate student and you are observing the big Orion Nebula from an airplane that has a good-sized infrared telescope built into it (there really is such a plane.) On an infrared image of the Nebula, what would particularly stand out? the clouds of the nebula that have a lot of dust in them Astronomers studying regions like the Orion Giant Molecular Cloud have observed that a wave of star formation can move through them over many millions of years. What sustains such a wave of star formation in a giant molecular cloud? when massive stars form, their ultraviolet radiation and later their final explosions compress the gas in the cloud and cause a new group of stars to form Why is it so difficult for astronomers to see new stars in the process of birth? -most stars are born inside dusty clouds, which block any light that may be coming from the stars -protostars which are not yet doing fusion do not give off a lot of visible light -the size of a newly forming star is typically quite small and thus hard to make out -birth happens very quickly, so it is hard to "catch" stars "in the act" Astronomers call a ball of matter that is contracting to become a star _____. a protostar A star whose temperature is increasing but whose luminosity is roughly constant moves in what direction on the H-R diagram? to the left
Astronomers identify the "birth" of a real star (as opposed to the activities of a protostar) with what activity in the star? when nuclear fusion reactions begin inside its core When a star settles down to a stable existence as a main-sequence star, what characteristics determines where on the main sequence in an H-R diagram the star will fall? its mass Which of these stars will take the SHORTEST time to go from the earliest protostar stage to the main sequence? a star ten times the mass of our Sun A graduate student is given the assignment to find stars with dusty disks around them. What kind of telescope would it be best for her to use for this purpose? a large telescope that detects infrared radiation What observations about disks of dusty material around young stars suggest that planets may be forming in such disks? the disks show lanes that are empty of dust within them Why do all stars spend most of their lives on the main sequence? because the fuel for energy production in this stage of the star's life is hydrogen; and that is an element every star has lots and lots of Which of the following types of stars will spend the longest time (the greatest number of years) on the main sequence? K Which of the following statements about the main sequence stage in the life of a star is FALSE? main sequence stars are rare in the Galaxy, so we are lucky to be living around one How long a main sequence star remains on the main sequence in the H-R diagram depends most strongly on _____. its mass Biologists tell us that life on Earth took billions of years to evolve into astronomy students and other examples of intelligent life. If we want to search for planets with intelligent life-forms that evolved over the same period of time that we did, what sorts of stars should we not bother searching around? O and B type stars The event in the life of a star that begins its expansion into a giant is _____. almost all the hydrogen in its core that was hot enough for fusion has been turned into helium When the outer layers of a star like the Sun expand, and it becomes a giant, which way does it move on the H-R diagram? toward the upper right A group of graduate students, bored during a cloudy night at the observatory, begin to make bets about the time different stars will take to evolve. If they have a cluster of stars which were all born at roughly the same time, and want to know which star will become a red giant first, which of the following stars should they bet on? a type O main sequence star A type of star cluster that contains mostly very old stars is _____. a globular star cluster A science fiction writer needs an environment for her latest story where stars are as crowded together as possible. Which of the following would be a good place to locate her story?
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