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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 _____.

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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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in a globular cluster As a cluster of stars begins to age, which type of star in the cluster will move off the main sequence of the H-R diagram first? the O and B type stars An astronomy student, for her PhD, really needs to estimate the age of a cluster of stars. Which of the following would be part of the process she would follow? plot an H-R diagram for the stars in the cluster On an H-R diagram of a cluster of stars, which characteristic of the diagram do astronomers use as a good indicator of the cluster's age? the point on the main sequence where stars begin to "turn off" -- to move toward the red giant region If you trace back the history of a carbon atom in your little finger through all of cosmic history, where did this atom most likely originate? it was fused from 3 helium nuclei in the core of a red giant star long before the Sun existed After it experiences a "helium flash" a star like the Sun will have a brief period of stability, fusing helium into carbon (and sometimes oxygen). During this brief stable stage, the star _____. -gets to be even larger in diameter than it was as a red giant -returns to the position on the H-R diagram that the star had in its main-sequence stage -increases tremendously in luminosity -is able to fuse many of the heaviest elements (such as iron and gold) in its superhot core -none of the above Why can a star with a mass like our Sun not fuse (produce) further elements beyond carbon and oxygen? because they just cannot get hot enough for the fusion of heavier nuclei Why is it easier for red giants to lose mass than main sequence stars? red giants are so big, the gravity at their surface (that holds material to the star) is less Really massive stars differ from stars with masses like the Sun in that they _____. can fuse elements beyond carbon and oxygen in their hot central regions Many names used by astronomers are misleading or outdated. A good example is the term planetary nebula, which astronomers use to refer to _____. the shell let go by a dying low-mass star In a planetary nebula, the shell of expelled material is glowing intensely. What is the main source of energy for this glow? ultraviolet radiation from the collapsing hot star at the center If most stars are low-mass stars, and low-mass stars typically eject a planetary nebula, why then do astronomers see relatively few planetary nebulae in the sky? planetary nebulae expand rapidly and soon become too faint to be visible Which of the following stages will our own Sun go through in the future? eventually fusing helium into carbon

giving off a planetary nebula expanding to become red giant spending a long time on the main sequence If stars with masses like our Sun's cannot make elements heavier than oxygen, where are heavier elements like silicon produced in the universe? heavier elements are made in the cores of significantly more massive stars than the Sun, which can get hotter in the middle How does a white dwarf differ from a neutron star? -A neutron star has a smaller radius than a white dwarf. -A white dwarf is less massive than a neutron star. -A neutron star is denser than a white dwarf. How do white dwarfs and neutron stars form? -Neutron stars form just before the parent star with an initial mass between 10 and 40 M Sun experiences a type II supernova explosion -White dwarfs form after the planetary nebula phase of the parent star with an initial mass of less than 10 M Sun . What keeps each (white dwarf and neutron star) from collapsing under its own weight? -Degeneracy pressure (electron degeneracy for white dwarfs and neutron degeneracy for neutron stars) prevents further collapse in the core. How do the two types of supernovae discussed in this chapter differ? -Type Ia are more luminous than type II. -Type Ia are observed in all types of galaxies. -Type Ia have a consistent value for maximum brightness. What kind of star gives rise to each type? -Type Ia supernovae are produced by white dwarf stars. -Type II supernovae are produced by massive stars. If the formation of a neutron star leads to a supernova explosion, explain why only three of the hundreds of known pulsars are found in supernova remnants. -Pulsars can be kicked from the site of the supernova which causes them to move away from the remnant at very high velocities. -Pulsars can remain visible through their radio emission long after the material from the supernova is no longer visible. What observations from SN 1987A helped confirm theories about supernovae? -Scientists detected neutrinos from the core collapse. Describe the evolution of a white dwarf over time, in particular how the luminosity, temperature, and radius change. How does the luminosity change? -The luminosity decreases over time.

How does the temperature change? -The temperature decreases over time. How does the radius change? -The radius is constant over time. Arrange the following stars in order of their evolution. A. a star with no nuclear reactions going on in the core, which is made primarily of carbon and oxygen B. a star of uniform composition from center to surface; it contains hydrogen but has no nuclear reactions going on in the core C. a star that is fusing hydrogen to form helium in its core D. a star that is fusing helium to carbon in the core and hydrogen to helium in a shell around the core E. a star that has no nuclear reactions going on in the core but is fusing hydrogen to form helium in a shell around the core -Oldest to youngest- BCEDA Would you expect to find any white dwarfs in the Orion Nebula? (See the chapter titled "The Birth of Stars and the Discovery of Planets" outside the Solar System to remind yourself of its characteristics.) Why or why not? -No. The stars in the Orion Nebula are about 2 million years old which is not enough time for stars to become white dwarfs. Suppose no stars more massive than about 2 M Sun had ever formed. Would life as we know it have been able to develop? Why or why not? -No. Stars with these masses only produce elements up to carbon and oxygen. More massive stars are needed to produce elements necessary for life such as calcium and iron. Would you be more likely to observe a type II supernova (the explosion of a massive star) in a globular cluster or in an open cluster? Why? -open cluster -The massive stars in globular clusters completed their evolutions long ago. The massive stars in open clusters might just be finishing their lives. Would you expect to observe every supernova in our own Galaxy? Why or why not? -No. Type II supernovae occur within the galactic disk which can be obscured by dust especially if they occur in more distant parts of the Galaxy. Type Ia supernovae can occur in older parts of the Galaxy that can also be hidden by the buildup of gas and dust. Look at the list of the nearest stars in Appendix I . Would you expect any of these to become supernovae? Why or why not? -No. The stars are all below 10 solar masses making it unlikely they will become a type II supernova and the Sirius and white dwarf companion system are too far apart to become a type Ia supernova. If most stars become white dwarfs at the ends of their lives and the formation of white dwarfs is accompanied by the production of a planetary nebula, why are there more white dwarfs than planetary nebulae in the Galaxy? -White dwarf stars are visible for a billion years before they completely cool and become undetectable as opposed to the 10,000 years it takes for the expelled gas from the planetary nebula disappear.

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You have discovered two star clusters. The first cluster contains mainly main-sequence stars, along with some red giant stars and a few white dwarfs. The second cluster also contains mainly main-sequence stars, along with some red giant stars, and a few neutron stars—but no white dwarf stars. What are the relative ages of the clusters? How did you determine your answer? -The first cluster is older. -The lack of white dwarf stars in the second cluster indicate that the lower mass stars have not yet evolved to the white dwarf stage. Based upon the evolution of stars, place the following elements in order of least to most common in the galaxy: gold, carbon, neon. -Gold is produced in supernovae, which only occur in the most massive stars making it rare -Carbon is formed in low-mass stellar fusion making it very common. -Neon is formed in much-higher-mass stellar fusion making it common. According to the general theory of relativity, the presence of mass _____. -causes a curvature (or warping) of spacetime Which of the following statements about the way the mass of a white dwarf affects spacetime is correct? -the white dwarf mass will curve spacetime; light has to follow that curvature When Einstein proposed his General Theory of Relativity, he suggested some pretty strange ideas about space, time, and gravity. How did scientists in 1919 show that Einstein's theory described the behavior of the real world and wasn't just a crazy hypothesis? -by observing starlight coming close to the Sun during an eclipse When a light wave leaves a region of strong gravity, compared to the same wave leaving a spaceship in empty space, the wave in strong gravity will have _____. -less energy -a longer wavelength -a lower frequency -a gravitational redshift From which of the following will a wave of light show the greatest gravitational redshift? -a white dwarf According to the general theory of relativity, light and other radiation coming from a white dwarf or a neutron star should (and experiments show that it does) exhibit _____. -a gravitational redshift To predict whether a star will ultimately become a black hole, what is the key property of the star we should look at? -mass The region around a black hole where everything is trapped, and nothing can get out to interact with the rest of the universe, is called _____. -the event horizon Deep inside a black hole (and hidden from our view) is the compressed center, where all the "stuff" of the star goes. Astronomer call this central point _____.

-a singularity In the far future, a starship becomes trapped inside the event horizon of a black hole. Although the crew discovers that their ship cannot get out, they at least want to send a message to other ships in the area to stay away from the danger zone. If they send out a message in the form of a radio wave, what will be its fate? -the message will never emerge from the event horizon The astronomer who first worked out the mathematical description of black hole event horizons was _____. -Karl Schwarzschild Once a black hole forms, the size of its event horizon is determined only by _____. -the mass inside the event horizon Which of the following can a black hole not "eat" (swallow)? -black holes can eat anything Suppose each of the following objects could collapse into a black hole. Each black hole would have a sphere around it that is the limit for escape—once you are inside this region, you cannot get away. For which object would this region be the largest in diameter? -an entire galaxy of stars (with about a billion stars in it) What type of main sequence star is most likely to become a black hole? -an O-type star Far away from a black hole (at the distance of another star), which of the following is a possible way to detect it? -search for flickering x-rays being given off from an accretion disk around the black hole, as it "eats" part of a neighbor star Which of the following objects do many astronomers believe is a black hole? -Cygnus X-1 When one member of a binary star system is a black hole, and astronomers detect flickering x-rays coming from the system, where are these x-rays usually coming from? -from a disk of material around the black hole (material that has been pulled from the companion star and is falling toward the black hole) In the first direct detection of gravitational waves by LIGO in 2015, the waves came from _____. -the merger of two black holes The luminosity of a star is the total amount of _______________ that is emitted by a star -energy The apparent brightness of a star is the amount of light that we receive here on Earth. -true In order to determine the luminosity of a star from its apparent brightness, we need to know its ___. -distance

In the astronomical magnitude scale, a larger value means that the star is brighter. -False One magnitude difference means a difference in brightness of: -2.512 times How much brighter is a magnitude 2 star compared to a magnitude 4 star? -6.31 times Looking at these two blackbody radiation curves of stars: Which of the following statements is correct? -Star A is the hotter star In terms of colors, which of the following statements is correct for these 2 stars: -Star A would appear bluer than Star B Cecilia Payne-Gaposchkin discovered that the appearance and strength of the absorption lines in stellar spectra only depend on the amounts of various chemical elements in the photospheres of these stars. In other words, a star with strong hydrogen absorption lines has much more hydrogen in its photosphere than stars with no or weaker hydrogen lines. -False For which spectral type are the hydrogen absorption lines in the visible (the Balmer lines) the strongest? -A For which spectral type are the absorption lines of ionized helium the strongest? -O Which spectral type is our Sun? -G The apparent change in position of a star with respect to more distant background stars is called ____ motion. -proper The radial velocity of a star as measured with the Doppler effect, is the velocity component perpendicular to the line of sight. -False Astronomers can determine what property/properties of a star by measuring the broadening of the spectral lines? -the pressure in the photosphere and the rotation speed of the star The Hertzsprung-Russell (HR) diagram is a simple plot of 2 fundamental stellar values, which two are plotted against each other? -luminosity vs photospheric temperature Correctly identify the 4 major regions in the HR diagram: - A = main sequence, B = red giants, C = supergiants, D = white dwarfs The main sequence phase of stars is defined by the stars being in ____________________ equilibrium due to stable hydrogen fusion in their cores - hydrostatic

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Which star(s) is/are intrinsically brighter than the Sun? - star A & star B Once astronomers have determined the luminosity of star and its temperature (by using spectroscopy) they can use the Stefan-Boltzmann law to calculate the ___________ of the star - radius Which star is the largest star? - star B Which star is the hottest main sequence star - star A What type of star is star C - a white dwarf Astronomers use Kepler's 3rd law to determine the total _____________ of the stars in binary systems. - mass You find a binary star system with an orbital period of 30 years and a semimajor axis of 10 astronomical units. What is the total (=combined) mass of the two stars? - about 1.1 solar masses Looking at the stellar mass function , which statement below is correct? - most stars have lower masses than the Sun An object that formed like a star but has insufficient mass to start nuclear fusion in its core is called a: - brown dwarf — A parsec is defined as: the distance to a star that has a parallax angle of 1 arcsecond The angle of stellar parallax for a star gets larger as the length of the baseline increases A star with a parallax angle of 1/6 of an arcsecond is at a distance of: 6 parsecs Henrietta Swan-Leavitt made a profound discovery in astronomy. By studying RR Lyrae and Cepheids in the Large Magellanic Cloud, she discovered that the ___________ of variability of these stars is tightly connected to their brightness. Period What is the period of variability of this Cepheid variable star? - about 6 days

You found a Cepheid variable star with a period of variability of 5 days. Using the period-luminosity relationship, what is the luminosity of the star that you have found? -about 1000 solar luminosities The way how the period-luminosity relationship works as a distance measurement tool is equivalent to knowing the wattage of a light bulb, that you see at an unknown distance, and then you can pull out this distance by comparing the luminosity (wattage) of this light bulb to its apparent brightness. - True The luminosity class of a star completes its spectral classification. Astronomers use roman numerals I -> V, and these numbers indicate the: -the size of the star What is the luminosity class of the Sun? - V (it's a main sequence star) Astronomers can determine the luminosity class of a star by looking at a spectrum of the star and: -measuring the amount of pressure broadening of the absorption lines The interstellar medium (ISM) is mostly made of - hydrogen and helium gas This picture shows the typical image of: - an emission nebula This image shows a typical - reflection nebula The horsehead nebula is caused by -extinction due to a dark dust lane in front of an emission nebula What is the consequence of this transition of hydrogen atoms in the ISM? -the emission of a photon with a wavelength of 21 cm (radio) The scattering of preferentially blue wavelengths by microscopic dust grains in the ISM causes ______________ of starlight that passes through the ISM -reddening — Stars form when cold and dense ISM clouds spontaneously collapse under their own gravity - False The fundamental force that forms stars out of ISM clouds is gravity. - True Which of the following is/are triggers of star formation? - these are all possible triggers of star formation Once a large section of an ISM clouds becomes gravitationally unstable, the first step/stage in the star formation process is ______. - Fragmentation

During the star formation process, the object that forms in the center and heats up due to the increase in pressure is called a ___________. This object already shines with light, but not because of nuclear fusion, but because of its own thermal emission (blackbody radiation) . -protostar A star is truly born when its center reaches a temperature high enough to start ______. - nuclear fusion Stars always form in large groups, never alone. - True Why is the luminosity of this protostar dropping so much between stage 4 and stage 6? - because it gets smaller Massive stars take longer to reach the main sequence than lower mass protostars - False What is the main difference between these two clusters? - their age Why do all stars eventually need to leave the main sequence? -because they run low on hydrogen to fuse in the core which leads to a loss of hydrostatic equillibrium Which one is the longest stage in the life of a star? - the main sequence phase At the end of its life, the Sun will explode in a spectacular supernova event! - False The Sun will end its life as a ________________ (fill in the blank with two words) - white dwarf Why is there such a large increase in luminosity between stage 8 and stage 9? -because the star increases its size significantly At stage 10 in this diagram, what is the correct structure of the core of the star? -in its center it is fusing helium into carbon, surrounded by a hydrogen fusion shell At stage 11, what is the correct structure of the core of the star? -it's mostly inert carbon, surrounded by a helium fusion shell, and then a hydrogen fusion shell What is happening during the planetary nebula phase? - the entire envelope of the star is expelled into space by violent pulsations, exposing the core that has finally shut down all fusion reactions What is the major difference in the post-main sequence evolution of massive stars compared to Sun-like stars? - Their contracting cores will fuse even heavier elements than helium Using the method of the main sequence turn off point, which one of the 3 HR-diagrams belongs to the youngest cluster? - B Which star will probably go supernova?

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-a 15 solar mass star Once an iron core emerges in the center of a very massive star, it will lead to a rapid collapse of the core, and a subsequent supernova explosion. Why? -because iron cannot be fused, the energy production is zero, and hydrostatic equillibrium is lost catastrophically During the core collapse of a type II supernova, something weird happens to the protons and the electrons inside the collapsing core. What happens? - they get pushed together so that they merge to form neutrons, releasing neutrinos in the process Why was supernova 1987A so important for our understanding of core-collapse supernovae? -because a neutrino pulse was detected as well, confirming our understanding of the physics of the core-collapse model Which bright star in our sky will certainly go supernova sometime in the next few thousands of years (maybe even during our life times)? -Betelgeuse Astronomers realized that there are two types of supernovae by comparing the ________________ of this explosions. - light curves How can a white dwarf go supernova? - when it somehow gains mass until gravity overpowers the electron degeneracy pressure, collapsing the WD, which leads to a rapid temperature increase that triggers the fusion of carbon What are the progenitors of type-I supernovae? -Either a white dwarf in a close binary system with mass transfer, or 2 merging WDs At peak luminosity a supernova can be as bright as all the remaining stars of a galaxy combined. - True White dwarfs are about the size of: - planet earth The vast majority of all stars, including our Sun, will end their lives as: - white dwarfs For the remainder of its life, a white dwarf is bascially doing nothing except to ____________ down. (Fill in the blank with a single word). - cool Neutron stars are about the size of - a city What is a pulsar? - a rapidly rotating neutron star that has 2 jets of radiation and particles along their magnetic field axis. Every time the jets sweep over the Earth, we receive a "pulse" Neutron stars are leftovers from -massive stars that underwent a core collapse supernova

When does a stellar mass black hole form? -Whenever the collapsing core during a type-II supernova has a mass higher than 3 solar masses What would happen to the orbits of the planets, if the Sun was replaced by a black hole with 1 solar mass? -The orbits would stay the same since the mass of the central object has not changed Why are black holes black? In other words, why is light also affected by the strong gravity of the singularity, and cannot escape even though light has no mass? -because gravity curves space-time and light follows the geometry of space-time The Schwarzschild radius of a black hole is the distance to the singularity where the escape velocity from the gravitational field of the singularity exceeds the speed of light. This radius defines the size of the ______ ________ of the black hole. - Event horizon Like shown in the movie "Interstellar" (starring Matthew Mcconaughey), an astronaut would actually survive the passage through the event horizon of a stellar-mass black hole. -False Nowadays astronomers detect and study black hole mergers by detecting the ________________ waves caused by these events. - gravitational

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