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Astro Midterm #3
| Question | Answer |
|---|---|
| Which of the following is not a method astronomers use to determine the physical conditions inside the Sun? | observing X-ray images of the solar interior using satellites |
| What is a geomagnetic storm? | An event initiated by a disturbance in the solar wind that includes radio blackouts, a depression of Earth's magnetic field, and an increase in the number of energetic particles in the solar wind |
| A star's color is related to it's surface temperature because _________. | stars emit thermal radiation |
| Consider a large molecular cloud that will give birth to a cluster of stars. Which of the following would you expect to be true? | A few massive stars will form, live, and die before the majority of the star's clusters even complete their protostar stage |
| What percentage of a star's total lifetime is spent on the main sequence? | 90% |
| Observationally, how can we tell the difference between a white-dwarf supernova and a massivestar supernova? | The spectrum of a massive-star supernova shows prominent hydrogen lines, while the spectrum of a white-dwarf supernova does not |
| What condition is necessary in an interstellar gas cloud for it to begin collapsing? | low temperature |
| What eventually halts the gravitational collapse of an interstellar gas cloud that forms an object that is not massive enough to become a star? | the crowding of electrons in the core |
| When does a star become a main-sequence star? | when the rate of hydrogen fusion in the star's core is high enough to sustain gravitational equilibrium |
| Imagine comparing a 1.2 solar mass white dwarf to a 1.0 solar mass white dwarf. Which of the following must be true? | The 1.2 solar mass white dwarf has a smaller radius. |
| What is the approximate diameter of the disk of the Milky Way? | 100,000 light-years |
| Which forces have physicists shown to be the same force under conditions of very high temperature, as confirmed by experiments in particle accelerators? | the electromagnetic and weak forces |
| Which of the following statements about comets and asteroids is true? | Comets are frozen balls of ice and dust. |
| Why aren't small asteroids spherical in shape? | The force of gravity in small asteroids is less than the resistance of the rock to deform |
| What type of energy transport is occurring just below the photosphere of the Sun? | convection |
| In the H-R diagram, which group represents stars that are classified as giants and supergiants? | D |
| Which of the following sequences correctly describes the stages of life for a one solar mass star? | protostar, main-sequence, red giant, white dwarf |
| Carbon fusion occurs in high-mass stars but not in low-mass stars because ________. | the cores of low-mass stars never get hot enough for carbon fusion |
| What is a planetary nebula? | the expanding shell of gas that is no longer gravitationally bound to the core of a star |
| What is the chief difficulty in attempting to detect planets around other stars? | Planets do not emit visible light, and they are typically at least a billion times fainter than their host stars. |
| Which new idea has been added into our theory of solar system formation as a result of the discoveries of extrasolar planets? | Planets can migrate from the orbits in which they are born |
| Compare the two radial velocity curves shown on the right for the stars 51 Pegasi (left) and Upsilon Andromedae (right). Why is the curve for Upsilon Andromedae more complex than that of 51 Pegasi? | Upsilon Andromedae has more than one planet |
| About how far is the Oort cloud from the Sun? | 10,000 to 100,000 AU |
| Gas flows from the Sun in the ____ , which can be considered an extension of the corona. | solar wind |
| What eventually halts the gravitational collapse of an interstellar gas cloud if it is massive enough to become a star? | the central object becoming hot enough to sustain nuclear fusion in its core |
| Which of the following statements about novae is not true? | Our Sun will probably undergo at least one nova when it becomes a white dwarf about 5 billion years from now. |
| This Hubble Space Telescope photo shows a planetary nebula. What is the white dot in the center (indicated by the arrow)? | A white dwarf |
| What produces the 21-cm radio emission line that can be used to map the Milky Way Galaxy? | atomic hydrogen |
| The galactic center lies in the direction of which constellation? | sagittarius |
| How would you expect a star that formed recently in the disk of the galaxy to differ from one that formed early in the history of the disk? | It should have a higher fraction of elements heavier than hydrogen and helium |
| Where are the X-rays produced that are emitted by quasars and other active galactic nuclei? | in the hot gas in an accretion disk around a central black hole |
| What are two key observational facts that led to widespread acceptance of the Big Bang model? | - the cosmic background radiation - the helium content of the universe |
| Which of the following is the phase of matter in the interior of the Sun? | Plasma |
| Based on its surface temperature of 6,000 K, most photons that leave the Sun's surface lie in which region of the electromagnetic spectrum? | visible |
| Hydrogen fusion in the Sun requires a temperature (in Kelvin) of | millions of degrees |
| How much mass does the Sun lose through nuclear fusion per second? | 4 million tons |
| What is the only force that can overcome the repulsion between two positively charged nuclei and bind them together? | the strong force |
| Since all stars begin their lives with the same basic composition, what characteristic is most important in determining how they will differ? | their initial mass |
| Star A and B have the same luminosity, but star B is 10 times farther from Earth than star A. How much brighter will star A appear? | 100 times |
| Which of the following statements about apparent and absolute magnitudes is true? | all of the above are true |
| In the H-R diagram shown on the right, which star has the largest radius? | Mu Cephei |
| In the H-R diagram shown on the right, which main sequence star has the longest lifetime? | Barnard's star |
| In the H-R diagram shown above, which star is in the last stage of its life cycle? | 40 Eridani B |
| What happens after the helium flash in the core of a star? | The core quickly heats up and expands as helium fusion begins all at once throughout the core. |
| Which element has the lowest mass per nuclear particle and therefore cannot release energy by either fusion or fission? | iron |
| What is the main idea captured by the graph on the right? | Low-mass stars are much more common than higher-mass stars |
| What is the ultimate fate of an isolated white dwarf? | It will cool down and become a cold black dwarf |
| From a physical standpoint, what is a pulsar? | a rapidly rotating neutron star |
| How do we know that pulsars must be neutron stars? | No massive object, other than a neutron star, could spin as fast as we observe pulsars to spin and remain intact |
| How does a black hole form from a massive star? | During a supernova, if the mass of the infalling core has enough gravity to overcome neutron degeneracy pressure, the core will collapse to a black hole. |
| Suppose you drop a clock toward a black hole. As you look at the clock from a high orbit, what will you notice? | Time on the clock will run slower as it approaches the black hole, and light from the clock will be increasingly redshifted |
| If you were to take a voyage through the entire disk of the Milky Way, what kind of material would you spend most of your time in? | rarefied clouds of atomic hydrogen |
| Over time, what is the net effect of the star-gas-star cycle in the Milky Way? | The total mass in the galaxy's interstellar medium is gradually reduced, and the remaining gas is continually enriched in heavy elements |
| Approximately how long does it take the Sun to orbit the center of the Milky Way Galaxy? | 200 million years |
| Where are most elements heavier than hydrogen and helium made? | in the cores of stars and in supernovae |
| Where does most star formation occur in the Milky Way today? | in the spiral arms |
| Why are we unlikely to find Earth-like planets around halo stars in the galaxy? | Halo stars formed from gas containing few heavy elements, which are needed to create terrestrial planets like the Earth. |
| Compared to spiral galaxies, elliptical galaxies are | redder and more round |
| Why are Cepheid variables important? | Cepheids variables are pulsating stars whose pulsation periods are directly related to their true luminosities. Therefore they can be used as distance indicators. |
| What is a standard candle? | an object whose luminosity we know |
| How was Edwin Hubble able to use his discovery of Cepheids in Andromeda to prove that the "spiral nebulae" were actually galaxies external to the Milky Way? | From the period-luminosity relation for Cepheids, he was able to determine the distance to Andromeda. He showed that it was far outside the Milky Way Galaxy |
| In words, what does the equation for Hubble's law state? | The recession velocity of a galaxy is directly proportional to its distance |
| Helium originates from | mostly from the Big Bang with a small contribution from stellar nucleosynthesis |
| How do observations of galaxies at different distances help us learn about galaxy evolution? | Observations of different distances show galaxies of different ages and therefore different stages of evolution |
| If the electromagnetic force is stronger than gravity, then why is it overpowered by gravity on large scales? | Most objects are electrically neutral |
| What do we mean by inflation? | a sudden expansion of the universe driven by the energy released when the strong and electroweak forces froze out from the GUT force |
| Evidence that the cosmic background radiation is the remnant of a Big Bang comes from predicting characteristics of remnant radiation from the Big Bang theory ... Four of the five statements below are real. Which one is fictitious? | The cosmic background radiation is expected to contain redshifted emission lines from hydrogen and helium, and it does |
| What does the diagram on the right represent? | A particle and antiparticle colliding and converting all their mass into photons |
| In principle, if we could see all the way back in time we could see the Big Bang taking place. However, our view is blocked for times prior to about 380,000 years after the Big Bang. Why? | Before that time, the gas in the universe was dense and ionized and therefore did not allow light to travel freely |
| Olbers' paradox is an apparently simple question, but its resolution suggests that the universe is finite in age. What is the simple question posed by Olbers' paradox? | why is the sky dark at night? |
| How can primitive meteorites be distinguished from processed meteorites? | Radiometric dating shows that primitive meteorites are slightly older |
| Where are the Trojan asteroids located? | along Jupiter's orbit, 60° ahead of and behind Jupiter |
| What surprising discovery did the New Horizons spacecraft make during its recent flyby of Pluto? | Pluto's surface shows signs of very recent geological activity |
| What causes the meteor showers we witness on Earth? | Earth passes through comet dust particles |
| In what way does the figure on the right give a poor representation of the asteroid belt? | For a correct scale, the asteroids should be drawn much smaller. This image gives the impression that the asteroid belt is very crowded. |
| What observations characterize solar maximum? | There are many sunspots visible on the surface of the Sun |
| What is a solar flare? | an eruption of solar material from the photosphere |
| In the diagram of the Sun's interior on the right, some arrows point outward, and some arrows point inward. Long arrows are near the center, short arrows near the outer surface. What do the different directions and different arrow lengths represent? | Inward arrows represent gravity; outward arrows represent pressure; longer arrows represent a stronger push or pull |
| In the late 1800s, Kelvin and Helmholtz suggested that the Sun stayed hot due to gravitational contraction. What was the major drawback to this idea? | It predicted that the Sun could shine for about 25 million years, but geologists had already found that Earth is much older than this |
| Star A and Star B have measured stellar parallax of 0.75 arc second and 0.10 arc second, respectively. Which star is closer? | Star A is closer |
| Hydrogen lines are weak in the spectra of hot O-type stars because ____. | most of the hydrogen is ionized to high energy levels |
| Which of the following is the most numerous type of main-sequence star? | M star |
| On the main sequence, stars obtain their energy | by converting hydrogen to helium |
| Which of the following comparisons between low-mass stars and high-mass main-sequence stars is true? | Low-mass stars are cooler and less luminous than high-mass stars |
| Helium fusion directly results in the formation of | carbon |
| This famous image from the Hubble Space Telescope shows what is sometimes called the "pillars of creation." Which of the following best describes what it shows? | The pillars are clouds of gas and dust in which many new stars are forming; the edges of the pillars are sculpted by ultraviolet radiation from stars outside the pillars |
| How do we think the "hot Jupiters" around other stars were formed? | They formed as gas giants beyond the frost line and then migrated inwards |
| The graph shows how a star's orbital speed (radial velocity) varies with time due to the gravitational tug of an orbiting planet. The amplitude of the radial velocity (how high and how low the radial velocity gets to be) is related to the planet's | mass |
| Using the transit method, how many times must a dip be observed before it can qualify as an actual planet detection? | 3 times |
| When Mercury or Venus passes in front of the disk of the Sun, we call this | transit |
| Brown dwarfs have | no habitable zones |
| What is the upper limit of a brown dwarf's size? | less than 0.08 solar masses |
| The fusion of helium into carbon will take a temperature of at least | 100 million degrees kelvin |
| Imagine that you are trying to stop neutrinos with a lead shield. How thick would you need to make this shield to ensure that it can stop an average neutrino? | about one light-year |
| The _____ is a type of hydrogen fusion that uses carbon, nitrogen, and oxygen atoms as catalysts | CNO cycle |
| _________ are thought to be caused by the formation of a black hole after a star explodes in a supernova explosion. | gamma ray bursts |
| Once a rock from space strikes the Earth's surface it's called a(n) | meteorite |
| At the end of its life, our Sun will leave behind an object similar to that shown by an arrow in the picture on the right. That object is called a | white dwarf |
| A star's luminosity is the | total amount of energy that the star radiates each second |
| If the distance between us and a star is doubled, with everything else remaining the same, its luminosity | remains the same, but its apparent brightness is decreased by a factor of four |
| A star's color is related to its surface temperature because _________. | stars emit thermal radiation |
| The faintest star visible to the naked eye has an apparent magnitude of about | 6 |
| Which of the following stars are brightest at ultraviolet wavelengths? | O and B stars |
| On a Hertzsprung-Russell diagram, where would you find red giant stars? | upper right |
| What did Carl Sagan mean when he said that we are all "star stuff"? | that the carbon, oxygen, and other elements essential to life were created by nucleosynthesis in stellar cores |
| The photo on the right shows the Pleaides, known in Japanese as Subaru. What kind of object is it? | open cluster of stars |
| Which of the following masses separates low mass stars from intermediate mass stars? | about 2 solar masses |
| Why is there an upper limit to the mass of a white dwarf? | The more massive the white dwarf, the greater the degeneracy pressure and the faster the speeds of its electrons. Near 1.4 solar masses, the speeds of the electrons approach the speed of light, and no more mass can be supported. |
| After a massive-star supernova, what is left behind? | either a neutron star or a black hole |
| From an observational standpoint, what is a pulsar? | an object that emits flashes of light several times per second (or even faster), with near perfect regularity |
| Which of the following statements about black holes is not true? | If the Sun magically disappeared and was replaced by a black hole of the same mass, the Earth would soon fall into the black hole. |
| Notice the well-defined spiral arms in this photograph of the galaxy M51. What makes the spiral arms so much brighter than regions between the arms? | The presence of many massive young stars in the spiral arms. |
| On a dark summer night in the northern hemisphere, we can see the Milky Way but we cannot see the center of the Milky Way. Why not? | Interstellar dust and gas absorbs and scatters visible light before it can reach us from the center |
| What makes up the interstellar medium? | gas and dust |
| What evidence supports the existence of a very massive black hole at the center of our galaxy? | The motions of the gas and stars at the center indicate that it contains 4 million solar masses within a region no larger than our solar system |
| How does the diameter of the disk of Milky Way Galaxy compare to its thickness? | The diameter is about 100 times as great as the thickness |
| If you looked at the field of view seen in this Hubble Space telescope (called the Hubble Deep Field) with your naked eye, about how big would it appear in the sky? | About the size of this period — . — viewed at arm's held at arm's length against the sky |
| Most active galactic nuclei are at large distances from us; relatively few nearby galaxies harbor active galactic nuclei. What does this imply? | Active galactic nuclei tend to become less active as they age |
| Why is the nucleosynthesis era so important in determining the chemical composition of the universe? | Except for a small amount of elements heavier than helium produced later by stars, the chemical composition of the universe is the same now as at the end of the nucleosynthesis era |
| Which of the following statements about the cosmic background radiation is not true? | It is the result of a mixture of radiation from many independent sources, such as stars and galaxies that formed within the first billion years of the Big Bang |
| Why do we expect the cosmic background radiation to be almost, but not quite, the same in all directions? | The overall structure of the universe is very uniform, but the universe must have contained some regions of higher density in order for galaxies to form |
| What does the diagram on the right represent? 2019 - 47 | A particle and antiparticle colliding and converting all their mass into photons. |
| We have direct experimental evidence (from large particle accelerators) for the physical conditions in the universe back to about ________ after the Big Bang. | 10^-10 seconds |
| Which of the following is not evidence for dark matter? | the expansion of the universe |
| Which of the following correctly describes how light will be affected as it tries to escape from a massive object? | Light escaping from a compact massive object, such as a neutron star, will be redshifted |
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