Deck 20: Stellar Evolution: The Death of Stars

Question 1

What is meant when a nuclear reaction is described as "thermonuclear"?

A)The temperature threshold is set by convention at 10 million K.
B)High-speed collisions bring nuclear particles with like electric charge close enough together that nuclear binding occurs.
C)The reaction can be triggered by heat alone; motion is not needed.
D)Thermonuclear is another word for fusion as opposed to fission reactions.

Answer 1

B

Question 2

During the first red giant phase, the source of a star's energy is:

A)shell hydrogen fusion only.
B)both shell hydrogen fusion and core helium fusion.
C)core helium fusion only.
D)the Kelvin-Helmholtz contraction.

Answer 2

A

Question 3

How bright will a star with the mass of the Sun become at the helium flash point, compared to its main-sequence luminosity?

A)10⁶ times brighter
B)about 50% brighter
C)10³ times brighter
D)10 times brighter

Answer 3

C

Question 4

The most important development in the interior of a star of moderate mass that leads to the red giant phase is:

A)a runaway nuclear furnace and eventual explosion in the star's interior, destroying the star and leaving only a rapidly expanding shell of gas.
B)the cessation of hydrogen "burning" in the core, leading to core contraction and overall star expansion.
C)expansion of the whole star, leading to lower temperatures throughout the star and the turnoff of the nuclear furnace in the core.
D)contraction of the whole star, which quenches the nuclear furnace in the core and leads to cooling of the surface.

Question 5

During which phase in the life of a solar mass star does helium shell burning occur?

A)first red giant phase
B)main sequence
C)horizontal branch
D)asymptotic giant branch

Question 6

The main source of energy for a solar mass star while it is on the horizontal branch is:

A)core hydrogen fusion.
B)core helium fusion.
C)shell hydrogen fusion.
D)shell helium fusion.

Question 7

The characteristics of red supergiant stars are a brightness of:

A)10,000 Suns and a diameter of about that of Earth's orbit.
B)Sun and size of about that of Mercury's orbit.
C)about 1 million Suns and a diameter of the whole solar system.
D)about 10,000 Suns and a diameter of 1/10 of that of the Sun.

Question 8

What will be the consequences for the planetary system when the Sun evolves to the asymptotic giant branch (AGB) phase of its evolution?

A)The whole planetary system will melt and vaporize.
B)The whole system will survive almost intact, but the planets will be driven outward from their present orbits by the intense solar wind.
C)The planetary system will be slowly drawn into the core of the Sun by the gravitational field of the high-density core.
D)The inner planets will melt and vaporize, but Jupiter and the outer planets will survive, after losing their outer atmospheres.

Question 9

A "carbon star" has more carbon on its surface than does the Sun. This is the result of:

A)neutrinos, which escape easily from the core of a star but react with the cool hydrogen at its surface to form carbon.
B)helium flash, in which the explosion blasts carbon from the core into the surface layers.
C)dredge-up, in which the convective envelope transports material from a star's core to its surface.
D)mass loss, which strips away the outer envelope from an old star and reveals the carbon-rich core.

Question 10

The one material transported from the core to the surface in all three dredge-ups is:

A)hydrogen.
B)helium.
C)carbon.
D)oxygen.

Question 11

Which of the energy-transporting processes in a star's interior also plays a role in moving heavy elements from their production region to the star's surface and from there into outer space?

A)Transport is not necessary because heavy elements are produced at the star's surface by fusion reactions in the late evolutionary phases of a star.
B)radiative diffusion, by radiation pressure
C)conduction
D)convection

Question 12

Dredge-ups occur when convection extends from deep within a star to the surface. Three of the following examples of deep convection can dredge up heavy elements to the surface. Which example does NOT dredge up heavy elements?

A)Convection in a red dwarf starts in the core.
B)Convection starts deep within a star when it becomes a red giant for the first time.
C)A dredge-up occurs when a star first becomes an asymptotic branch star.
D)For stars more massive than about 2 M ? , a dredge-up occurs late in the asymptotic branch period.

Question 13

Convection that reaches from the core to the surface of a star is important in each of the following EXCEPT ONE. Which is the exception?

A)main-sequence stars of one solar mass
B)red dwarfs
C)giant stars
D)carbon stars

Question 14

Asymptotic branch stars are important for enriching the interstellar medium with heavy elements and molecules. Three of the following statements correctly describe reasons for this enrichment, and one does NOT. Which is the EXCEPTION?

A)These stars have relatively cool surfaces.
B)These stars have strong stellar winds.
C)These stars have convective zones that extend deep into the star.
D)These stars have shell fusion occurring just beneath the surface.

Question 15

What process is necessary for a star with twice the Sun's mass to become a carbon star?

A)dredge-up by convection of carbon that has been produced in the stellar core by nuclear fusion
B)nuclear fusion reactions on the surface of the star, producing carbon nuclei by combination of three helium nuclei during the helium flash
C)ejection of the outer hydrogen and helium layers by the helium flash in the star's interior, revealing the stellar carbon core
D)nuclear fission reactions splitting magnesium nuclei (²⁴Mg, charge = 12) into carbon nuclei (¹²C, charge = 6) on the star's surface

Question 16

A star of twice the mass of the Sun reaches the asymptotic giant branch (AGB) phase of its life and begins to eject large amounts of carbon and heavier elements into space. What process occurs to make this component easy to detect, particularly at radio wavelengths?

A)The carbon grains (soot!) are heated by radiation and reemit a cool thermal continuum spectrum whose peak is at radio wavelengths.
B)The carbon nuclei acquire electrons and become neutral atoms in an excited state, which then emit an easily detected spectrum as they de-excite.
C)Carbon and oxygen nuclei have such high velocities that they undergo nuclear fusion in a shell surrounding the star to produce heavier elements whose emissions are at radio wavelengths.
D)Carbon and oxygen combine chemically to produce the easily detected CO molecule.

Question 17

Interstellar dust contains graphite dust, made of pure carbon, along with other materials. Most of this carbon probably:

A)came from T Tauri stars through stellar winds.
B)came from carbon stars, through stellar mass loss.
C)was produced in the Big Bang at the time of the creation of the universe.
D)was produced in supernova explosions.

Question 18

During its lifetime, the Sun will experience all of the following energy sources EXCEPT:

A)Kelvin-Helmholtz gravitational contraction.
B)hydrogen burning.
C)helium burning.
D)carbon burning.

Question 19

All of the ¹²C in the universe, including that in our bodies, is believed to come from:

A)the proton-proton chain of nuclear reactions in main-sequence stars.
B)the triple alpha process in helium burning in giants.
C)supernova explosions.
D)many important sources, not just one.

Question 20

A planetary nebula is:

A)a contracting spherical cloud of gas surrounding a newly formed star, in which planets are forming.
B)the expanding nebula formed by the supernova explosion of a massive star.
C)an expanding gas shell surrounding a hot, white dwarf star.
D)a disk-shaped nebula of dust and gas from which planets will eventually form, easily photographed around relatively young stars.

Question 21

A planetary nebula is created:

A)over several hundred years, during mass transfer in a close binary star system.
B)in seconds, during the helium flash in a low-mass star.
C)slowly over 10,000 years or more, due to thermal pulses in a low-mass star.
D)in hours or less, during the explosion of a massive star.

Question 22

The major source of light in the expanding shell of gas in a planetary nebula is:

A)fluorescence of the atoms, caused by UV light from the hot central white dwarf star.
B)reflection and scattering of the light of the central white dwarf star from dust and gas in the shell.
C)thermal heating of the dust grains by radiation from the hot central star.
D)thermonuclear reactions in this hot gas, caused by the underlying explosion.

Question 23

The typical diameter of a planetary nebula is:

A)about 1000 ly.
B)about 1 au.
C)about 3 to 5 stellar diameters.
D)about 1 ly.

Question 24

The core of a planetary nebula has an initial surface temperature of 100,000 K. If it radiates like a blackbody, in what electromagnetic region will the peak of its radiation spectrum lie?

A)radio
B)visible
C)ultraviolet
D)gamma ray

Question 25

The fraction of the mass of a red giant that is ejected as a shell in a planetary nebula is:

A)almost the whole star, greater than 90%.
B)substantial, up to 40%.
C)extremely small, less than 1 part in 10⁴.
D)small, close to 0.01, or 1%.

Question 26

What will be the mass of the Sun at the end of its asymptotic giant branch (AGB) phase, as a result of mass loss to space by its stellar wind?

A)0.6 M ?
B)almost 1 M ? , because mass loss is negligible for a low-mass star like the Sun
C)0.1 and 0.2 M ? , because the Sun will lose most of its mass
D)0.9 M ?

Question 27

The shell of a planetary nebula is measured by the Doppler shift of emission lines to be expanding outward at a speed of 10 km/s and its diameter is measured to be 1 ly. How long has the shell been expanding?

A)50,000 years
B)50 million years
C)50 years
D)9.5 × 10¹¹ years

Question 28

Out of all matter ejected into the interstellar medium each year by stars, planetary nebula shell ejections account for:

A)almost 100%.
B)just over half.
C)less than 1%.
D)about 15%.

Question 29

What fraction of the material returned to the interstellar medium of a galaxy by stars is contributed by planetary nebulae as they eject their shells?

A)about 15%
B)only about 1 part in a million
C)less than about 1%
D)about 80%, or most of the returned material

Question 30

Which of the following important components does a planetary nebula contribute to the interstellar medium?

A)nuclei of elements like carbon, nitrogen, and oxygen, which are major components of planets such as our own
B)UV light that photoionizes hydrogen, which produces the red Balmer- light we see in emission nebulae
C)rotational motion from the original star, which serves to concentrate interstellar matter into new stars and planetary systems
D)new hydrogen nuclei, replenishing those which are lost when stars form

Question 31

What causes thermal pulses in low-mass stars?

A)Thermal pulses are the end result of the 5-minute vibrations like those we observe on the Sun.
B)The internal structure of these low-mass stars becomes unstable as they cross the instability strip on the main sequence.
C)A helium shell flash occurs. This re-ignites the helium shell and triggers an expansion of the outer layers.
D)Spent nuclear "ash" from the helium burning shell falls onto the hydrogen-burning core, causing it to flare up.

Question 32

The energy to produce a thermal pulse comes from:

A)hydrogen core fusion.
B)the helium flash.
C)helium core fusion.
D)the helium shell flash.

Question 33

The object seen at the center of a planetary nebula is:

A)an accretion disk around a black hole.
B)a planet in the process of forming.
C)composed almost entirely of neutrons and is spinning rapidly.
D)the former core of a red giant star, now a white dwarf star.

Question 34

According to our theories of stellar evolution, the Sun will probably end its active life as a:

A)red giant.
B)black hole.
C)pulsar.
D)white dwarf.

Question 35

In which order will a single star of about 1 M ? progress through its various stages of evolution?

A)T Tauri, main sequence, planetary nebula, and white dwarf
B)planetary nebula, main sequence, neutron star, and black hole
C)T Tauri, red giant, white dwarf, and neutron star
D)planetary nebula, main sequence, red giant, and white dwarf

Question 36

One particular characteristic of a white dwarf star is its:

A)very high surface temperature.
B)extremely low mass for a star, about 1/100 of 1 M ?
C)very low surface temperature, because it is at the end of its life.
D)spectrum, consisting simply of emission lines from hydrogen, helium, carbon, and oxygen.

Question 37

A white dwarf star is supported from collapse under gravity by:

A)electron degeneracy pressure in the compact interior.
B)centrifugal force due to rapid rotation.
C)pressure of the gas, heated by nuclear fusion reactions in a shell around its core.
D)pressure of the gas, heated by nuclear fusion reactions in its core.

Question 38

When the small but dense white dwarf is formed from a star like the Sun, the core temperature is not high enough to permit the carbon and oxygen to participate in further energy-producing nuclear reactions. But why does this dense core not produce energy by the Kelvin-Helmholtz contraction?

A)Electron degeneracy holds up the star and prohibits contraction.
B)Neutron degeneracy holds up the star and prohibits contraction.
C)The Kelvin-Helmholtz contraction only applies to hydrogen.
D)The Kelvin-Helmholtz contraction only applies to hydrogen and helium, not to metals.

Question 39

As a red giant evolves, its temperature decreases but its luminosity increases. As a white dwarf ages, its temperature decreases, and its luminosity also decreases. Why this difference?

A)A red giant experiences nuclear fusion on its surface while a white dwarf no longer experiences fusion.
B)A white dwarf expels much soot, and this blocks its increasing luminosity from being seen.
C)A red giant expands greatly as it cools, but a white dwarf shrinks as it cools.
D)A red giant expands greatly as it cools. Because of degeneracy pressure, a white dwarf maintains the same size as it cools.

Question 40

Because it has ceased nuclear burning in its interior and therefore no longer generates energy, why is it that a white dwarf does not shrink rapidly under the force of gravity as it cools?

A)The rapid reduction of radius before the white-dwarf phase produces a very rapid rotation, thereby generating a large centrifugal force that prevents the star from shrinking.
B)The very low luminosity of a white dwarf means that it cools slowly and maintains a high temperature and therefore a high internal pressure that opposes gravity.
C)The star has lost so much mass in earlier phases that the remaining mass generates insufficient gravitational force to produce further shrinkage.
D)The electrons within it are in a degenerate state and will not allow further shrinkage.

Question 41

The stars that eventually become white dwarfs are those that start life with masses of less than about

A)1.4 M ?
B)3 M ?
C)25 M ?
D)4 M ?

Question 42

A white dwarf star, as it evolves, undergoes which of the following changes?

A)Luminosity and size decrease, while its temperature remains constant.
B)Its temperature remains constant, but its radius and luminosity decrease.
C)It shrinks in size, the resulting release of gravitational energy maintaining both luminosity and temperature constant.
D)Luminosity and temperature decrease, but its size remains constant.

Question 43

If two white dwarf stars have the same composition, then the more massive one always has:

A)a smaller radius than the less massive one.
B)a larger radius than the less massive one.
C)the same radius as the less massive one, since electron degeneracy causes all white dwarfs to be the same size.
D)a size that depends critically on composition (metallicity) rather than mass.

Question 44

White dwarf stars of different masses have radii or sizes for which the:

A)more massive the star, the larger it is.
B)sizes are the same for all stellar masses.
C)sizes start out the same for all masses, but the more massive stars shrink fastest.
D)more massive the star, the smaller it is.

Question 45

If you were to increase the mass of a white dwarf star in some way:

A)its radius would become smaller.
B)it would reject the mass, because the electrons within the star are in a degenerate state and generate a very large outward pressure.
C)its radius would become larger.
D)its radius would remain the same, because the electrons in the stellar material are in a degenerate state.

Question 46

Why can we not have a white dwarf of 2 M ? ?

A)Even the largest main-sequence stars always lose enough mass so that they reduce to 1.4 M ? after they form a planetary nebula.
B)A white dwarf of more than 1.4 M ? could not be supported by electron degeneracy.
C)When a star reaches the white dwarf stage with more than 1.4 M ? , the intensity of nuclear radiation in its core makes it explode.
D)A white dwarf is like a giant nucleus; if it has a mass of more than 1.4 M ? , it fissions into two parts like an unstable nucleus.

Question 47

One peculiar feature of the evolution of a white dwarf star is that:

A)its size or radius slowly increases as it cools, until it becomes a red giant star.
B)it heats as it shrinks because of the release of gravitational energy, ending up as a very hot but very small star.
C)it shrinks as it cools, eventually to become a cold black hole in space.
D)its size remains constant as it cools and dies.

Question 48

As a white dwarf evolves, the direction of its motion on the Hertzsprung-Russell diagram below the main sequence is upper right to lower left, which means that:

A)its size must be increasing; therefore, its luminosity is also increasing.
B)the release of gravitational energy heats the star as it shrinks, and it will die as a hot but very small star.
C)its size or radius remains constant as it cools and becomes less luminous.
D)it shrinks as it cools, becoming a cold neutron star.

Question 49

There is a mass limit for a star in the white dwarf phase, the Chandrasekhar limit, beyond which the star can no longer support its own weight. This mass limit, in terms of solar mass, is:

A)1.4.
B)14.
C)30.
D)0.2.

Question 50

How much fainter than a supernova is a typical nova?

A)1 million times
B)10 times
C)100 million times
D)10,000 times

Question 51

What is the mass limit above which the self-gravity of stars can overcome electron degeneracy pressure?

A)0.05 M ?
B)1.4 M ?
C)14 M ?
D)There is no limit, because nothing in nature can overcome this quantum mechanical limit.

Question 52

A star of 25 M ? spends roughly what percentage of its life as a main-sequence star? (See Table 20-1 of Universe, 11th ed.)

A)about 10%
B)91%
C)very little, less than 1%
D)about 50%

Question 53

Is it possible for an alpha particle (helium nucleus) to fuse with an iron nucleus to form a heavier nucleus?

A)No, iron is the heaviest stable nucleus.
B)No, iron is not the heaviest stable nucleus, but the iron nucleus already has a complete set of protons and refuses to accept the constituents of an additional alpha particle.
C)Yes, adding an alpha particle to iron is the next energy-generating reaction after silicon fusion.
D)Yes, but this reaction does not take place spontaneously in stars, because it requires energy rather than producing energy.

Question 54

Which of the following nuclei can act as "fuel" in a core fusion reaction?

A)²⁸Si (silicon)
B)⁵⁶Fe (iron)
C)¹⁹⁷Au (gold)
D)²⁰⁸Pb (lead)

Question 55

The majority of massive stars, at the point in time when they explode as supernovae, have become: (See Figure 20-14 of Universe, 11th ed.(in the text))

A)red supergiants.
B)neutron stars.
C)blue supergiants.
D)white dwarfs.

Question 56

When an iron core is formed in a massive star, what ultimately keeps the core from collapsing to form a black hole?

A)the pressure generated by fusion reactions within the iron core
B)electron degeneracy
C)neutron degeneracy
D)Nothing; the iron core must collapse all the way to a black hole.

Question 57

A shock wave is produced during a supernova explosion, caused by the outward movement of hot inflating bubbles. Where do these bubbles get their energy?

A)Kelvin-Helmholtz contraction
B)energy from the final fusion reaction: iron + hydrogen = nickel
C)gamma radiation from the core
D)neutrinos

Question 58

Just before the outer layers of a high-mass star are thrown into space in a supernova explosion, the density in its core is about that:

A)of iron metal, 7.5 × 10³ kg/m³.
B)of degenerate gases in white dwarf stars, about 10³ kg/m³.
C)at the center of the Sun, about 1.5 × 10⁵ kg/m³.
D)of nuclear matter in a normal nucleus, about 4 × 10¹⁷ kg/m³.

Question 59

It is now thought that most elements in the universe heavier than iron in the periodic table:

A)are produced by the nuclear reactions in the central cores of high-mass stars a few hundred years before the star explodes into a supernova.
B)were produced in the Big Bang explosion at the beginning of our universe.
C)are produced by the successive capture of high-speed neutrons within low-mass star cores because neutrons are uncharged and can approach other nuclei without electrostatic repulsion.
D)are produced by nuclear reactions in the shock wave regions surrounding supernova explosions.

Question 60

Very heavy elements beyond iron in the periodic table are formed in the interiors of massive stars by what process in general?

A)triple alpha process
B)neutron capture
C)splitting of heavier elements by nuclear fission
D)CNO cycle of nuclear fusion

Question 61

Supernovae are of particular importance in theories of stellar evolution because:

A)they are one of the few known mechanisms for producing the heaviest elements.
B)the Sun will probably go through a supernova phase.
C)there are several supernovae in the immediate vicinity of the Sun.
D)they always result in black holes.

Question 62

The next element in the periodic table after tin (chemical symbol Sn) is antimony (chemical symbol Sb). If ¹²²Sn captures a neutron, the result will be:

A)¹²³Sn.
B)¹²³Sb.
C)¹²¹Sn.
D)¹²²Sb.

Question 63

Supergiants are huge stars of 25 or 30 M ? , which are undergoing core nuclear fusion involving nuclei heavier than carbon. These supergiants:

A)are always red.
B)are always blue.
C)are always white.
D)may be any of a variety of colors.

Question 64

The star that exploded to form the supernova SN 1987A probably had, before it blew up, a mass of:

A)about 20 M ε
B)less than 1 M ε
C)about 1.4 M ε
D)about 40 to 50 M ε

Question 65

A blue (hot) supergiant is 10 times larger in diameter than the Sun, but a red (cool) supergiant of the same luminosity has a diameter 1000 times that of the Sun. How does the surface temperature of the blue supergiant compare to that of the red supergiant?

A)They have the same luminosity, so they must have the same surface temperature.
B)The surface temperature of the blue supergiant is 10 times the surface temperature of the red supergiant.
C)The surface temperature of the blue supergiant is 100 times the surface temperature of the red supergiant.
D)The surface temperature of the blue supergiant is 1000 times the surface temperature of the red supergiant.

Question 66

The progenitor of SN 1987A was a smaller blue supergiant rather than a larger red supergiant when the core erupted as a supernova. What difference did this make?

A)Blue indicates a hotter surface temperature. Thus, this was a brighter supernova than it would have been if the progenitor had been a red supergiant.
B)Blue supergiants are inherently more massive and more compact than red supergiants. Thus, this was a brighter supernova than it would have been if the progenitor had been a red supergiant.
C)The more compact state of the blue supernova meant that more energy was required to lift the outer layers away from the core, and thus, less energy remained to increase the luminosity.
D)Blue supergiants are younger than red supergiants. Thus, fewer metals had been formed in the supergiant, and the resulting supernova remnant contained fewer metals than it would have had it erupted while a red supernova.

Question 67

In 1990, the HST first examined SN 1987A and found a ring of glowing gas surrounding it. In 1994, HST found three rings of glowing gas. What happened during this interval?

A)The core of SN 1987A bounced not once but twice, producing two additional rings.
B)Radioactivity in the original ring heated it and caused it to expand and split.
C)The shock wave from SN 1987A triggered the formation of planetary nebulae in two nearby stars.
D)The optics on HST were repaired, resulting in better resolution.

Question 68

What was the effective "neutrino energy luminosity" (energy carried away by neutrinos) of the SN 1987A supernova for a few seconds following the explosion compared to relatively well-known electromagnetic luminosities?

A)1 million times greater than that of the observable universe
B)10 times greater than the total observable universe
C)10 times greater than the total output of the stars in our galaxy
D)1 million times greater than the luminosity of the Sun

Question 69

Scientists were able to calculate the number of neutrinos released by SN 1987A over a 10-s period from the number ascertained at the detectors. In order to make this calculation, it was necessary to know the distance to the supernova and which one of the following pieces of information?

A)the energy of the neutrinos
B)the speed of the neutrinos
C)the size of SN 1987A
D)the efficiency of the neutrino detectors

Question 70

If SN 1987A produces 10⁵⁷ neutrinos per second (uniformly in all directions), and it is at a distance of 51,500 pc (=1.59 × 10²¹ m), what would be the flux of neutrinos at an Earth-based detector with an area of 1 square meter?

A)3.15 × 10¹³
B)3.96 × 10¹⁴
C)6.29 × 10³⁵
D)3.18 × 10¹⁰⁰

Question 71

Neutrino detectors for astronomical purposes are built and operated deep underground to:

A)shield them against other high-energy radiation from space or from natural radioactivity.
B)reduce the effect of rotational speed produced by Earth's rotation, because the sensitive neutrino detection techniques are adversely affected by Doppler shifts.
C)utilize the gravitational focusing of neutrinos that occurs in Earth's core.
D)ensure that they are absolutely light-tight, because they depend on the detection of very faint flashes of light.

Question 72

What is Cherenkov radiation?

A)x-ray radiation emitted directly from supernova eruptions
B)gamma radiation emitted directly from neutrinos
C)the shock wave emitted when neutrinos move faster than the speed of light in water
D)the shock wave emitted when recoil positrons move faster than the speed of light in water

Question 73

A Type Ib supernova is believed to result from the:

A)collapse of a blue supergiant star to form a black hole.
B)explosion of a massive star that has lost its hydrogen-rich outer layers through a stellar wind or by mass transfer in a binary star system.
C)explosion of a white dwarf in a binary star system after matter transferred to it from its companion has increased its mass above the Chandrasekhar limit.
D)explosion of a massive, hydrogen-rich star after silicon burning has produced a core of iron nuclei.

Question 74

A supernova is observed with no emission or absorption lines of hydrogen, helium, or silicon. What type of supernova is it?

A)Ia
B)Ib
C)Ic
D)II

Question 75

A supernova of Type Ia has a much higher peak luminosity than a supernova of Type II. When we measure luminosity, we are including the energy of all of the following EXCEPT:

A)ultraviolet radiation.
B)gamma rays.
C)neutrinos.
D)x rays.

Question 76

Measurements from distant galaxies indicate that supernovae should occur at a rate of two per century in a spiral galaxy such as the Milky Way, but only three have been recorded in our galaxy in the past 1000 years. Why is this?

A)Most supernovae produce x rays and radio waves but not visible light, and hence were invisible to earlier observers.
B)The majority of supernovae must have occurred in the plane of the Milky Way, and hence were hidden from Earth by the dense gas and dust in the Milky Way plane.
C)Observers were not watching the sky carefully enough, particularly through the Dark Ages and over the past few centuries.
D)The Milky Way Galaxy is somehow different, with much lower numbers of very massive stars in general, so many fewer stars have undergone supernova explosions.

Question 77

More supernovae have been recorded, on average, in distant galaxies than in the Milky Way Galaxy. Why is this?

A)The Milky Way is a relatively small galaxy, so we would expect fewer supernovae.
B)We can only view our own galaxy's central plane from within that plane, where dust obscures our view. We view other galaxies from a variety of orientations, not just along the central plane.
C)The Milky Way is a particularly old galaxy, and the production rate of supernovae has decreased.
D)Much of the radiation emitted from supernovae is not visible. The radiation from other galaxies, however, is often Doppler shifted into the visible, where it is more easily detected.

Question 78

Supernova remnants are LEAST likely to be discovered when observers are attempting to detect them by looking for:

A)radio waves.
B)x rays.
C)visible light.
D)neutrinos.