astronomy test 2
Which statement about Jupiter's moon Io is true?
It is the most volcanically active body in our solar system.- At least one eruption seems to be occurring at almost all times.
Why does the Moon have a layer of "powdery soil" on its surface?
It is the result of countless tiny impacts by small particles striking the Moon.- On Earth, these particles burn up in the atmosphere (creating what we see as meteors).
In the late 1800s, Kelvin and Helmholtz suggested that the Sun stayed hot because of 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.- This was indeed a major drawback to the idea. Clearly, Earth could not have had life before the Sun was shining.
According to modern science, approximately how old is the Sun?
4 1/2 billion years.- The Sun is the same age as the rest of our solar system.
The overall result of the proton-proton chain is:
4 H becomes 1 He + energy.- The helium nucleus has a slightly lower mass than the four hydrogen nuclei combined, so the difference is converted to energy in accord with E = mc2.
The Sun will exhaust its nuclear fuel in about ______.
5 billion years.- The Sun's total life time is about 10 billion years, and the Sun is only about halfway through this lifetime at present.
The Sun's average surface (photosphere) temperature is about ______.
5,800 K
What is the Sun's approximate composition (by mass)?
70% hydrogen, 28% helium, 2% other elements 90% dark matter, 10% ordinary matter.- This is also the approximate overall chemical composition of the Milky Way Galaxy.
Which of the following best distinguishes the Kuiper belt from the Oort cloud?
Both are made up of many comets, but Kuiper belt comets orbit within a donut-shaped region just beyond the orbit of Neptune, while Oort cloud comets orbit much farther from the Sun and can have orbits of all inclinations.- Kuiper belt comets orbit much closer to the Sun than Oort cloud comets, except when a comet from the Oort cloud has had its orbit disturbed so that it travels inward toward the Sun.
All the following statements about Mars are true. Which one might have led to a significant loss of atmospheric gas to space?
Mars lost any global magnetic field that it may once have had.- This allowed the solar wind to strip atmospheric gas into space.
Which of the following general statements about Earth's atmosphere is not true?
The nitrogen and oxygen in Earth's atmosphere keep the surface pleasantly warm.- Only greenhouse gases influence surface temperature, and neither oxygen nor nitrogen is a greenhouse gas.
Which jovian planet has the most extreme seasonal changes, and why?
Uranus, because of its extreme axis tilt.- this gives it seasons that each last about 1/4 of its 84-year orbit.
What is Eris?
an icy object that orbits in the Kuiper belt and is more massive than Pluto, it is classified as a dwarf planet.
Overall, current data suggest that __________ of stars are orbited by one or more planets.
at least about 70%
What major feature of Saturn's moon Enceladus led scientists to suspect it might have a subsurface ocean?
fountains of ice spraying out from the surface into space.- Subsequent study of these fountains and other evidence has strongly suggested that the subsurface ocean is global on Enceladus.
You observe a star and you want to plot it on an H-R diagram. You will need to measure all of the following, except the star's _________.
mass
Which statement about planetary rings is not true?
Saturn's rings formed along with its moons 4.6 billion years ago.- The rings must be more recent than that, because ring particles cannot survive for billions of years.
Every second, the Sun converts about 600 million tons of hydrogen into 596 million tons of helium. The remaining 4 million tons of mass is _________.
converted to an amount of energy equal to 4 million tons times the speed of light squared.- Einstein's famous equation, E = mc2, explains what happens to the "missing" four million tons of mass.
From center to surface, which of the following correctly lists the interior layers (defined by density) of a terrestrial world?
core, mantle, crust -These are the layers defined by density; by rock strength, we speak instead of the lithosphere, the stiff layer of rigid rock that encompasses the crust and the top of the mantle.
Suppose we have determined the orbital period of a planet around its star. If we also know the mass of the star, then we can use the planet's orbital period and the star's mass to calculate __________.
the planet's average orbital distance.-We do this using Newton's version of Kepler's third law. Note that we can generally calculate orbital distance for any planet detected by either the Doppler, astrometric, and transit methods, because all three methods tell us a planet's orbital period and we usually have good estimate of stellar masses (from the spectral types of the stars).
A star with a parallax angle of 1/20 arcsecond is _________.
20 parsecs (≈ 65.2 light-years) away.- distance in parsecs = 1/(parallax angle in arcsecond).
The approximate main-sequence lifetime of a star of spectral type O is _________.
3 million years.- O stars are the shortest-lived of all stars, because they are the most massive and therefore the most luminous.
What is the approximate chemical composition (by mass) with which all stars are born?
3/4 hydrogen, 1/4 helium, no more than about 2% heavier elements.- Our Sun has this same chemical composition.
Which of the following best describes the primary difference between asteroids and comets?
Asteroids are rocky and comets are ice-rich.- This difference in composition is the primary way that we classify asteroids and comets
Which of the following is not a piece of evidence supporting the idea that Europa may have a subsurface ocean?
Astronomers have detected small lakes of liquid water on Europa's surface.- There is no evidence of liquid water on Europa's surface, and even if liquid water emerged the cold temperatures would ensure that it would quickly freeze.
Why is the sky blue (on Earth)?
Because molecules scatter blue light more effectively than red light.- Therefore blue light from the Sun is scattered all over the sky. At sunset or sunrise, when we see the Sun through more atmosphere (because it is on the horizon), so much blue light is scattered away that we are left with a red sunset (or sunrise).
How can we measure the strength of magnetic fields on the Sun?
By looking for the splitting of spectral lines in the Sun's spectrum.- Magnetic fields can cause some spectral lines to split, so we can measure the Sun's magnetic field strength by measuring the splitting of spectral lines. (This splitting of spectral lines is often called the Zeeman effect.)
How did astronomers discover the relationship between spectral type and mass for main-sequence stars?
By measuring the masses and spectral types of main-sequence stars in binary systems.- We can only measure mass directly for stars in binary systems, so this is how the relationship was first discovered.
Assuming that we can measure the apparent brightness of a star, what does the inverse square law for light allow us to do?
Calculate the star's luminosity if we know its distance, or calculate its distance if we know its luminosity.-
Which statement is not thought to be true of all comets in our solar system?
Comets always have tails.- Only the rare comets that enter the inner solar system have tails, and only when they are close to the Sun.
From center outward, which of the following lists the "layers" of the Sun in the correct order?
Core, radiation zone, convection zone, photosphere, chromosphere, corona
Why didn't a planet form where the asteroid belt is now located?
Early in the solar system's history there should have been plenty of material to form a planet here, but Jupiter's gravity prevented it. Over time, most of this material has left this region (crashing to other planets or ejected from solar system), leaving the asteroid belt of today with much less mass than would be needed to form a planet.
The Huygens probe took numerous pictures as it descended to the surface of Saturn's moon Titan in 2005. What did the pictures show?
Features or erosion, including what appeared to be dry river valleys and lakebeds.- Titan is far too cold for liquid water, so these features probably were carved by liquid methane or ethane.
Which statement about asteroids is not true?
If we could put all the asteroids together, they would make an object about the size of Earth.- The total mass of the asteroids is much less than the mass of a terrestrial planet.
Which of the following might explain the lack of "water worlds" (planets made mostly of water or other hydrogen compounds) in our own solar system?
In our solar system, the solar wind did not clear out gas until relatively late.- As an example, consider Uranus and Neptune. They are thought to have formed from ice-rich planetesimals that then collected hydrogen and helium gas from the solar nebula. If the solar wind had cleared the nebula earlier, these planetesimals would not have collected that gas, and the resulting planets would have been made mostly of hydrogen compounds.
Scientists suspect that Mars once had a global magnetic field. Assuming this is true, which of the following could explain why Mars today lacks a global magnetic field like that of Earth?
Mars's interior has cooled so much its molten core layer no longer undergoes convection.- Convection of the molten core is required for a global magnetic field.
Which of the following best describes the lunar maria?
Relatively smooth, flat plains on the Moon. -These plains are the dark, circular regions visible on the face of the Moon.
What do we mean when we say that the Sun is in energy balance?
The amount of energy released by fusion in the Sun's core equals the amount of energy radiated from the Sun's surface into space.- Together, energy balance and gravitational equilibrium keep the Sun stable.
What makes us think that Mars must once have had an atmosphere that was warmer and had higher surface pressure?
The atmosphere is too cold and thin for liquid water today, yet we see evidence that water flowed on the surface in the past.- If water really did flow, the atmospheric conditions must have been different.
Which of the following is not an example of tectonics?
The gradual disappearance of a crater rim as a result of wind and rain. This is an example of erosion, not tectonics.
What do we mean when we say that the Sun is in gravitational equilibrium?
There is a balance within the Sun between the outward push of pressure and the inward pull of gravity.- This balance is important to understanding the Sun and all other stars.
Which moon has a thick atmosphere made mostly of nitrogen?
Titan.- It's nitrogen content is similar to that of Earth, but it has no oxygen and is extremely cold.
What key evidence suggests that Triton is a captured moon?
Triton orbits Neptune in a direction opposite that of Neptune's rotation.- This "backward" orbit is a tell-tale sign of a captured moon.
Which one gives the primary reason why the surface of Venus today is some 450°C hotter than the surface of Earth?
Venus has a much stronger greenhouse effect than Earth.- Without this greenhouse effect, Venus's surface temperature would be below the freezing point
All the following statements about Venus are true. Which one offers evidence of a global repaving about a billion years ago?
Venus has relatively few impact craters and these craters are distributed fairly evenly over the entire planet.- This suggests that older impact craters were covered over everywhere on the planet.
Which of the following correctly describes how the process of gravitational contraction can make a star hot?
When a star contracts in size, gravitational potential energy is converted to thermal energy.- This process is called gravitational contraction> because gravity is what makes the star contract.
Suppose you find a meteorite made almost entirely of metal. According to current science, which of the following statements must be true?
Your meteorite is a fragment from the core of a large asteroid that shattered in a collision.- Only a large asteroid would have undergone differentiation to make a metal core, so this must be the source of the meteorite.
On average, how fast do the plates move on the Earth?
a few centimeters per year.- This sounds slow, but over tens of millions of years it completely rearranges the map of Earth's continents.
Which of the following is the most basic definition of a greenhouse gas?
a gas that absorbs infrared light.- Greenhouse gases can therefore slow the escape of heat from a planet, because planets radiate infrared light.
In general, which type of planet would you expect to cause the largest Doppler shift in the spectrum of its star?
a massive planet that is close to its star.- Doppler shifts measure velocity, and the star will move faster if it experiences a greater gravitational tug from its planet. A massive, close-in planet will cause the strongest gravitational tug.
What do we mean by a primitive meteorite?
a meteorite that is essentially unchanged since it first condensed and accreted in the solar nebula almost 4.6 billion years ago.- It's called primitive because the word means "first."
Scientists suspect that Mars once had abundant liquid water on its surface. For this to have been true, early Mars must have had __________ than Mars today.
a much higher atmospheric pressure and much stronger greenhouse effect.- Higher atmospheric pressure was necessary for liquid water to remain stable, while a stronger greenhouse effect was necessary to make it warm enough for liquid water.
Jupiter's magnetic field is __________ Earth's magnetic field.
about 20,000 times as strong as.- It is the strongest magnetic field of any planet in our solar system.
Suppose we represent Earth with a basketball. On this scale, most of the air in Earth's atmosphere would fit in a layer that is _________.
about the thickness of a sheet of paper.- The atmosphere is remarkably thin compared to our entire planet.
Among discovered meteorites, we have found some with all the following origins except _________.
being a fragment from Comet Halley.- No meteorites have characteristics suggesting that they came from Comet Halley. Moreover, any pieces of that comet that have reached Earth have likely been small enough and icy enough to have burned up completely (leaving no meteorite) as they passed through our atmosphere.
On an H-R diagram, stellar masses _________.
can be determined for main sequence stars but not for other types of stars.- Along the main sequence, mass decreases from upper left to lower right; but there is no clear pattern to masses for stars that are not on the main sequence.
The intricate patterns visible in an X-ray image of the Sun generally show _________.
extremely hot plasma flowing along magnetic field lines
Earth has been gradually warming in recent decades, and very strong evidence indicates that the cause of this warming is _____.
human activities that are increasing the concentration of greenhouse gases in Earth's atmosphere.- The increase in greenhouse gas concentration is well established as a result of human activity, and the basic physics of the greenhouse effect and detailed climate models leave little room for doubt that this is the cause of the recent and ongoing warming.
The processes responsible for virtually all surface geology are _________.
impact cratering, volcanisms, tectonics, and erosion.- Virtually all surface features trace to one or more of these four processes.
Which of the following is most unlikely to be found on Titan?
lakes of liquid water in the warmer equatorial regions.- Titan's surface temperature is never anywhere close to being warm enough for liquid water, even at its equator.
If star A is closer to us than star B, then Star A's parallax angle is _________.
larger than that of Star B.- A larger parallax angle means the star is closer.
The axes on a Hertzsprung-Russell (H-R) diagram represent _________.
luminosity and surface temperature.- We plot luminosity on the vertical axis and surface temperature (or spectral type) on the horizontal axis.
The transit method allows us in principle to find planets around __________.
only a small fraction of stars that have planets.- We can see transits only if the planetary orbits are nearly precisely edge-on as viewed from Earth, which means that most planetary systems cannot be detected through transits. TheKepler mission overcame this limitation by studying a large number of stars (about 150,000).
To calculate the masses of stars in a binary system, we must measure their _________.
orbital period and average orbital distance.- then apply Newton's version of Kepler's third law to find the sum of their masses.
On an H-R diagram, a red supergiant would be located in the __________.
upper right.- It is up high on the diagram because of its high luminosity, and to the right because of its low temperature.
Which of the following correctly lists two key pieces of evidence that, together, indicate that we should expect human activity to cause global warming?
(1) Carbon dioxide tends to make planets warmer than they would be otherwise.(2) Measurements demonstrate that human activity is raising the carbon dioxide concentration in the atmosphere.- Given these two indisputable facts, we should expect that Earth will warm up as we continue to add carbon dioxide to the atmosphere.
The two most important processes in heating the interiors of the terrestrial worlds are:
(1) heat deposited by the process of formation(2) heat released by radioactive decay.- The first was important only in Earth's early history; radioactive decay continues to be important today, though at a lower level than in the past.
In the context of plate tectonics, what is a subduction zone?
A place where a seafloor plate is sliding under a continental plate.- This process returns seafloor crust to the mantle.
What do we mean by a "super Earth"?
A planet made of metal and rock that is larger in mass than Earth.
What is an extrasolar planet?
A planet that orbits a star that is not our own Sun
Suppose we use a baseball to represent Earth. On this scale, the other terrestrial worlds (Mercury, Venus, the Moon, and Mars) would range in size approximately from that of _________.
A golf ball is about right for the Moon (1/4 Earth's diameter) and the baseball would work for Venus, since it is nearly the same size as Earth.
Which of the following best describes how the greenhouse effect works?
A planet's surface absorbs visible sunlight and returns this absorbed energy to space as infrared light. Greenhouse gases slow the escape of this infrared radiation, which thereby heats the lower atmosphere.- This is the basic mechanism of the greenhouse effect, and it explains why more greenhouse gases mean more warming.
Which of the following is the best answer to the question, "Why does the Sun shine?"
As the Sun was forming, gravitational contraction increased the Sun's temperature until the core become hot enough for nuclear fusion, which ever since has generated the heat that makes the Sun shine.
Suppose you are using the Doppler method to look for planets around another star. What must you do?
Compare many spectra of the star taken over a period of many months or years.- We look for Doppler shifts in the star's spectrum that, over time, shift back and forth to indicate the influence of an orbiting planet.
Which of the following will allow you to learn something about a transiting planet's atmospheric composition?
Compare spectra obtained before and during an eclipse.- The spectrum before the eclipse is that of the star and planet combined. The spectrum during eclipse, when the planet is behind its star, is that of the star alone. The difference between the two spectra therefore represents the planet's spectrum, from which we learn about atmospheric composition.
Suppose Earth were to cool down a little. How would the carbon dioxide cycle tend to restore temperatures to normal?
Cooler temperatures lead to slower formation of carbonate minerals in the ocean, so carbon dioxide released by volcanism builds up in the atmosphere and strengthens the greenhouse effect.- This feedback mechanism would indeed warm the planet back up.
How is the sunspot cycle directly relevant to us here on Earth?
Coronal mass ejections and other activity associated with the sunspot cycle can disrupt radio communications and knock out sensitive electronic equipment.- This is the only direct effect of the sunspot cycle that has been recognized, though it may also have subtle influences in other ways, such as in some climate cycles.
What is Pluto's moon Charon thought to have in common with our own Moon?
It probably formed as a result of a giant impact.- That seems to be the most likely explanation for Charon's relatively large size in comparison to Pluto, and the giant impact may also explain the two smaller moons of Pluto.
Which of the following best describes the geological histories of the Moon and Mercury?
Early in their histories, they suffered many impacts and experienced some volcanism and tectonics, but they now have little geological activity at all.- For most practical purposes, the Moon and Mercury are now geologically dead.
Why does Earth have so little carbon dioxide in its atmosphere compared to Venus?
Earth has just as much carbon dioxide as Venus, but most of it is locked up in carbonate rocks rather than being free in the atmosphere.- The carbon dioxide became locked up like this on Earth after it dissolved in water and combined with minerals to make carbonate rock. This does not occur on Venus because there is no liquid water.
When we see a meteor shower, it means that _________.
Earth is crossing the orbit of a comet.- The meteor shower is caused by particles that have been shed by the comet along its orbit.
Suppose our Sun were suddenly replaced by a supergiant star. Which of the following would be true?
Earth would be inside the supergiant.- This is true because a supergiant's radius is typically much more than 1 AU.
Satellites in low-Earth orbits are more likely to crash to Earth when the sunspot cycle is near solar maximum because _________.
Earth's upper atmosphere tends to expand during solar maximum, exerting drag on satellites in low orbits.- The increase in solar X rays and high energy particles from the Sun that occur during solar maximum causes Earth's upper atmosphere to expand.
What happens to energy in the Sun's convection zone?
Energy is transported outward by the rising of hot plasma and sinking of cooler plasma.- The rising of hot gas and falling of cool gas is an example of convection, which is why this region is called the convection zone.
Which of the following is not a piece of evidence supporting the idea that an impact caused the mass extinction that occurred 65 million years ago?
Fossilized dinosaur bones contain fragments of rock from the impact.- We do not see any such thing.
Why does the Sun emit neutrinos?
Fusion in the Sun's core creates neutrinos.- Neutrinos are a byproduct of a fusion reaction.
If the Sun suddenly stopped emitting neutrinos, what might we infer (after checking that our neutrino detectors were still operational)?
Fusion reactions in the Sun have ceased within the past few minutes. Neutrinos are produced by fusion reactions and, because they interact so rarely with matter, they travel directly to Earth at nearly the speed of light, reaching Earth about 8 minutes after they are produced. So if we stopped receiving neutrinos, we would know that fusion reactions had stopped.
Our Sun is a star of spectral type _________.
G.- Because the spectral sequence is OBAFGKM, the Sun's spectral type of G means that the Sun's surface is hotter than that of K and M stars, but cooler than that of O, B, A, and F stars.
All the following statements are true. Which one is most important in explaining the tremendous tidal heating that occurs on Io?
Io orbits Jupiter on an elliptical orbit as a result of orbital resonances with other moons.- On an elliptical orbit, tides vary and flex Io's shape.
From the viewpoint of an alien astronomer, how does Jupiter affect observations of our Sun?
It causes the Sun to move in a small ellipse with an orbital period of about 12 years.- Jupiter exerts a gravitational tug on the Sun, causing the Sun to move around their mutual center of mass with the same orbital period as Jupiter.
Suppose we discover a new comet on an orbit that brings it closer to the Sun than Mercury every 125 years. What can we conclude?
It has been on its current orbit for only a very short time compared to the age of our solar system.- Comets cannot survive more than a couple hundred passages through the inner solar system because they lose mass with each pass. So this comet cannot have been on its current orbit for more than a few tens of thousands of years.
What is Charon?
It is much larger than the other moons of Pluto, and is thought to have formed in a giant impact with Pluto (much as our own Moon is thought to have formed in a giant impact with Earth).
What do we mean by the main-sequence turnoff point of a star cluster, and what does it tell us?
It is the spectral type of the hottest main sequence star in a star cluster, and it tells us the cluster's age.
What is the importance of the carbon dioxide (CO2) cycle?
It regulates the carbon dioxide concentration of our atmosphere, keeping temperatures moderate.- But remember that this natural cycle operates over time scales of hundreds of thousands of years, and hence cannot help us with the problem of human-induced global warming.
When is/was gravitational contraction an important energy generation mechanism for the Sun?
It was important when the Sun was forming from a shrinking interstellar cloud of gas.- The shrinkage of the cloud meant that particles were losing gravitational potential energy, which could be converted into thermal energy.
Suppose there were no solar wind. How would the appearance of a comet in our inner solar system be different?
It would have only one tail instead of two.- There would be no plasma tail without the solar wind. The dust tail would still be present, since it responds to radiation pressure rather than the solar wind.
According to the inverse square law of light, how will the apparent brightness of an object change if its distance to us triples?
Its apparent brightness will decrease by a factor of 9.- The inverse square law for light tells us that the light gets dimmer with increasing distance by the square of the distance, so tripling the distance decreases the brightness by a factor of 32 = 9.
What would happen to Jupiter if we could somehow double its mass?
Its density would increase, but its diameter would barely change.- You can understand why by thinking about the pillow stack analogy described in Section 8.1 of The Essential Cosmic Perspective.
Which of the following is a key factor in explaining why many jovian moons have been more geologically active than the Moon or Mercury?
Jovian moons contain much more ice that can melt or deform at lower temperatures than can the rock and metal that make up the Moon and Mercury.- This "ice geology" means that icy worlds need far less internal heat for geological activity than rocky worlds.
Jupiter's colors come in part from its three layers of clouds. Which of the following is not the primary constituent of one of Jupiter's cloud layers?
Jupiter does not have sulfuric acid clouds. (Sulfuric acid clouds are found on Venus.)
Which of the following best describes why the Sun emits most of its energy in the form of visible light?
Like all objects, the Sun emits thermal radiation with a spectrum that depends on its temperature, and the Sun's surface temperature is just right for emitting mostly visible light.- The Sun's 5,800 K surface temperature means it emits most of its radiation as visible light.
Which of the following statements about the moons of the jovian planets is true?
Most of the moons have the potato-like shapes of asteroids, but a few are large enough to be spherical in shape.- Only the relatively large and medium size moons are spherical. These are far outnumbered by small moons, and small moons generally are not spherical.
Suppose that large jovian planets had never formed in our solar system. Which of the following would most likely be true?
Neither the asteroid belt nor Oort cloud would exist.- Jupiter's gravity is responsible for the existence of the asteroid belt, and the Oort cloud is thought to consist of comets ejected from the region of the jovian planets by their gravity. So without the jovian planets, neither the asteroid belt nor the Oort cloud would be likely to exist.
A salesman attempts to convince you to purchase a "solar neutrino" shield to protect you and your family. (It's even "on sale" !) Why do you turn down this excellent offer?
Neutrinos rarely, if ever, interact with other matter.- About a thousand trillion neutrinos pass through your body every second, but neutrinos interact so weakly with other matter that they will do no damage at all. It would therefore be pointless to have a neutrino shield, even if such a shield were possible (which it's not, since the neutrinos won't interact with the shield either).
Suppose we had a device that allowed us to see Earth's interior. If we looked at a typical region of the mantle, what would we see happening?
Not much: on human time scales, the mantle looks like solid rock. Most of the mantle is solid. Mantle convection is the slow flow of this solid rock, but it occurs too slowly to notice on human time scales. (The cycling time for rock from the bottom to the top of the mantle is about 100 million years.)
Which of the following best explains why nuclear fusion requires bringing nuclei extremely close together?
Nuclei normally repel because they are all positively charged and can be made to stick only when brought close enough for the strong force to take hold.- The strong force acts only across extremely short distances - such as the distance across a nucleus - and so the particles must be brought very close together before they can fuse.
From hottest to coolest, the order of the spectral types of stars is _________.
OBAFGKM
Based on everything you have learned about the formation of our solar system, which of the following statements is probably not true?
Only a tiny percentage of stars are surrounded by spinning disks of gas during their formation.- Spinning disks are a natural consequence of how we think stars form, which is one reason we expect planets to be quite common; the fact that we have now discovered thousands of extrasolar planets lends further support to the idea that disks and planets should both be common.
Which of the following objects are probably not located in the same general region of the solar system in which they originally formed?
Oort cloud comets probably formed between the jovian planets are were kicked out to their current large distances from the Sun.
Assuming that our ideas about how "hot Jupiters" ended up on their current orbits are correct, why didn't our own solar system end up with any hot Jupiters?
Our solar nebula must have been blown into space shortly after the formation of the jovian planets.- This would prevent much inward migration from occurring.
According to current models of solar system formation, why did Uranus and Neptune end up to be much less massive than Jupiter and Saturn?
Particles in the solar nebula were more spread out at greater distances, so that accretion took longer and there was less time to pull in gas before the solar wind cleared the nebula.- Based on this idea, larger planets might have formed if the solar nebula had taken longer to clear.
Based on discoveries to date, which of the following conclusions is justified?
Planetary systems are common and planets similar in size to Earth are also common.- The statistics of extrasolar planets known to date strongly support these conclusions.
Which new process was added to the nebular theory of solar system formation to account for the existence of Jupiter-mass planets that orbit close to their stars (the "hot Jupiters")?
Planets can migrate from the orbits in which they are born.- Computer models confirm that, at least in some cases, migration should be expected given the complex dynamics of planetary formation.
Overall, what do current data suggest about planetary types in other planetary systems?
Planets come in a wider range of types than the planets in our solar system.- For example, there may be other planets that are Jupiter-like in size but much higher or lower in density, or planets that are Earth-like in size but made mostly of water.
All the following statements are true. Which one makes it possible to learn of the existence of a planets without seeing the planets themselves?
Planets exert gravitational tugs on their stars that cause stars to orbit around the center of mass of their planetary systems.- This fact means that we can learn of the existence of a planet by detecting the motion it causes its star to make around the center of mass, which is the key to both the Doppler and astrometric methods.
What are the two geological features that appear to set Earth apart from other terrestrial worlds in our solar system?
Plate tectonics and a high level of erosion.- Neither of these features appears to exist on any other terrestrial world in our solar system.
Why won't Pluto collide with Neptune?
Pluto orbits the Sun exactly 2 times for every 3 Neptune orbits.- This stable orbital resonance means that Pluto and Neptune never come close together.
Based on all we know about the terrestrial worlds, what single factor appears to play the most important role in a terrestrial planet's geological destiny?
Size determines how long the planet can retain internal heat, which drives geological activity.
The choices describe four hypothetical planets. Which one would you expect to have the most features of erosion? (Assume the planets orbit a star just like the Sun and that they are all the same age as the planets in our solar system.)
Size: same as Venus.Distance from Sun: same as Mars.Rotation rate: once every 25 hours.- This planet is large enough to have had outgassing make an atmosphere, and rotates fast enough to drive winds.
Sirius is a star with spectral type A star and Rigel is a star with spectral type B star. What can we conclude?
Rigel has a higher surface temperature than Sirius. Sirius has a higher core temperature than Rigel.- This is true because the spectral sequence from hot to cool is OBAFGKM, which means spectral type B is hotter than spectral type A.
The choices describe four hypothetical planets. Which one's surface would you expect to be most crowded with impact craters? (Assume the planets orbit a star just like the Sun and that they are all the same age as the planets in our solar system.)
Size: same as the Moon.Distance from Sun: same as Mars.Rotation rate: once every 10 days.- The smallest size means the least geological activity to have erased past impact craters, so lots of craters would still be present.
The choices describe four hypothetical planets. Which one would you expect to have the hottest interior? (Assume the planets orbit a star just like the Sun and that they are all the same age as the planets in our solar system.)
Size: twice as big as Earth.Distance from Sun: same as Mercury.Rotation rate: once every 6 months.- Largest size means the highest interior temperature.
Star A has an apparent magnitude = 3 and star B has an apparent magnitude = 5. Which star is brighter in our sky?
Star A.- The magnitude scale is "backward" so that smaller numbers mean greater apparent brightness.
Which of the following statements comparing open and globular star clusters is true?
Stars in open clusters are relatively young, while stars in globular clusters are very old.- Open clusters are found in the disk of the galaxy, where star formation is ongoing; globular clusters are found in the halo, where star formation ceased long ago, so that all remaining stars are very old.
Which of the following is a real piece of evidence (from the New Horizons mission) pointing to recent or ongoing geological activity on Pluto?
Surface regions that suggest glacial movement of frozen ices of nitrogen and methane.- Apparently, Pluto has enough internal heat to drive flows of these ices, which exist in solid form only at very low temperatures.
Uranus and Neptune have methane clouds but Jupiter and Saturn do not. Which factor explains why?
Temperatures on Jupiter and Saturn are too high for methane to condense.- But methane can condense on Uranus and Neptune because they are farther from the Sun and hence colder.
How can we best observe the Sun's chromosphere and corona?
The chromosphere is best observed with ultraviolet telescopes and the corona is best observed with X-ray telescopes.- The chromosphere is hot enough to emit mostly ultraviolet light and the corona to emit mostly X rays.
When a comet passes near the Sun, part of it takes on the appearance of a large, bright ball from which the tail extends. This part is called the _____.
The coma is essentially a large atmosphere of gas and dust released by the comet.
If the Sun's core suddenly shrank a little bit, what would happen in the Sun?
The core would heat up, fusion rates would increase, the core would re-expand.- This is why the "solar thermostat" is so effective at maintaining a steady temperature.
Why are there fewer large impact craters on Earth's seafloor than on the continents?
The crust on seafloors is younger than on continents, so it has had less time in which to suffer impacts.- Seafloor crust is continually recycled, so that the seafloor is nearly everywhere younger than about 200 million years. Therefore, any evidence of earlier impacts has been erased as the seafloor crust was recycled.
The choices below each describe the appearance of an H-R diagram for a different star cluster. Which cluster is the youngest?
The diagram shows main-sequence stars of every spectral type except O, along with a few giants and supergiants.- Only O stars have died, so the cluster is no more than a few million years old.
The choices below each describe the appearance of an H-R diagram for a different star cluster. Which cluster is most likely to be located in the halo of our galaxy?
The diagram shows main-sequence stars of spectral types G, K, and M, along with numerous giants and white dwarfs.- This is an old cluster so it must be a globular cluster, and most globular clusters are in the halo of the galaxy.
Many scientists suspect that Venus has a stronger and thicker lithosphere than Earth. If this is true, which of the following could explain it?
The high surface temperature that has "baked out" all the liquid water from Venus's crust and mantle.- Without water, the rock would be stronger and the lithosphere could become thicker.
Jupiter and the other jovian planets are sometimes called "gas giants." In what sense is this term misleading?
They actually contain relatively little material in a gaseous state.- Much more of their mass is liquid, metallic, or in strange high-pressure states that we don't naturally find on Earth.
Which large jovian moon is thought to have been captured into its present orbit?
The key clue is the fact that Triton orbits in the opposite direction of Neptune's rotation, indicating that it did not form within the "miniature solar nebula" that gave birth to Neptune. That is why it seems likely that Triton once orbited the Sun independently, in which case its size (larger than Pluto) would have qualified it as a dwarf planet.
Consider a planet orbiting a Sun-like star that has been detected by the Doppler and that has an orbital period of one year. The larger the velocity changes measured for the star, the __________.
The larger the mass of the planet.- the one-year orbital period and Sun-like star mean this planet orbits at about 1 AU from its star. Therefore, the more massive it is, the stronger its gravitational tug on the star, which will give the star a higher velocity.
Which of the following is a major reason why it so difficult to obtain direct images of extrasolar planets?
The light of the planets is overwhelmed by the light from their star.- A Sun-like star is about a billion times brighter than the light from a Jupiter-size planet orbiting it.
Which of the following best explains why we see horizontal "stripes" in images of Jupiter and Saturn?
The light stripes are regions of high clouds, and the dark stripes are regions where we can see down to deeper, darker clouds.- And the reason these stripes stretch all the way around these planets is the strong Coriolis force that comes from the planets' rapid rotation.
What atmospheric constituent is responsible for the blue color of Uranus and Neptune?
The low temperatures of these distant planets allow for the formation of methane clouds that contribute to the blue color.
Most of the Moon's surface is densely covered with craters, but we find relatively few craters within the lunar maria. What can we conclude?
The maria formed after the heavy bombardment ended.- They contain few craters because they formedaftermost impacts had occurred.
Why were scientists surprised when they first discovered the existence of planets with Jupiter-like masses orbiting close to their stars (the "hot Jupiters")?
The nebular theory predicts that jovian planets should only form at much larger distances from their stars.- This posed a challenge to the nebular theory, demanding an explanation of how it might be possible to find jovian planets close to their stars.
If we use the Doppler method to measure the period with which a star alternately moves toward and away from us due to an orbiting planet, then we also know the __________?
The orbital period of the planet (around the star) is the same as the orbital period of the star around the center of mass.
You observe a star similar to our own Sun in size and mass. This star moves slightly back and forth in the sky once every 4 months, and you attribute this motion to the effect of an orbiting planet. What can you conclude about the orbiting planet?
The planet must be closer to the star than Earth is to the Sun.- According to Kepler's third law, a planet with a shorter orbital period must be closer to the star, assuming the star has the same mass as the Sun.
According to current understanding, which of the following is required in order for a planet to have rings?
The planet must have many small moons that orbit relatively close to the planet in its equatorial plane.- Ring particles come from small and large impacts on these small moons.
Which of the following statements comparing the jovian interiors is not thought to be true?
They all have the same exact set of internal layers, though these layers differ in size.- The layers on Uranus and Neptune are significantly different from those on Jupiter and Saturn.
Why is the radiation so intense in the region that traces Io's orbit around Jupiter (the Io torus)?
The region is full of gases that become ionized after they are released from volcanoes on Io, because the low gravity allows these gases to escape
Which statement about Saturn's rings is not true?
The rings must look much the same today as they did shortly after Saturn formed.- Ring particles cannot last long compared to the age of the solar system, so the rings much change their appearance with time.
Which of the following statements about spectral types of stars is not generally true?
The spectral type of a star can be used to determine its distance.- Spectral type tells us surface temperature, but to calculate the star's distance we need to know its luminosity and apparent brightness (and then use the inverse square law for light).
Astronomers can measure a star's mass in only certain cases. Which one of the following cases might allow astronomers to measure a star's mass?
The star is a member of a binary star system.- If we can measure the orbital properties of the star around its companion, then we can measure the mass with Newton's version of Kepler's third law.
Which of the following is not a characteristic of the 11-year sunspot cycle?
The sunspot cycle is very steady, so that each 11-year cycle is nearly identical to every other 11-year cycle.- The sunspot cycle is not steady at all. Even the "11 years" is only an average, as cycle length varies between about 7 and 15 years. The number of sunspots at solar maximum also varies significantly, and it appears there have been times (such as the Maunder minimum) when the sunspot cycle virtually ceased to operate.
Why does Jupiter have three distinct layers of clouds?
The three cloud layers are ammonia, ammonium hydrosulfide, and water. Each of these gases condenses at a different temperature and hence at a different altitude in Jupiter's atmosphere.
Which of the following moons is considered likely to have a deep, subsurface ocean of liquid water?
There is strong evidence for a subsurface ocean on Europa. Note, however, that it is not the only moon with evidence for a subsurface ocean. There is also strong evidence for a subsurface ocean on Saturn's moon Enceladus, and at least some evidence for subsurface oceans on several other moons including Ganymede, Callisto, Titan, and Triton.
Current evidence suggests that some massive jovian planets orbit at close orbital distances to their stars. How do we think these planets ended up on these close orbits?
These planets migrated inward after being born on orbits much farther from their stars.- We still think that jovian planets must form in cold, outer regions of their star systems, which is why the idea of migration was first proposed. Although work is still preliminary, models suggest that this type of migration is indeed likely.
Why do sunspots appear dark in pictures of the Sun?
They actually are fairly bright, but appear dark against the even brighter background of the surrounding photosphere.- In other words, sunspots are hot enough to emit visible light; they are just cooler (and hence less bright) than surrounding plasma.
What do sunspots, solar prominences, and solar flares all have in common?
They are all strongly influenced by magnetic fields on the Sun.- These magnetic fields can bind up or release vast amounts of energy, producing the phenomena of solar weather.
How do typical wind speeds in Jupiter's atmosphere compare to typical wind speeds on Earth?
They are much faster than hurricane winds on Earth.- Typical wind speeds on Jupiter are nearly double those found in hurricanes.
When a comet is within the inner solar system, its visible tails point __________.
They are pushed away from the Sun by radiation and the solar wind.
Comets with orbits that take them through the inner solar system shed sand- to pebble-size particles that then follow the comet around its orbit. How do these particles affect Earth?
They are the particles that produce meteor showers.- In other words, each annual meteor shower occurs when Earth passes through the orbit of some comet.
What is the common trait of all main-sequence stars?
They generate energy through hydrogen fusion in their core.- A star becomes a main-sequence star when it first starts fusing hydrogen into helium, and it ends its main-sequence life when it exhausts its central core supply of hydrogen for fusion.
Why are neutrinos so difficult to detect?
They have a tendency to pass through just about any material without any interactions.- This makes them difficult to detect because we need to see an interaction to know something passed through.
Which of the following best describes why scientists seek to identify even very small asteroids that could potentially hit Earth?
To try to prevent an impact that might destroy a town or city.- The hope is that if we can first identify potential threats, then we can find a way to divert their paths before they hit Earth.
In which of the following cases can we determine the precise mass and density of an extrasolar planet?
We get a precise mass from the Doppler method only if the planet's orbit is edge-on to us, and if it is edge-on, then we can also detect transits of the planet. The transits then tell us the planet's radius, so that we can combine mass and radius to determine density.
Which of the following best explains what scientists think happened to outgassed water vapor on Venus?
Ultraviolet light split the water molecules, and the hydrogen then escaped to space.- This did not occur on Earth, because the temperatures allowed the water vapor to condense into liquid water and rain down to the surface.
Which of the following is an example of convection?
Warm air expanding and rising while cooler air contracts and fall.- Convection is any type of heat-driven circulation like this.
The Sun's surface seethes and churns with a bubbling pattern. Why?
We are seeing hot gas rising and cool gas falling as a result of the convection that occurs beneath the surface.- You can see this idea by studying the figure of the Sun's structure in your textbook, in which you'll see that the convection zone lies just beneath the photosphere.
Which of the following choices is not a way by which we can study the inside of the Sun?
We can send a space probe into the Sun's photosphere.
Suppose a planet is discovered by the Doppler method and is then discovered to have transits. In that case, we can determine all the following about the planet except ______________.
We do not get any direct information about rotation period. (However, if the planet is very close to its star, we can sometimes conclude that it has synchronous rotation, in which case the rotation period and orbital period are the same.)
To date, we've found very few planets orbiting their stars at distances comparable to the distances of the jovian planets in our solar system. Why do astronomers think this is the case?
We have not yet been searching for planets at such distances for a long enough time.- Remember that we must observe at least something close to one complete orbit, and preferably more, before we can be confident of an extrasolar planet discovery. Jupiter takes 12 years to orbit the Sun, and Neptune takes 165 years, so we would need observations over similarly long periods to detect planets in these types of orbits around other stars.
What do we mean when we say that the terrestrial worlds underwent differentiation?
When their interiors were molten, denser materials sank toward their centers and lighter materials rose toward their surfaces. Differentiation therefore occurred very early in each planet's history, before it cooled enough for much of its interior to solidify.
The light radiated from the Sun's surface reaches Earth in about 8 minutes, but the energy of that light was released by fusion in the solar core about _________.
a few hundred thousand years ago.- The energy takes a long time to make its way from the core to the surface (essentially because it bounces around so much within the solar interior), but once at the surface it travels through space to Earth at the speed of light.
What do we mean by a runaway greenhouse effect?
a greenhouse effect that keeps getting stronger until all of a planet's greenhouse gases are in its atmosphere.- This is presumably what happened on Venus.
The reason that small planets tend to lose interior heat faster than larger planets is essentially the same as the reason that _________.
a large baked potato takes longer to cool than a small baked potato.- A smaller object has proportionally more surface area through which to lose its interior heat.
According to current evidence, Pluto is best described as ______.
a large member of the Kuiper belt.- Pluto is not even the most massive known member, because Eris is larger in mass (but about the same size).
A terrestrial world's lithosphere is ________.
a layer of relatively strong, rigid rock, encompassing the crust and part of the mantle- The depth of the lithosphere varies among the different worlds, with larger worlds (like Venus and Earth) having thinner lithospheres.
Which of the following statements best describes the size of the largest asteroid, Ceres?
a little less than half the diameter of our Moon.- This size is large enough for Ceres to qualify as a dwarf planet.
What is the Great Red Spot?
a long-lived, high-pressure storm on Jupiter.- It has been seen ever since telescopic observations of Jupiter began some 400 years ago.
What do we mean by a "hot Jupiter"?
a planet that is Jupiter-like in size but orbits very close to its star
What is a magnetosphere?
a region of space around a planet in which the planet's magnetic field can trap charged particles.- Therefore, having a magnetosphere requires having a global magnetic field.
What is tidal heating?
a source of internal heat caused by varying tidal forces acting on a moon or other object, it is very important on numerous jovian moons, including Jupiter's moons Io, Europa, and Ganymede.
What is the solar wind?
a stream of charged particles flowing outward from the surface of the Sun.- The solar wind blows outward in all directions and has important effects on the planets, especially through interactions with planetary magnetospheres.
The asteroid belt is located _________.
between the orbits of Mars and Jupiter.- It is a "belt" because asteroids have a variety of orbits in this region.
Star A is identical to Star B, but Star A is twice as far from us as Star B. Therefore:
both stars have the same luminosity, but the apparent brightness of Star B is four times that of Star A.- Luminosity is an intrinsic property, so two identical stars will always have the same luminosity. But apparent brightness follows an inverse square law with distance, so the fact that Star A is twice as far away makes it 22 = 4 times dimmer.
Which of the following best describes the internal layering of Jupiter, from the center outward?
core of rock, metal, and hydrogen compounds thick layer of metallic hydrogen layer of liquid hydrogen layer of gaseous hydrogen cloud layer.- You can see the relative depths of these layers in the figure of Jupiter's interior in Section 8.1 ofThe Essential Cosmic Perspective.
Suppose you could float in space just a few meters above Saturn's rings. What would you see as you looked down on the rings?
countless icy particles, ranging in size from dust grains to large boulders.- And each particle orbits Saturn independently.
Which of the following correctly states the number of comets that scientists estimate to be located in the Oort cloud and the number that have actually been observed in the Oort cloud (with telescopes)?
estimated number in Oort cloud: 1 trillion, observed in Oort cloud: zero.- The estimated number is extrapolated from the number of comets that we see in the inner solar system with orbits indicating they came from the Oort cloud. However, no comets have been directly observed in the Oort cloud, because it is too far way for them to be detected with current technology.
Which of the following was a new discovery about Pluto by the New Horizons mission?
evidence of recent geological activity.- Few people had suspected that Pluto would be so active given its small size.
What two physical processes balance each other to create the condition known as gravitational equilibrium in stars?
gravitational force and outward pressure.- This dynamic is important for understanding stellar structure and stellar evolution.
How does the Sun's mass compare to Earth's mass?
he Sun's mass is about 300,000 times the mass of the Earth.- the Sun's mass is more than 1,000 times that of all the planets combined.
According to models of planetary interiors, the differences in mass among the four jovian planets (Jupiter, Saturn, Uranus, and Neptune) are due primarily to differences __________.
in the amounts of material in the layers extending from the tops of their cores to their surfaces.- Their cores are all thought to be of similar mass (about 10 times the mass of Earth), so the differences must be in layers above their cores.
On an H-R diagram, stellar radii _________.
increase diagonally from the lower left to the upper right.- Small radius white dwarfs are down near the lower left, while large radius supergiants are in the upper right .
What can we infer, at least roughly, from a star's luminosity class?
its size (radius).- the major luminosity classes run from the supergiants (luminosity class I) to the main sequence stars (luminosity class V).
What are the appropriate units for the Sun's luminosity?
luminosity measures energy output per second, which corresponds to units of watts.
The total amount of power (in watts, for example) that a star radiates into space is called its _________.
luminosity- of the Sun is 3.8 ( 1026 watts.
Which two factors are most important to the existence of plate tectonics on Earth?
mantle convection and a thin lithosphere.- Mantle convection helps move the plates, and the plates probably exist because the lithosphere was thin enough to break into these plates.
All stars are born with the same basic composition, yet stars can differ greatly in appearance. Which two factors are most important in determining the current appearance of a star?
mass and stage of life.- Mass determines the star's life progression, so the star's current place along that progression depends on its current stage of life.
The cores of the terrestrial worlds are made mostly of metal because ______.
metals sunk to the centers a long time ago when the interiors were molten throughout.- This sinking was part of the process called differentiation.
A rock found on Earth that crashed down from space is called a(n) ______.
meteorite
The transit method searches for extrasolar planets by ____________.
monitoring stars for slight dimming that might occur as unseen planets pass in front of them
If we could put all the asteroids in the asteroid belt together, their total mass would be _________.
much less than the mass of Mercury.- There may once have been enough material in the asteroid belt to assemble a terrestrial planet, but there is no longer enough today.
High-mass stars have __________ lifetimes than low-mass stars.
much shorter.- This is true because massive stars use their nuclear fuel at a much greater rate than less massive stars.
The fundamental nuclear reaction occurring in the core of the Sun is _________.
nuclear fusion of hydrogen into helium.- Each complete reaction converts four hydrogen nuclei (protons) into 1 helium nucleus, although the reaction proceeds through several smaller steps.
The source of energy that keeps the Sun shining today is _________.
nuclear fusion.- The Sun shines by fusing hydrogen into helium, a process in which a small amount of the mass is converted into energy.
If the hypothesis tracing the extinction of the dinosaurs to an impact is correct, the dinosaurs died off largely because ______.
of global climate effects initiated by dust and smoke that entered the atmosphere after the impact
Objects the size of the one that exploded over Chelyabinsk (Russia), causing significant damage and injuries to more than 1,000 people, probably hit Earth at least __________.
once a century
Gaps in the asteroid belt (often called Kirkwood gaps) are caused by ______.
orbital resonances with Jupiter.- Asteroids with orbital periods that are simple fractions (such as ½, 1/3, ¼) of Jupiter's period receive repeated tugs with each orbit, and these tugs tend to nudge them out of those orbits, clearing a gap.
A typical meteor is created by a particle about the size of a _________.
pea.- We do not see the particle itself, but rather the flash of light it generates as it enters (and burns up in) the atmosphere at high speed.
The Sun's visible surface (that is, the surface we can see with our eyes) is called the _________.
photosphere.- the prefix photo means "light" - usually taken to imply visible light - so the photosphere is the visible surface (sphere) of the Sun.
Which two factors are critical to the existence of the carbon dioxide (CO2) cycle on Earth?
plate tectonics and liquid water oceans.- Carbon dioxide dissolves in the oceans and becomes incorporated into carbonate rock; plate tectonics recycles the carbonate rock into the mantle, where it melts and releases its gas back to the atmosphere.
The astrometric method looks for planets with careful measurements of a star's _________.
position in the sky
Which of the following terms is given to a pair of stars that we can determine are orbiting each other only by measuring their periodic Doppler shifts?
spectroscopic binary.- We measure Doppler shifts by looking for shifts in the wavelengths of spectral lines.
A star's color is related to its surface temperature because _________.
stars emit thermal radiation.- The color of a star depends primarily on where its thermal radiation spectrum peaks. (Recall that thermal radiation peaks at a wavelength that depends on temperature.)
The first confirmed detections of extrasolar planets occurred in ____________.
the 1990s.- Scientists had long suspect that extrasolar planets exist, but technology did not meet the challenge of confirming this fact until the mid-1990s.
When we say that the Sun is a ball of plasma, we mean that _________.
the Sun consists of gas in which many or most of the atoms are ionized (missing electrons)- A plasma is an ionized gas, and the Sun is so hot that most of its gas is ionized.
Which of the following correctly compares the Sun's energy generation process to the energy generation process in human-built nuclear power plants?
the Sun generates energy by fusing small nuclei into larger ones, while our power plants generate energy by the fission (splitting) of large nuclei.- That is, the Sun produces energy through nuclear fusion while our nuclear power plants produce energy through nuclear fission.
Overall, Jupiter's composition is most like that of _________.
the Sun.- Both are made mostly of hydrogen and helium.
We can essentially rule out the Sun as a cause of global warming over the past few decades because __________.
the amount of incoming sunlight has decreased while Earth's average temperature has increased.- This is the opposite of what we would expect if the Sun were the cause of the temperature increase.
Which of the following is not a key line of evidence supporting the hypothesis that Mars once had abundant liquid water on its surface?
the canals of Mars mapped by Percival Lowell.- These don't really exist.
What do we mean by a mass extinction?
the extinction of a large fraction of the world's plant and animal species in a relatively short period of time
When you see the bright flash of a meteor, what are you actually seeing?
the glow of heated air surrounding a small particle as it burns up in our atmosphere, you do not see the particle itself, but only the effects it has on the surrounding air as it burns up.
Complete this statement: The larger the decrease in the star's brightness as a planet transits in front of its star, __________ of the planet.
the larger the size (radius)
What is Ceres?
the largest known asteroid.- It is large enough to be round in shape and is therefore classified as a dwarf planet.
What observational evidence supports the idea that Mercury once shrank by some 20 kilometers in radius?
the presence of many long, tall cliffs. -These cliffs formed as the crust shrank and buckled.
Energy balance in the Sun refers to a balance between _________.
the rate at which fusion generates energy in the Sun's core and the rate at which the Sun's surface radiates energy into space.- Together, energy balance and gravitational equilibrium keep the Sun stable.
In the context of planetary geology, what do we mean by outgassing?
the release by volcanism of gases that had been trapped in a planetary interior.- Outgassing therefore released all the gases from Earth's interior that ultimately became our atmosphere or condensed to form the oceans.
What is a white dwarf?
the remains of a star that ran out of fuel for nuclear fusion.- Once the nuclear fuel runs out, the star can no longer shine; for relatively low-mass stars, gravity then compresses its remaining core into a white dwarf.
In science fiction movies, spaceships are often shown dodging through large numbers of closely spaced, boulder-size objects. Which of the following real things in our solar system would look most like such science fiction dangers?
the rings of Saturn.- The rings consist of countless small particles ranging in size from dust grains to city blocks, so it would be quite a challenge to dodge them.
The proton-proton chain is _________.
the specific set of nuclear reactions through which the Sun fuses hydrogen into helium.- That is, the proton-proton chain is just one pathway by which hydrogen can be fused into helium. Note: the "energy" is carried away in forms that include gamma rays and neutrinos, and the neutrinos have proven very important to confirming that nuclear fusion really does occur in the Sun.
Careful measurements reveal that a star maintains a steady apparent brightness at most times, except that at precise intervals of 73 hours the star becomes significantly dimmer for about 2 hours. The most likely explanation is that _________.
the star is a member of an eclipsing binary star system.- The dimming comes during periodic eclipses.
To estimate the central temperature of the Sun, scientists _________.
use computer models to predict interior conditions.- Computer models are used to model internal core temperatures in the Sun and to make testable predictions about the Sun's observable properties.
Which units are appropriate for measurement of apparent brightness?
watts per square meter.- Apparent brightness is defined as the power of starlight reaching us per unit area. Power has units of watts and area has units of square meters.
Suppose that Earth's atmosphere had no greenhouse gases. Then Earth's average surface temperature would be ________.
well below the freezing point of water.- Without the greenhouse effect, Earth's average temperature would be about -16°C, which is well below freezing (0°C). Thanks to the greenhouse effect, the actual global average temperature is about 15°C.
