ASTR 101 - HW 6
Approximately what is the parallax angle of a star that is 20 light-years away?
0.16 arcsecond (You can use the parallax formula d (in light-years) = 3.26 × (1/p[in arcseconds]) or p (in arcseconds) = 3.26 × (1/d[in light-years]) Using a distance of d = 20 light-years, this formula gives the parallax angle to be p=3.26/20=0.163 arcsecond. )
Suppose that a star had a parallax angle of exactly 1 arcsecond. Approximately how far away would it be, in light-years?
3.3 light-years (A more precise animation would show that the distance of an object with a parallax angle of 1 arcsecond is 3.26 light-years, which astronomers call a parsec (short for "parallax second"). One parsec is defined as the distance of an object with a parallax angle of exactly 1 arcsecond, and its value in light-years is 1 parsec = 3.26 light-years. This is the reason why there is the 3.26 in the parallax formula given at the end of Part E.)
The overall result of the proton-proton chain is ________.
4 H becomes 1 He + energy
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 = mc 2.)
How much mass does the Sun lose through nuclear fusion per second?
4 million tons
According to modern science, approximately how old is the Sun?
4.5 billion years (The Sun is the same age as the rest of our solar system, which from earlier chapters is 4 ½ billion years.)
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 (Note that this is a temperature at which most thermal radiation is in the visible part of the spectrum - which is why the Sun radiates most of its energy as visible light.)
What is the Sun made of (by mass)?
70% hydrogen, 28% helium, 2% other elements
What is the Sun made of (by mass)?
70% hydrogen, 28% helium, 2% other elements (This is also the approximate overall chemical composition of the Milky Way Galaxy and the rest of the universe today.)
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.
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.))
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
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.
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 the cause of stellar parallax?
Earth's orbit around the Sun. (Stellar parallax occurs because we see stars from different vantage points as Earth orbits the Sun. This causes the positions of nearby stars to appear to shift relative to the positions of more distant objects.)
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.)
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.)
Red arrows point outward, green arrows point inward. Long arrows are near the center, short arrows near the outer surface. The arrows in this diagram are meant to show how gravitational equilibrium works in the Sun. What do the different colors and different arrow lengths represent?
Green arrows represent gravity; red arrows represent pressure; longer arrows represent a stronger push or pull. (That is, gravity pulls inward while pressure pushes outward. The overlying weight becomes greater with depth, which is why the opposing forces become stronger.)
Which of the following is a valid way of demonstrating parallax for yourself?
Hold up your hand in front of your face, and alternately close your left and right eyes. (Notice that parallax is an apparent shift caused by a change in the way you are looking at your hand, not an actual motion. Parallax is an apparent motion, not an actual motion.)
This photograph shows a small portion of the Sun's photosphere. What is going on in the bright regions (such as the bright region indicated by the arrow)?
Hot gas is rising up from the solar interior. (The bubbling pattern is caused by the underlying convection, and the bright spots are where hot gas is bubbling upward.)
In the late 1800s, Kelvin and Helmholtz suggested that the Sun stayed hot due to gravitational contraction. What was the major drawback to this idea?
It predicted that the Sun could shine for about 25 million years, but geologists had already found that Earth is much older than this.
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.)
If the sun's surface cooled, how would its appearance change?
It would appear more red.
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.
A solar model is used to calculate the expected temperature and density at all depths within the Sun. These results are then used to calculate the expected fusion rate within the Sun. We have confidence that the model is correct because it agrees with the observed characteristics of the Sun. Which of the following observations can be used to check that we really do know the Sun's internal fusion rate?
Measurements of the Sun's total energy output into space & Observations of neutrinos coming from the Sun (The Sun shines with energy generated by fusion, so the total rate at which the Sun emits energy into space must be equal to the rate at which it generates energy by fusion in the core. Neutrinos are a product of fusion reactions, so they provide direct evidence concerning fusion in the core.)
A computer accessory salesman attempts to convince you to purchase a "solar neutrino" shield for your new computer. (It's even "on sale"!) Why do you turn down this excellent offer?
Neutrinos rarely, if ever, interact with your computer. (In fact, neutrinos rarely interact with matter of any kind, so it would 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).)
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.)
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.)
How does the Sun's mass compare to Earth's mass?
The Sun's mass is about 300,000 times the mass of the Earth.
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.)
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.)
One statement about the Sun from Part A is "The corona is hotter than the photosphere." Which of the following statements provides observational evidence for this claim?
The corona primarily emits X rays while the photosphere primarily emits visible light. (In general, higher temperature gas emits higher energy light. The fact that the corona emits primarily in X rays therefore indicates that it consists of higher temperature gas than visible-light-emitting photosphere.)
What is the solution to the solar neutrino problem?
The electron neutrinos created in the Sun's core change into another type of neutrino that we did not detect.
Look again at the loop of hot gas in this X-ray image. Suppose we took another photo looking at the same place one hour later. What would we see?
The loop would look about the same as it does in this photo.
Study this figure and its axis labels. What is this graph showing us?
The number of sunspots on the Sun tends to increase and decrease with an approximately 11-year cycle.
Which photo shows Earth correctly scaled in comparison to the Sun?
The one where Earth appears the smallest (The Sun's diameter is more than 100 times Earth's diameter.)
What would happen to the core of the sun if its temperature rose slightly?
The rate at which fusion occurs would increase, leading to an expansion of the core, which would in turn cause the temperature to drop back down.
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.)
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.
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.)
Why do sunspots appear dark?
They are regions that are significantly cooler than the rest of the photosphere.
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.)
The Sun's surface seethes and churns with a bubbling pattern. Why?
We are seeing hot gas rising and cool gas falling due to the convection that occurs beneath the surface.
Now consider the statements in Part A that are inferred from models. A solar model is used to calculate interior conditions based on certain "known" characteristics of the Sun, such the Sun's total mass. How do we know the Sun's mass?
We can calculate it by applying Newton's version of Kepler's third law with Earth's orbital period (1 year) and Earth's average distance from the Sun (1 AU). (Recall that Newton's version of Kepler's third law allows us to calculate the mass of any object if we know the orbital period and distance of a much smaller object that orbits it. This means we can use not only Earth's period and distance to calculate the Sun's mass but the period and distance of any planet or even of a spaceship that we put into solar orbit.)
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.
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.
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.)
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 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.)
Approximately how many neutrinos pass through your body each second?
about a thousand trillion
Imagine that you are trying to stop neutrinos with a lead shield. How thick would you need to make this shield to ensure that it can stop a neutrino?
about one light year
How does the number of neutrinos passing through your body at night compare with the number passing through your body during the day?
about the same
What do we need to measure in order to determine a star's luminosity?
apparent brightness and distance
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 = mc 2, explains what happens to the "missing" four million tons of mass.)
What layer of the Sun are we seeing in this photograph, and in what wavelength band was it photographed?
corona, photographed in X rays
The intricate patterns visible in an X-ray image of the Sun generally show __________.
extremely hot plasma flowing along magnetic field lines (This very hot gas produces X rays and hence is what we see in X-ray images.)
If the star Alpha Centauri were moved to a distance 10 times farther than it is now, its parallax angle would ________.
get smaller
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.)
At the center of the Sun, nuclear fusion converts hydrogen into ___________,
helium, gamma rays, and neutrinos
The Sun generates energy by fusing hydrogen into helium in its core: observation or inferences from a model?
inference from a model
The composition of the photosphere is the same as that of the gas cloud that gave birth to our solar system: observation or inferences from a model?
inference from a model
The convection zone is cooler than the radiation zone: observation or inferences from a model?
inference from a model
The core temperature is 10 million K: observation or inferences from a model?
inference from a model
If star A is closer to us than star B, then Star A's parallax angle is _________.
larger than that of Star B
By what process do nuclear power plants on the Earth generate energy?
nuclear fission
By what process does the Sun generate energy?
nuclear fusion
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.)
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 corona is hotter than the photosphere: observation or inferences from a model?
observation
The photosphere emits mostly visible light: observation or inferences from a model?
observation
The photosphere is made mostly of hydrogen and helium: observation or inferences from a model?
observation
The sun emits neutrinos: observation or inferences from a model?
observation
The Sun's surface, as we see it 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.)
What layer of the Sun are we seeing in this photo?
photosphere (The photosphere is the visible surface of the Sun.)
What do the yellow regions represent in this photograph?
regions where very hot gas is emitting a lot of X rays
Suppose you try to bring two protons close together. Because of the electromagnetic force, the two protons will __________.
repel one another
The light radiated from the Sun's surface reaches Earth in about 8 minutes. However, the energy of this light was released by fusion in the Sun's core about _____________.
several hundred thousand years ago
The more distant a star, the __________.
smaller its parallax angle (Just as moving your finger farther from your face makes its parallax smaller as you alternately close each eye, a more distant star has a smaller amount of stellar parallax than a star that is closer to Earth.)
Listed following is an event or phenomena that occurs during either the part of the sunspot cycle known as solar minimum or the part known as solar maximum. Match this item to the correct part of the sunspot cycle: auroras are most likely in Earth's skies.
solar maximum
Listed following is an event or phenomena that occurs during either the part of the sunspot cycle known as solar minimum or the part known as solar maximum. Match this item to the correct part of the sunspot cycle: occurs about 11 years after a solar maximum (on average).
solar maximum
Listed following is an event or phenomena that occurs during either the part of the sunspot cycle known as solar minimum or the part known as solar maximum. Match this item to the correct part of the sunspot cycle: orbiting satellites are most at risk.
solar maximum
Listed following is an event or phenomena that occurs during either the part of the sunspot cycle known as solar minimum or the part known as solar maximum. Match this item to the correct part of the sunspot cycle: solar flares are most common.
solar maximum
Listed following is an event or phenomena that occurs during either the part of the sunspot cycle known as solar minimum or the part known as solar maximum. Match this item to the correct part of the sunspot cycle: sunspots are most numerous on the Sun.
solar maximum
Listed following is an event or phenomena that occurs during either the part of the sunspot cycle known as solar minimum or the part known as solar maximum. Match this item to the correct part of the sunspot cycle: occurs about 5 to 6 years after a solar maximum (on average).
solar minimum
The dark spots in this photo (such as the one indicated by the arrow) represent what we call:
sunspots
This X-ray image shows a loop of hot gas above the surface of the Sun. If we took a visible light photo that looked in the Sun's photosphere just under the two points where the loop of gas comes down (arrows), what would we find?
sunspots (The loop is shaped by magnetic fields, and these loops generally are anchored to the strong magnetic fields of sunspots at each end.)
Before we can use parallax to measure the distance to a nearby star, we first need to know __________.
the Earth-Sun distance (The Earth-Sun distance, or 1 AU, forms the baseline used in parallax measurements. We measure the Earth-Sun distance by using radar within our solar system to measure the distance to a planet, such Venus, from which we can then calculate the Earth-Sun distance.)
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.)
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.)
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.)
What is the only force that can overcome the repulsion between two positively charged nuclei to bind them into an atomic nucleus?
the strong force
What two pieces of information would you need in order to measure the masses of stars in an eclipsing binary system?
the time between eclipses and the average distance between the stars
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.)
What are the appropriate units for the Sun's luminosity?
watts (Luminosity measures energy output per second, which corresponds to units of watts.)
