ASTR 101 Exam II - HW 5
Which of the following statements about thermal radiation is always true?
A hot object emits more radiation per unit surface area than a cool object. (This is part of the first law of thermal radiation (the Stefan-Boltzmann law).)
Type of telescope used to make this statement (INFRARED TELESCOPE, VISIBLE LIGHT TELESCOPE, or X-RAY TELESCOPE): Look for high-energy radiation from a supernova.
X-ray telescope
Type of telescope used to make this statement (INFRARED TELESCOPE, VISIBLE LIGHT TELESCOPE, or X-RAY TELESCOPE): Observe the hot (1-million K) gas in the Sun's corona.
X-ray telescope
The _________ is the layer of the Sun between its core and convection zone.
radiation zone
The world's largest is 1-meter in diameter: REFLECTING or REFRACTING telescopes?
refracting telescopes
Very large telescopes become "top-heavy": REFLECTING or REFRACTING telescopes?
refracting telescopes
Type of observation you would perform to answer this question (IMAGING, SPECTROSCOPY, or TIMING): Is the star Vega moving toward us or away from us?
spectroscopy
Type of observation you would perform to answer this question (IMAGING, SPECTROSCOPY, or TIMING): What is the chemical composition of the Crab Nebula?
spectroscopy
In order for an atom to absorb a photon (a particle of light),
the photon must have energy matching the difference in energy between energy levels in the atom OR the photon must have enough energy to remove an electron from the atom.
Which of the following statements is true of green grass?
It absorbs red light and reflects green light. (We see the reflected green light.)
Which of the following is not an advantage of the Hubble Space Telescope over ground-based telescopes?
It is closer to the stars. (Distance to the stars has absolutely nothing to do with it, as should be apparent if you consider the scale of the solar system and the distances to stars to scale (as discussed in Chapter 1).)
______________ separate the various colors of light, allowing astronomers to determine stellar composition and many other stellar properties.
Spectrographs
A 10-meter telescope has a larger ______________ than a 4-meter telescope.
light-collecting area
How do we know how old the Sun is?
from ages of solar system meteorites, based on radioactive elements
Type of observation you would perform to answer this question (IMAGING, SPECTROSCOPY, or TIMING): Are stars in the Orion Nebula surrounded by dusty disks of gas?
imaging
A gas heated to millions of degrees would emit
mostly X-rays.
The source of energy that keeps the Sun shining today is _________.
nuclear fusion
Most commonly used by professional astronomers today: REFLECTING or REFRACTING telescopes?
reflecting telescopes
The Hubble Space Telescope: REFLECTING or REFRACTING telescopes?
reflecting telescopes
The large research observatories on Mauna Kea use giant ______________.
reflecting telescopes
World's largest telescope: REFLECTING or REFRACTING telescopes?
reflecting telescopes
Without telescopes or other aid, we can look up and see the Moon in the night sky because it
reflects visible light.
Galileo's telescope designs using lenses were examples of ____________.
refracting telescopes
Galileo's telescopes: REFLECTING or REFRACTING telescopes?
refracting telescopes
Incoming light passes through glass: REFLECTING or REFRACTING telescopes?
refracting telescopes
Type of observation you would perform to answer this question (IMAGING, SPECTROSCOPY, or TIMING): What is the temperature of Jupiter's atmosphere?
spectroscopy
Type of observation you would perform to answer this question (IMAGING, SPECTROSCOPY, or TIMING): Does the star Mira vary in brightness?
timing
Type of observation you would perform to answer this question (IMAGING, SPECTROSCOPY, or TIMING): Is the X-ray emission from the galactic center steady or changing?
timing
Which of the following is not a good reason to place observatories on remote mountain tops?
to be able to observe at radio wavelengths
In hydrogen, the transition from level 2 to level 1 has a rest wavelength of 121.6 nm. Find the direction for a star in which this line appears at wavelength 121.1 nm.
toward us (less than wavelength at rest)
In hydrogen, the transition from level 2 to level 1 has a rest wavelength of 121.6 nm. Find the direction for a star in which this line appears at wavelength 120.4 nm.
toward us (less than wavelength at rest)
Suppose you decide to make a graph of intensity against wavelength for the spectrum shown here. Which of the following shows what the graph will look like?
upward curve with spikes dipping down (2) (Notice that the downward spikes on the graph occur at wavelengths corresponding to the dark lines in the spectrum.)
In hydrogen, the transition from level 2 to level 1 has a rest wavelength of 121.6 nm. Find the speed for a star in which this line appears at wavelength 121.1 nm. Express your answer to three significant figures and include the appropriate units.
v = 1230 km/s (find by [ wavelength shifted - wavelength at rest / wavelength at rest ] * [speed of light - 300,000])
In which case will the woman see a just a spectrum that is almost entirely black except for few bright emission lines?
The hot cloud emits light only at specific wavelengths, producing an emission line spectrum. (3)
In which case will the woman see a rainbow of color interrupted by a few dark absorption lines?
The hot light source produces a continuous spectrum, but the cool gas absorbs light at specific wavelengths, so that she sees an absorption line spectrum. (2)
In hydrogen, the transition from level 2 to level 1 has a rest wavelength of 121.6 nm.Find the speed for a star in which this line appears at wavelength 122.1 nm. Express your answer to three significant figures and include the appropriate units.
v = 1230 km/s (find by [ wavelength shifted - wavelength at rest / wavelength at rest ] * [speed of light - 300,000])
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.
According to modern science, approximately how old is the Sun?
4.5 billion years
Assume the woman in the figure uses her prism to look at a spectrum of light coming from the object(s) shown. In which case will she see a continuous rainbow of thermal radiation?
A hot light source produces a continuous spectrum (of thermal radiation). (1)
Which of the following changes would cause the fusion rate in the Sun's core to increase?
An increase in the core temperature & A decrease in the core radius (An increase in the core temperature increases the fusion rate because the fusion rate is very sensitive to temperature. A decrease in the core radius causes the core to heat up and increase in density, which therefore leads to an increased fusion rate.)
Each of the following describes an "Atom 1" and an "Atom 2." In which case are the two atoms different isotopes of the same element?
Atom 1: nucleus with 7 protons and 8 neutrons, surrounded by 7 electrons; Atom 2: nucleus with 7 protons and 7 neutrons, surrounded by 7 electrons (They are isotopes both atoms have the same atomic number but different atomic mass numbers.)
The diagram represents energy levels in a hydrogen atom. The labeled transitions (A through E) represent an electron moving between energy levels. Which labeled transition represents an electron that absorbs a photon with 10.2 eV of energy?
B: Arrow stops at 10.2 level. (You can tell because the electron jumps up from 0 eV to 10.2 eV.)
Which of the following statements about electrons is not true?
Electrons orbit the nucleus rather like planets orbiting the Sun.
When the temperature of the Sun's core goes down, what happens next?
Fusion reactions slow down; the core shrinks and heats.
Which of the following must occur for a star's core to reach equilibrium after an initial change in fusion rate?
If the fusion rate initially increases, then the core expands. & If the fusion rate initially decreases, then the core contracts. (Increasing the fusion rate releases more energy into the core, which raises the temperature and increases the internal pressure, causing the core to expand. Decreasing the fusion rate means less energy is released, so the temperature and internal pressure decreases. As you can see in the interactive figure, the solar thermostat keeps the fusion rate fairly steady in a star like the Sun because a temperature increase causes the core to expand while a temperature decrease causes the core to contract.)
Consider the spectra of the four objects shown beneath the laboratory spectrum. Based on these spectra, what can you conclude about Object 1?
It is moving away from us. (Notice that the lines in Object 1's spectrum are all to the right of those in the laboratory spectrum. You can tell that this indicates a redshift - which means the object is moving away - because red is on the right side of these spectra (and blue is on the left).)
Betelgeuse is the bright red star representing the left shoulder of the constellation Orion. All the following statements about Betelgeuse are true. Which one can you infer from its red color?
Its surface is cooler than the surface of the Sun. (Red light has lower energy than yellow or white light, so the red color of Betelgeuse tells us that its peak thermal radiation comes at lower energy than the peak thermal radiation of the yellow/white Sun. A lower energy of peak radiation means a lower temperature.)
Now consider Object 2. What can you say about Object 2 in comparison to Object 1?
Object 2 is moving away from us faster than Object 1. (Object 2's lines are shifted farther to the right than Object 1's, which means it has a greater redshift and is moving away faster.)
Suppose you see two stars: a blue star and a red star. Which of the following can you conclude about the two stars? Assume that no Doppler shifts are involved. (Hint: Think about the laws of thermal radiation.)
The blue star has a hotter surface temperature than the red star.
Which of the following conditions lead you to see an absorption line spectrum from a cloud of gas in interstellar space?
The cloud is cool and lies between you and a hot star. (Atoms or molecules in the cloud therefore absorb specific wavelengths of light from the hot star.)
This photo shows the visible light spectrum of the Sun. Why does it have all those dark lines on it?
The dark lines represent wavelengths of light at which atoms near the Sun's surface absorb radiation from the hotter solar interior. (In other words, the Sun has an absorption line spectrum because its surface (photosphere) acts like a cooler cloud over the much hotter interior.)
Suppose that Star X and Star Y both have redshifts, but Star X has a larger redshift than Star Y. What can you conclude?
Suppose that Star X and Star Y both have redshifts, but Star X has a larger redshift than Star Y. What can you conclude? (The redshifts mean that both stars are moving away from us, and a larger redshift means a faster speed.)
How does the light-collecting area of an 8-meter telescope compare to that of a 2-meter telescope?
The 8-meter telescope has 16 times the light-collecting area of the 2-meter telescope. (The 8-meter telescope is 4 times larger in diameter, so its light collecting area is 42 = 16 times greater.)
Which of the following statements is an inference from a model (rather than an observation)?
The Sun's core is gradually turning hydrogen into helium.
What would happen if the fusion rate in the core of the Sun were increased but the core could not expand?
The Sun's core would start to heat up and the rate of fusion would increase even more. (As you know from Part B, an increase in the fusion rate will cause the core to expand in a normal (or main-sequence) star like the Sun, and this expansion will restore core equilibrium. But if for some reason the core could not expand, the higher temperature would make the fusion rate increase even more, creating a positive feedback loop in which the fusion rate and temperature would keep getting higher and higher. (That is, there would be no equilibrium.) In fact, as you'll learn when you study stellar life cycles, this is essentially what will occur when the Sun is near the end of its life.)
Consider an atom of gold in which the nucleus contains 79 protons and 118 neutrons. What is its atomic number and atomic mass number?
The atomic number is 79, and the atomic mass number is 197.
The diagram represents energy levels in a hydrogen atom. The labeled transitions (A through E) represent an electron moving between energy levels. Suppose that an electron in a hydrogen atom absorbs 10.2 eV of energy, so that it moves from level 1 to level 2. What typically happens next?
The electron returns to level 1 by emitting an ultraviolet photon with 10.2 eV of energy. (Electrons typically return to the ground state unless something else interferes with this return.)
Which of the following best describes why we say that light is an electromagnetic wave?
The passage of a light wave can cause electrically charged particles to move up and down. (That is, electrically charged particles can respond to (or generate) electromagnetic radiation.)
The angular separation of two stars is 0.1 arcseconds and you photograph them with a telescope that has an angular resolution of 1 arcsecond. What will you see?
The photo will seem to show only one star rather than two. (Because the angular separation of the stars is smaller than the telescope's angular resolution, the light of the two stars will be blurred together to look like a single star.)
If we observe one edge of a planet to be redshifted and the opposite edge to be blueshifted, what can we conclude about the planet?
The planet is rotating.
The first telescopic photo shows what appears to be a single star. The second photo shows the same object, now revealed to be two distinct stars. What is the difference between the two photos?
The second photo has better (smaller) angular resolution than the first photo. (Better angular resolution means we can see (resolve) the two individual sources that are blurred together in the first photo.)
Laboratory measurements show hydrogen produces a spectral line at a wavelength of 486.1 nanometers (nm). A particular star's spectrum shows the same hydrogen line at a wavelength of 486.0 nm. What can we conclude?
The star is moving toward us. (The wavelength is shifted from 486.1 to 486.0 nm, which means a shift to a shorter wavelength. A shorter wavelength means a shift to the blue end of the spectrum (a blueshift) so that the object is moving toward us.)
Studying a spectrum from a star can tell us a lot. All of the following statements are true except one. Which statement is NOT true?
The total amount of light in the spectrum tells us the star's radius. (We cannot measure radius from a spectrum without additional information.)
Which of the following is always true about images captured with X-ray telescopes?
They are always shown with colors that are not the true colors of the objects that were photographed. ("True colors" make sense only for visible light, not X rays.)
Suppose you watch a leaf bobbing up and down as ripples pass it by in a pond. You notice that it does two full up and down bobs each second. Which statement is true of the ripples on the pond?
They have a frequency of 2 hertz. (Remember that hertz are units meaning "cycles per second.")
The Chandra X-ray Observatory must operate in space because:
X rays do not penetrate Earth's atmosphere. (To detect X rays, the observatory must be above Earth's atmosphere.)
Which of the following statements about X rays and radio waves is not true?
X rays travel through space faster than radio waves. (All light travels at the same speed.)
This figure shows idealized thermal radiation spectra from several stars and a human. Based on this graph, at about what wavelength does a 15,000 K star emit its most intense light?
about 100 nanometers (The blue curve represents the 15,000 K star and it peaks close to 102 = 100 nm on the wavelength axis.)
The Hubble Space Telescope obtains higher-resolution images than most ground-based telescopes because it is:
above Earth's atmosphere.
If you heat a gas so that collisions are continually bumping electrons to higher energy levels, when the electrons fall back to lower energy levels the gas produces
an emission line spectrum.
The _____________ of the Hubble Space Telescope is better for shorter (bluer) wavelengths of light than for longer (redder) wavelengths of light.
angular resolution
In hydrogen, the transition from level 2 to level 1 has a rest wavelength of 121.6 nm. Find the direction for a star in which this line appears at wavelength 122.1 nm.
away from us (more than wavelength at rest)
In hydrogen, the transition from level 2 to level 1 has a rest wavelength of 121.6 nm. Find the direction for a star in which this line appears at wavelength 122.9 nm.
away from us (more than wavelength at rest)
Most of the Sun's ultraviolet light is emitted from the narrow layer called the ____________ where temperature increases with altitude.
chromosphere
Energy moves through the Sun's __________ by means of the rising of hot gas and the falling of cooler gas.
convection zone
Nuclear fusion of hydrogen into helium occurs in the ________.
core
We can see the Sun's _________ most easily during total solar eclipses.
corona
Following are the different layers of the Sun's atmosphere. Rank them based on the order in which a probe would encounter them when traveling from Earth to the Sun's surface, from first encountered to last.
corona, chromosphere, photosphere
Rank the layers of the Sun's atmosphere based on their temperature, from highest to lowest.
corona, chromosphere, photosphere (Scientists were quite surprised when they first learned that the temperature increases with altitude in the Sun's atmosphere, and even today the heating mechanism is not fully understood. However, we know that magnetic fields play an important role in transporting heat upward, making the chromosphere hotter than the photosphere and the corona hotter still.)
Rank the layers of the atmosphere based on the energy of the photons that are typically emitted there, from highest to lowest.
corona, chromosphere, photosphere (The energy of the emitted light rises with temperature, so the ranking for this question is the same as that of Part C. In fact, the photosphere emits primarily visible light, the chromosphere primarily ultraviolet light, and the corona primarily X rays. This is why astronomers study the corona with X-ray telescopes and the chromosphere with ultraviolet telescopes, while we observe the photosphere with visible-light telescopes.)
If our eyes were sensitive only to X rays, the world would appear __________.
dark because X-ray light does not reach Earth's surface (Because X rays from the Sun do not reach Earth's surface, eyes that were sensitive only to X rays would have nothing to see.)
What two physical processes balance each other to create the condition known as gravitational equilibrium in stars?
gravitational force and outward pressure
The Chandra X-ray observatory focuses X-rays with ____________ mirrors.
grazing incidence
Type of observation you would perform to answer this question (IMAGING, SPECTROSCOPY, or TIMING): How large is the Andromeda Galaxy?
imaging
Type of observation you would perform to answer this question (IMAGING, SPECTROSCOPY, or TIMING): What are the major surface features of Mars?
imaging
Telescope time is a valuable commodity in astronomy, so astronomers plan their observations carefully. One of the first steps is to choose what type of observation to make. Recall that three basic types of astronomical observation are ________ (taking a photograph of an object), _____________ (spreading an object's light into a spectrum), and ________ (measuring how an object's light varies with time).
imaging, spectroscopy, timing
If you had only one telescope and wanted to take both visible-light and ultraviolet pictures of stars, where should you locate your telescope?
in space (While visible light can be observed from the ground, ultraviolet light can be easily observed only from space. Indeed, the capability of observing ultraviolet light is a major advantage of the Hubble Space Telescope over larger ground-based telescopes.)
Type of telescope used to make this statement (INFRARED TELESCOPE, VISIBLE LIGHT TELESCOPE, or X-RAY TELESCOPE): Determine the surface temperature of Venus.
infrared telescope
Type of telescope used to make this statement (INFRARED TELESCOPE, VISIBLE LIGHT TELESCOPE, or X-RAY TELESCOPE): Study a dense cloud of cold gas in space.
infrared telescoped
The twin 10-m Keck telescopes can work together to obtain better angular resolution through a technique known as ______________.
interferometry
Nearly all the visible light we see from the Sun is emitted from the _________.
photosphere
Which of these layers of the Sun is coolest?
photosphere
Rank the layers of the Sun's atmosphere based on their density, from highest to lowest.
photosphere, chromosphere, corona (As your answer correctly indicates, the density of the Sun's atmosphere decreases with altitude above the photosphere.)
What kinds of light are these telescopes designed to detect?
radio waves (This is the Very Large Array in New Mexico; the many radio telescopes are used together for interferometry)
In hydrogen, the transition from level 2 to level 1 has a rest wavelength of 121.6 nm. Find the speed for a star in which this line appears at wavelength 120.4 nm. Express your answer to three significant figures and include the appropriate units.
v = 2960 km/s (find by [ wavelength shifted - wavelength at rest / wavelength at rest ] * [speed of light - 300,000])
In hydrogen, the transition from level 2 to level 1 has a rest wavelength of 121.6 nm. Find the speed for a star in which this line appears at wavelength 122.9 nm. Express your answer to three significant figures and include the appropriate units.
v = 3210 km/s (find by [ wavelength shifted - wavelength at rest / wavelength at rest ] * [speed of light - 300,000])
Which of the following forms of light can be observed with telescopes at sea level?
visible light and radio waves (Both visible light and radio waves pass almost freely through Earth's atmosphere, and therefore are easily observed with ground-based telescopes. The only other light that can be observed with ground-based telescopes is infrared, but it can be detected only at high altitudes (such as mountaintops) and even then only in selected portions of the infrared spectrum.)
Type of telescope used to make this statement (INFRARED TELESCOPE, VISIBLE LIGHT TELESCOPE, or X-RAY TELESCOPE): Measure the brightness of a star that is similar to our Sun.
visible light telescope
Type of telescope used to make this statement (INFRARED TELESCOPE, VISIBLE LIGHT TELESCOPE, or X-RAY TELESCOPE): Obtain a spectrum of the sunlight reflected by Mars.
visible light telescope