Physics 7 Midterm #2
Suppose a photon has a frequency of 300 million hertz (300 megahertz). What is its wavelength?
1 meter
A star of spectral type O lives approximately how long on the main sequence?
1 million years
A star of spectral type G lives approximately how long on the main sequence?
10 billion years Submit
At approximately what temperature can helium fusion occur?
100 million K
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.
What types of stars end their lives with supernovae?
stars that are at least several times the mass of the Sun
Five stars are shown on the following H-R diagrams. Rank the stars based on their surface temperature from highest to lowest. If two (or more) stars have the same surface temperature, drag one star on top of the other(s).
Spectral type is related to surface temperature, with stars of spectral type O having the highest surface temperature and stars of spectral type M having the lowest surface temperature. In other words, spectral type increases to the left on the H-R diagram.
Blue light has higher frequency than red light. Thus, blue light has:
higher energy and shorter wavelength than red light.
If you wanted a radio telescope to achieve the same angular resolution as a visible-light telescope, it would need to be:
much larger.
Click "show" for the emission line spectrum, then click "choose gases" and study the emission line spectrum for neon. The neon "OPEN" sign appears reddish-orange because __________.
neon atoms emit many more yellow and red photons than blue and violet photons
Listed following is a set of statements describing individual stars or characteristics of stars. Match these to the appropriate object category. WHITE DWARFS
not much large in radius than Earth very hot but very dim
Which two energy sources can help a star maintain its internal thermal pressure?
nuclear fusion and gravitational contraction Submit
What two forces are balanced in what we call gravitational equilibrium?
outward pressure and gravity
Solar energy leaves the core of the Sun in the form of
photons.
A spectral line that appears at a wavelength of 321 nmnm in the laboratory appears at a wavelength of 328 nmnm in the spectrum of a distant object. We say that the object's spectrum is:
redshifted
What do astronomers mean when they say that we are all "star stuff"?
that the carbon, oxygen, and many elements essential to life were created by nucleosynthesis in stellar cores
Notice that the Sun's spectrum appears brightest (or most intense) in the yellow-green region. This fact tells us __________.
the approximate temperature of the Sun's surface Submit
The interstellar clouds called molecular clouds are _______.
the cool clouds in which stars form
What is a planetary nebula?
the expanding shell of gas that is no longer gravitationally held to the remnant of a low-mass star
Listed following is a set of statements describing individual stars or characteristics of stars. Match these to the appropriate object category. MAIN SEQUENCE STARS
the majority of stars in our galaxy the sun for example the hottest and most luminous stars
Most interstellar clouds remain stable in size because the force of gravity is opposed by _______ within the cloud.
thermal pressure Submit
Use the labels to identify what kinds of stars inhabit each region of the the H-R diagram.
top right: supergiants, red giants Middle main sequence Bottom white dwarfs
The diagonal lines on the H-R diagram represent lines along which all stars would have the same radius. Label the three white lines with the correct values for the radii of stars that fall on them.
top right: 1000 solar radii middle: 1 solar radius bottom: 1 earth radius
The figure shows a standard Hertzsprung-Russell (H-R) diagram. Label the horizontal and vertical axes using the two blanks nearest the center of each axis, and label the extremes on the two axes using the blanks on the ends of the axes.
top to bottom: brighter, luminosity, fainter, hotter right to left: surface temp, colder
The position of a star along the main sequence tells you both its mass and its hydrogen-burning lifetime. Label the indicated blanks on the main sequence with the approximate lifetimes of stars at those positions.
top to bottom: 10 million year lifetime 10 billion year lifetime 100 billion year lifetime
Is it moving toward or away from us? (PART B)
toward us
Is it moving toward or away from us? (PART D)
toward us
Which of the following wavelength regions cannot be studied with telescopes on the ground?
ultraviolet and X rays
On a Hertzsprung-Russell diagram, where would we find red giant stars?
upper right
On a Hertzsprung-Russell diagram, where would we find stars that are cool and luminous?
upper right
Listed following is a set of statements describing individual stars or characteristics of stars. Match these to the appropriate object category. RED SUPER GIANTS STARS
very cool but very luminous found in the upper right of the H-R diagram
Gamma rays have a very small ______.
wavelength
The diffraction limit is a limit on:
A telescope's angular resolution.
If you heat a rock until it glows, its spectrum will be:
a thermal radiation spectrum.
Scientists estimate the central temperature of the Sun using
mathematical models of the Sun.
The twinkling of stars is caused by:
motion of air in our atmosphere. Submit
Suppose you have a 100-watt light bulb that you leave turned on for one minute. How much energy does it use?
6,000 joules.
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.
Five stars are shown on the following H-R diagrams. Rank the stars based on their surface temperature from highest to lowest. If two (or more) stars have the same surface temperature, drag one star on top of the other(s).
All five stars appear at the same place along the horizontal axis showing spectral type. Because spectral type is related to surface temperature, all five stars must have the same surface temperature. Now proceed to Part C to determine how these stars vary in luminosity.
Five stars are shown on the following H-R diagrams. Rank the stars based on their luminosity from highest to lowest; notice that these are the same five stars shown in Part D. If two (or more) stars have the same luminosity, drag one star on top of the other(s).
All five stars have the same luminosity because they are all at the same height along the vertical (luminosity) axis. Continue to Parts F and G for more practice in reading surface temperature and luminosity on the HR diagram.
Five stars are shown on the following H-R diagrams; notice that these are the same five stars shown in Part F. Rank the stars based on their luminosity from highest to lowest. If two (or more) stars have the same luminosity, drag one star on top of the other(s).
As always, the H-R diagram shows surface temperature along the horizontal axis and luminosity along the vertical axis.
Shown following are the primary mirror arrangements and total light-collecting area of five different telescopes. Notice that although the arrangements look similar to those in Part B, the light-collecting areas are not the same. Also listed is an amount of time (exposure time) that each telescope will be pointed at the same distant galaxy. Again assume that the quality of these mirrors, the detectors, and the observing conditions are identical. Rank the telescopes from left to right based on the brightness of the image each telescope will take of the galaxy in the time indicated, from brightest to dimmest. To rank two (or more) telescopes as equal, drag one on top of the other(s).
As your answer correctly indicates, it is the product of the light-collecting area and the exposure time that determines the total amount of light collected from a distant object.
Each diagram below shows a pair of spectra with a set of spectral lines. The top spectrum always shows the lines as they appear in a spectrum created in a laboratory on Earth ("Lab") and the bottom spectrum shows the same set of lines from a distant star. The left (blue/violet) end of each spectrum corresponds to shorter wavelengths and the right (red) end to longer wavelengths. Rank the five stars based on the Doppler shifts of their spectra, from largest blueshift, through no shift, to largest redshift.
As your answer correctly indicates, lines that are shifted to the left (toward the blue/violet) compared to the laboratory spectrum represent blue shifts, and lines shifted to the right (toward the red) represent redshifts.
The diagrams below show the same set of energy levels as in Parts A and B, but with a different set of electron transitions (notice that the arrows are now different). Assuming that these electron transitions were caused by the absorption of a photon, rank the atoms based on the energy of the absorbed photon, from highest to lowest.
As your answer correctly indicates, the atom in which the electron leaves (the atom is ionized) corresponds with the highest-energy photon, and the atom with the shortest arrow indicates the case where the absorbed photon had the lowest energy.
The circles in the diagrams below represent energy levels in an atom, and the arrows show electron (blue dot) transitions from one energy level to another. (The spacing between circles represents differences in energy: A larger spacing means a greater difference in energy.) Assuming that the transitions occur as photons are emitted, rank the atoms based on the photon energy, from highest to lowest.
As your answer correctly shows, the emitted photon must have exactly the same amount of energy that the electron loses in moving from the higher to the lower energy level. Therefore the ranking of the photon energies must be in the same order as the amounts of energy lost by the electrons, and longer arrows mean greater changes in energy.
The diagrams below each show the motion of a distant star relative to Earth (not to scale). The red arrows indicate the speed and direction of the star's motion: Longer arrows mean faster speed. Rank the stars based on the Doppler shift that we would detect on Earth, from largest blueshift, through no shift, to largest redshift.
As your correct answer indicates, the star moving fastest toward Earth will have the greatest blueshift, the star moving across our line of sight will have no shift at all, and the star moving fastest away from us will have the greatest redshift.
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
Suppose the star Betelgeuse (the upper left shoulder of Orion) were to become a supernova tomorrow (as seen here on Earth). What would it look like to the naked eye?
Betelgeuse would remain a dot of light but would suddenly become so bright that, for a few weeks, we'd be able to see this dot in the daytime. Submit
Consider an atom of oxygen in which the nucleus contains 8 protons and 8 neutrons. If it is doubly ionized, what is the charge of the oxygen ion and how many electrons remain in the ion?
Charge = +2; number of remaining electrons = 6.
Suppose we want to know what the Sun is made of. What should we do?
Compare the wavelengths of lines in the Sun's spectrum to the wavelengths of lines produced by chemical elements in the laboratory.
Which of the following statements about electrons is not true?
Electrons orbit the nucleus rather like planets orbiting the Sun.
A radio telescope and an optical telescope of the same size have the same angular resolution.
False
The Hubble Space Telescope is famous because, at least at the time of its launch, it was the largest visible light telescope ever built.
False
The diagrams below are the same as those from Part A. This time, rank the atoms based on the wavelength of the photon emitted as the electrons change energy levels, from longest to shortest.
From Part A, you already know the ranking of the photons by energy. Because higher energy means shorter wavelength, you have correctly found that the ranking for Part B is the reverse of that from Part A.
In this diagram, red balls represent protons and gray balls represent neutrons. What reaction is being shown?
Fusion of helium into carbon.
The most abundant molecule in molecular clouds is
H2.
Shown following are the primary mirror arrangements and total light-collecting area of five different telescopes. Each mirror uses a different segmented arrangement, but assume that they are all equivalent in quality and in their ability to focus light. Also assume that the telescopes use identical detectors and have the same observing conditions. Rank the telescopes from left to right based on their ability to detect very dim objects, from greatest to least. To rank two (or more) telescopes as equal, drag one on top of the other(s).
If all else is equal (such as mirror quality, detector, and observing conditions), the ability to detect dim objects depends only on light-collecting area. Because all the telescopes shown have the same light-collecting area, they all can detect dim objects equally well. The arrangement of the mirrors does not matter, as long as they are arranged and shaped so they bring light to a perfect focus.
Which of the following statements is true of green grass?
It absorbs red light and reflects green light.
Suppose you had molecular oxygen (O2) chilled enough so that it was in liquid form. Which of the following best describes the phase changes that would occur as you heated the liquid oxygen to high temperature?
It would evaporate into a gas, then the molecules would dissociate into individual oxygen atoms, then the atoms would become increasingly ionized as you continued to raise the temperature.
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.
Listed following are the names and mirror diameters for six of the world's greatest reflecting telescopes used to gather visible light. Rank the telescopes from left to right based on their light-collecting area from largest to smallest. For telescopes with more than one mirror, rank based on the combined light-collecting area of the mirrors.
Larger mirrors have a larger light-collecting area. Remember that the light-collecting area is proportional to the square of the mirror diameter. For example, if Mirror A has twice the diameter of Mirror B, then Mirror A has 2x2 = 4 times the light-collecting area of Mirror B.
Five stars are shown on the following H-R diagrams; notice that these are the same five stars shown in Part B. Rank the stars based on their luminosity from highest to lowest. If two (or more) stars have the same luminosity, drag one star on top of the other(s).
Luminosity is shown along the vertical axis, with stars higher up more luminous than those lower down. Note that each tickmark along the luminosity axis represents a change by a factor of 10 from the prior tickmark, so the range of luminosities is quite large. Continue to Parts D and E to investigate surface temperature and luminosity for a different set of five stars.
Consider the four stars shown following. Rank the stars based on their surface temperature from highest to lowest.
Notice that temperature is related to color, and follows the order of the colors in the rainbow: Blue (or violet) stars are the hottest, while red stars are the coolest. In the parts that follow, the H-R diagrams show the correlation between color and temperature on the horizontal axis.
An important line of hydrogen occurs at a rest wavelength (as measured in a laboratory) of 656 nmnm (a nanometer (nmnm) is a billionth of a meter). Each diagram below has this line labeled with its wavelength in the spectrum of a distant star. Rank the motion of the stars along our line of sight (radial motion) based on their speed and direction, from moving fastest toward Earth, through zero (not moving toward or away from Earth), to moving fastest away from Earth.
Notice that your correct answer ranks the stars in wavelength order. The first two stars are moving toward us, because their lines have wavelength shorter than the rest wavelength of 656 nm. The last two stars are moving away from us, because their lines have wavelength longer than the rest wavelength of 656 nm.
Suppose that two stars are separated in the sky by 0.1 arcsecond. If you look at them with a telescope that has an angular resolution of 0.5 arcsecond, what will you see?
One point of light that is the blurred image of both stars.
A typical adult uses about 2,500 CaloriesCalories of energy each day. (Hint: 1 CalorieCalorie = 4184 JJ) Use this fact to calculate the typical adult's average power requirement, in watts.
P =120W
Which of the following procedures would allow you to make a spectrum of the Sun similar to the one shown, though with less detail?
Pass a narrow beam of sunlight through a prism. Submit
The following figures show various stages during the life of a star with the same mass as the Sun. Rank the stages based on when they occur, from first to last.
Remember that these stages take very different amounts of time. A one-solar-mass star spends about ten billion years as a hydrogen-burning main-sequence star, making this by far the longest stage of its life.
Five stars are shown on the following H-R diagrams. Rank the stars based on their surface temperature from highest to lowest. If two (or more) stars have the same surface temperature, drag one star on top of the other(s).
Spectral type is related to surface temperature, with stars of spectral type O having the highest surface temperature and stars of spectral type M having the lowest surface temperature. In other words, spectral type increases to the left on the H-R diagram. Now proceed to Part E to determine how these stars compare in luminosity.
Suppose that Star X and Star Y both have redshifts, but Star X has a larger redshift than Star Y. What can you conclude?
Star X is moving away from us faster than Star Y.
Which of the following statements best describes the two principal advantages of telescopes over eyes?
Telescopes can collect far more light with far better angular resolution.
Which of the following best describes why the Sun's spectrum contains black lines over an underlying rainbow?
The Sun's hot interior produces a continuous rainbow of color, but cooler gas at the surface absorbs light at particular wavelengths. Submit
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.
Assume that all four H-R diagrams below represent a star in different stages of its life, after it starts to fuse hydrogen in its core. Rank the HR diagrams based on when each stage occurs, from first to last.
The diagram at the left represents the Sun (or any other one-solar-mass star) as a hydrogen-burning main-sequence star, with spectral type G and one solar luminosity. The next diagram shows the Sun after it has exhausted its core hydrogen and left the main sequence, making it a subgiant with energy generated by hydrogen burning in a shell around an inert helium core. The third diagram shows the Sun a little later; its energy source is still hydrogen shell burning, but at this point it has expanded in size so much that it is a red giant. The final diagram (far right) shows the white dwarf corpse of a one-solar-mass star; it is hot because it is the exposed core of the dead star, but dim because it is small in size.
Suppose that two stars are identical in every way - for example, same distance, same mass, same temperature, same chemical composition, and same speed relative to Earth - except that one star rotates faster than the other. Spectroscopically, how could you tell the stars apart?
The faster rotating star has wider spectral lines than the slower rotating star. Submit
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.
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.
Suppose you are listening to a radio station that broadcasts at a frequency of 97 Mhz (megahertz). Which of the following statements is true?
The radio waves from the radio station are causing electrons in your radio's antenna to move up and down 97 million times each second. Submit
All of the following statements about the Sun's corona are true. Which one explains why it is a source of X rays?
The temperature of the corona's gas is some 1 to 2 million Kelvin.
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.
The planet Neptune is blue in color. How would you expect the spectrum of visible light from Neptune to be different from the visible-light spectrum of the Sun?
The two spectra would have similar shapes, except Neptune's spectrum would be missing a big chunk of the red light that is present in the Sun's spectrum.
What prevents the pressure from increasing as a cloud contracts due to its gravity?
Thermal energy is converted to radiative energy via molecular collisions and released as photons.
Which of the following statements is probably true about the very first stars in the universe?
They were made only from hydrogen and helium.
Listed following are various physical situations that describe how light interacts with matter. Match these to the appropriate category.
Transmission: - cell phone signals pass through walls - visible light meets clear glass Absorption: -blue light hits a red sweatshirt -visible does not pass through a black wall Reflection or scattering: -White light hits a white piece of paper -red light hits a red sweatshirt Emission: -light comes from a light bulb -light comes from your computer screen
Clouds that appear dark in visible light often glow when observed at long infrared wavelengths
True
Calculate the speed of Star B. (PART C)
Va = -740 km/s
In hydrogen, the transition from level 2 to level 1 has a rest wavelength of 121.6 nmnm. Suppose you see this line at a wavelength of 120.7 nmnm in Star A and at 121.3 nmnm in Star B. (PART A)
Va= -2200 km/s
Which of the following statements about X rays and radio waves is not true?
X rays travel through space faster than radio waves.
Which of these stars has the largest radius?
a supergiant M star Submit
What kind of gas cloud is most likely to give birth to stars?
a cold, dense gas cloud
Radio waves are:
a form of light.
No object produces a perfect thermal radiation spectrum, but many objects produce close approximations. Which of the following would not produce a close approximation to a thermal radiation spectrum?
a hot, thin (low-density, nearly transparent) gas Submit
Study the graph of the intensity of light versus wavelength for continuous spectra, observing how it changes with the temperature of the light bulb. Recall that one of the laws of thermal radiation states that a higher-temperature object emits photons with higher average energy (Wien's law). This law is illustrated by the fact that for a higher temperature object, the graph peaks at __________.
a shorter wavelength Submit
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
The Hubble Space Telescope obtains higher-resolution images than most ground-based telescopes because it is:
above Earth's atmosphere. Submit
What type of visible light spectrum does the Sun produce?
an absorption line spectrum
The absorption line spectrum shows what we see when we look at a hot light source (such as a star or light bulb) directly behind a cooler cloud of gas. Suppose instead that we are looking at the gas cloud but the light source is off to the side instead of directly behind it. In that case, the spectrum would __________.
be an emission line spectrum
On the main sequence, stars obtain their energy
by converting hydrogen to helium. Submit
The set of spectral lines that we see in a star's spectrum depends on the star's:
chemical composition.
Compared to the Sun, a star whose spectrum peaks in the infrared is:
cooler
Compared to the star it evolved from, a red giant is
cooler and brighter.
The thermal pressure of a gas depends on
density and temperature.
The gravitational force in a molecular cloud depends on
density only.
Which of these groups of particles has the greatest mass?
four individual protons Submit
How much greater is the light-collecting area of a 6-meter telescope than a 3-meter telescope?
four times
Which part of the electromagnetic spectrum generally gives us our best views of stars forming in dusty clouds?
infrared
Which element has the lowest mass per nuclear particle and therefore cannot release energy by either fusion or fission?
iron
Compared to an atom as a whole, an atomic nucleus:
is very tiny but has most of the mass.
What type of star is our Sun?
low-mass star
On a Hertzsprung-Russell diagram, where would we find white dwarfs?
lower left
On a Hertzsprung-Russell diagram, where would we find stars that are cool and dim?
lower right
In the illustration of the solar spectrum, the upper left portion of the spectrum shows the __________ visible light.
lowest frequency
