Physics 7 Midterm #2
Consider the speed of star A
-2700 km/s
Suppose a photon has a frequency of 300 million hertz (300 megahertz). What is its wavelength?
1 meter 1m X 300 million= 300 million m/s
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
At approximately what temperature can helium fusion occur?
100 million K
Use this fact to calculate the typical average adult's power requirement, in watts
120 W
Calculate the speed of star B
1200 km/s
How many helium nuclei fuse together when making carbon?
3
This Hubble Space Telescope photo shows a planetary nebula. What is the white dot in the center (indicated by the arrow)?
a white dwarf
What kind of star is most likely to become a white-dwarf supernova?
a white dwarf star with a red giant binary companion
As the falling rocket plunges toward the event horizon, an observer in the orbiting rocket would see that the falling rocket
slows down as it approaches the event horizon and never actually crosses the event horizon.
What types of stars end their lives with supernovae?
stars that are at least several times the mass of the Sun
What makes us think that the star system Cygnus X-1 contains a black hole?
t emits X rays characteristic of an accretion disk, but the unseen star in the system is too massive to be a neutron star.
If we could see our own galaxy from 2 million light-years away, it would appear _________.
as a flattened disk with a central bulge and spiral arms
If you were inside the rocket that falls toward the event horizon, you would notice your own clock to be running
at a constant, normal rate as you approach the event horizon
The most common form of gas in the disk of the Milky Way galaxy is _________.
atomic hydrogen gas
Where would you be the most likely place to find an ionization nebula?
in a spiral arm
Where are most of the Milky Way's globular clusters found?
in the halo
Where would you least expect to find an ionization nebula?
in the halo
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
Which two energy sources can help a star maintain its internal thermal pressure?
nuclear fusion and gravitational contraction
Red and orange stars are found evenly spread throughout the galactic disk, but blue stars are typically found _________.
only in or near star-forming clouds
The Schwarzschild radius of a black hole depends on ________.
only the mass of the black hole
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
Which of the following wavelength regions cannot be studied with telescopes on the ground?
radio waves and ultraviolet
A spectral line that appears at a wavelength of 321 nm in the laboratory appears at a wavelength of 328 nm in the spectrum of a distant object. We say that the object's spectrum
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
Suppose you want to observe and study the radiation from gas inside an interstellar bubble created by a supernova. Which of the following observatories will be most useful?
the Chandra X-ray observatory
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
Most interstellar dust grains are produced in
the atmospheres of red giant stars
The interstellar clouds called molecular clouds are _______.
the cool clouds in which stars form
Which of these things has the smallest radius?
the event horizon of a 3 M sun black hole
What is.a planetary nebula?
the expanding shell of gas that is no longer gravitationally held to the remnant of a low-mass star
Applying the Newton's version of Kepler's third law (or the orbital velocity law) to the a star orbiting 40,000 light-years from the center of the Milky Way galaxy allows us to determine ______.
the mass of the Milky Way Galaxy that lies within 40,000 light-years of the galactic center
We measure the mass of the black hole at the galactic center from:
the orbits of stars in the galactic center
Most interstellar clouds remain stable in size because the force of gravity is opposed by _______ within the cloud.
thermal pressure
Spiral arms appear bright because
they contain more hot young stars than other parts of the disk
Is it moving toward or away from us
toward us
Is it moving toward or away from us?
toward us
Clouds that appear dark in visible light often glow when observed at long infrared wavelengths
true
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
Any spectrum can be displayed either in photographic form as shown to the left or as a graph. Which of the following graphs could represent a portion of the Sun's visible light spectrum.
upside down with lines
Gamma rays have a very small
wavelength which means they have the highest energy form of light and high frequency
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
A teaspoonful of neutron star material on Earth would weigh
more than Mt. Everest
The twinkling of stars is caused by
motion of air in our atmosphere
This H-R diagram shows the life track of a 1 MSun star from the time it first becomes a main-sequence star. Which numbered point represents the star when it has both hydrogen-burning and helium-burning shells around an inert carbon core?
4
Suppose you have a 100-watt light bulb that you leave turned on for one minute. How much energy does it use?
6000 joules 1 watt= 1 joule/s so a 100-watt light bulb uses 100 joules of energy per second. In 1 minute it uses 60*100= 6000 joules of energy
What is the typical hydrogen content of stars that are forming right now in the vicinity of the Sun?
70 % hydrogen
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 photo shows the famous Crab Nebula. What is it?
An expanding cloud of remains from a star that died in a supernova.
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
What do we mean by the event horizon of a black hole?
It is the point beyond which neither light nor anything else can escape.
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 degeneracy pressure is not true?
Degeneracy pressure varies with the temperature of the star.
Which of the following statements about stages of nuclear burning (i.e., first-stage hydrogen burning, second-stage helium burning, etc.) in a massive star is not true?
Each successive stage lasts for approximately the same amount of time.
What would happen if the Sun suddenly became a black hole without changing its mass?
Earth's orbit would not change.
Which of the following statements about electron degeneracy pressure and neutron degeneracy pressure is true?
Electron degeneracy pressure is the main source of pressure in white dwarfs, while neutron degeneracy pressure is the main source of pressure in neutron stars.
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
In this diagram, red balls represent protons and gray balls represent neutrons. What reaction is being shown?
Fusion of helium into carbon
How does the interstellar medium affect our view of most of the galaxy?
It prevents us from seeing most of the galactic disk with visible and ultraviolet light.
Which of the following statements correctly summarize key differences between the disk and the halo?
Gas and dust are abundant in the disk but not the halo Disk stars come in a broad range of masses and colors, while halo stars are mostly low mass and red Stars in the disk all orbit in the same direction and nearly the same place, while halo stars have more randomly oriented orbits Clusters of young stars are found only in the disk
If you could watch a time-lapse movie of the interstellar medium over hundreds of millions of years, what would you see?
Gas that is often moving at high speed, particularly after one or more supernovae, and constantly changing form between molecular clouds, atomic hydrogen, and hot, ionized bubbles and superbubbles.
The most abundant molecule in molecular clouds is
H2
Blue light has higher frequency than red light. Thus, blue light has
Higher energy and shorter wavelength than red light
According to this diagram, how much more abundant is hydrogen in the universe than nitrogen?
Hydrogen is about 10,000 times as abundant as nitrogen.
This is the HR diagram for stars in a 12-billion-year-old globular cluster. Based on our understanding of stellar lives, which of the four labeled stars is currently generating energy through hydrogen fusion in a shell around an inert (non-burning) helium core.
III
Which of the following statements about black holes is not true?
If the Sun magically disappeared and was replaced by a black hole of the same mass, Earth would soon be sucked into the black hole.
Which of the following statements is true of green gas?
It absorbs red light and reflects green light. We see the reflected green light.
What happens to the core of a star after a planetary nebula occurs?
It becomes a white dwarf.
Which of the following statements is not true of the object known as Sgr A* in the center of our Galaxy?
It is by far the brightest source of visible light lying in the direction of the galactic center.
Most nearby stars move relative to the Sun at speeds below about 30 km/s. Suppose you observe a nearby star that is moving much faster than this (say, 300 km/s). Which of the following is a likely explanation for its high speed?
It is probably a halo star that is currently passing through the disk.
What do we mean by the singularity of a black hole?
It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions.
How would you expect a star that formed recently in the disk of the galaxy to differ from one that formed early in the history of the disk?
It should have a higher fraction of elements heavier than hydrogen and helium.
The arrow in the photo on the left shows the star that we see as a supernova in the photo on the right. What can we conclude about this star?
It was a high-mass star with at least 8 times the mass of the Sun.
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 disassociate into individual oxygen atoms, then the atoms would become increasingly ionized as you continued to raise the temperature.
What happens when a star exhausts its core hydrogen supply?
Its core contracts, but its outer layers expand and the star becomes bigger and brighter.
How would a flashing red light appear as it fell into a black hole?
Its flashes would shift to the infrared part of the spectrum.
From Part B, you know that from afar you'll never see the in-falling rocket cross the event horizon, yet it will still eventually disappear from view. Why?
Its light will become so redshifted that it will be undetectable.
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.
How does the gravity of an object affect light?
Light coming from a compact massive object, such as a neutron star, will be redshifted.
Suppose you had looked with your naked eye at the supernova on the night it was photographed in the "after" photo. Assuming that it was bright enough to see (it was), what would it have looked like to your naked eye?
Like a single star in the night sky.
What would your estimate be for the mass of the object that the star is orbiting?
Mobj = 3.1×106 MSunMSun
Which of the following statements comparing halo stars to our Sun is not true?
Most stars in the halo have either died or are in their final stages of life, while the Sun is only in about the middle of its lifetime.
Imagine that our Sun were magically and suddenly replaced by a black hole of the same mass (1 solar mass). What would happen to Earth in its orbit?
Nothing—Earth's orbit would remain the same.
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.
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.
A mini-black hole with the mass of the Moon.
RSchwarzschild = 0.11mm
A 2×108 M Sun black hole in the center of a quasar.
RSchwarzschild =6.0×108km
Estimate the Schwarzschild radius (in kilometers) for a mini-black hole formed when a super advanced civilization decides to punish you (unfairly) by squeezing you until you become so small that you disappear inside your own event horizon. (Assume that the your weight is 50 kg)
RSchwarzschild =7×10−29 km
A 6 M Sun black hole that formed in the supernova of a massive star.
RSchwarzschild= 18 km
Suppose that Star X and Star Y both have redshifts, but Star X has a larger redshift than Star. What can you conclude?
Star X is moving away from us faster than Star Y.
If we could watch spiral arms from a telescope situated above the Milky Way over 500 million years, what would we see happen?
Stars will move through the spiral arms, bunching up closer as they pass through. Young hot stars will form and die within the arms before having a chance to move out.
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
Suppose a scientist holds a press conference at which he claims that 10% of the matter in the Milky Way is in the form of dust grains. Does his claim seem reasonable? Why or why not?
The 10% figure is too high because there are not enough heavy elements to make that much dust.
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.
Which of the following conditions lead you to see an absorption line spectrum from a cloud of gas in the interstellar space?
The cloud is cool and lies between you and a hot star.
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.
Consider Earth and the Moon. As you should now realize, the gravitational force that Earth exerts on the Moon is equal and opposite to that which the Moon exerts on Earth. Therefore according to Newton's second law of motion
The moon has a larger acceleration than Earth , because it has a smaller mass.
Why is there an upper limit to the mass of a white dwarf?
The more massive the white dwarf, the greater the degeneracy pressure and the faster the speeds of its electrons. Near 1.4 solar masses, the speeds of the electrons approach the speed of light, so more mass cannot be added without breaking the degeneracy pressure
Why do we believe that most of the mass of the Milky Way is in the form of dark matter?
The orbital speeds of stars far from the galactic center are surprisingly high.
What is the best evidence for an extremely massive black hole in the center of the Milky Way?
The orbits of stars in the center of the galaxy indicate that the presence of 3 to 4 million solar mass object in a region no larger than our Solar System.
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.
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 towards us.
Suppose a particular star has a core that is undergoing several stages of fusion simultaneously, as shown in this diagram. Which of the following statements about this star is most likely to be true?
The star will explode as a supernova within a few days.
How did star formation likely proceed in the protogalactic cloud that formed the Milky Way?
The stars that formed first could orbit the center of the galaxy in any direction at any inclination.
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 to 1 to 2 million Kelvin.
How should we expect the Milky Way's interstellar medium to be different in 50 billion years than it is today?
The total amount of gas will be much less than it is today.
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.
A typical neutron star is more massive than our Sun and about the size (radius) of _________.
a small asteroid (10 km in diameter)
Which of the following statements is probably true about the very first stars in the universe?
They were made only from hydrogen and helium.
From the viewpoint of an observer in the orbiting rocket, what happens on time on the other rocket as it falls toward the event horizon of the black hole?
Time runs increasingly slower as the rocket approaches the black hole.
Our Sun will end its life in a planetary nebula and become a white dwarf.
True
What observational evidence supports the galactic fountain model (which describes how gas cycles between the disk of the galaxy and regions high above the disk)?
We see hot gas high above the region of the disk near our solar system, along with cool gas that appears to be raining down from the halo.
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
Which of these objects has the smallest radius?
a 1.2 M sun white dwarf
What kind of gas cloud is most likely to give birth to stars?
a cold, dense gas cloud
A teaspoonful of white dwarf material on Earth would weigh
a few tons
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
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 objects 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
Suppose a particular star has a core that is undergoing several stages of fusion simultaneously, as shown in this diagram. Based on your understanding of stellar lives, the mass of this star is probably:
about 10 (or more) solar masses
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
If you were inside the rocket that falls toward the event horizon, from your own viewpoint you would
accelerate as you fall and cross the event horizon completely unhindered
Which kind of star is most likely to be part of the spheroidal population
an M star
Which of these binary systems is most likely to contain a black hole?
an X-ray binary containing an O star and another object of equal mass
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
Why do disk stars bob up and down as they orbit the galaxy?
because the gravitational pull of other disk stars always pulls them toward the disk
On the main sequence stars obtain their energy
by converting hydrogen to helium
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= 10
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
After a supernova event, what is left behind?
either a neutron star or a black hole
The heaviest element produced by stars or in supernovae is silicon
false
The most massive stars generate energy at the end of their lives by fusing iron in their cores.
false
Which of these groups of particles has the greatest mass?
four individual protons
How much greater is the light-collecting area of a 6-meter telescope than a 3-meter telescope?
four times
How do we determine the Milky Way's mass outside the Sun's orbit?
from the orbits of stars and gas clouds orbiting the galactic center at greater distances than the Sun.
All the following types of objects are found almost exclusively in the disk (rather than the halo) of the Milky Way except _________.
global clusters
Over time, the star-gas-star cycle leads the gas in the Milky Way to _________.
have a greater abundance of heavy elements
Which part of the electromagnetic spectrum generally gives us our best views of stars forming in dusty clouds?
infrared
Which of these forms of radiation passes most easily through the disk of the Milky Way?
infrared light
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 atom nucleus
is very tiny but has the 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
Scientists estimate the central temperature of the Sun using
mathematical models of the Sun