NEW
Font size
WorksheetsExam 3 - Stars and Galaxies
Total questions: 28
Worksheet time: 14mins
Where do stars form and why?
Stars form in hot gas clouds because stars have to be hot to start fusion reactions.
Stars form around giant planets because planets provide the "seeds" of star formation.
Stars form in cold gas clouds because gravity has to be stronger than gas pressure.
The buildup of thermal energy in a collapsing gas cloud leads to thermal pressure that can halt the gravitational collapse. What must happen for the gravitational collapse to continue?
The cloud must have a very strong magnetic field.
The cloud must get a push from the radiation of nearby hot, young stars.
The cloud must radiate away some of its thermal energy.
Which stars spend the shortest amount of time as protostars?
the most massive stars
the least massive stars
mass doesn't matter
What key event marks the transition of a protostar into a true (main-sequence) star?
the onset of sustained nuclear fusion in its core
the onset of a strong stellar wind
the clearing of the gas from the surrounding gas cloud into interstellar space
What is the primary source of energy for protostars (that have not yet become hot enough for fusion in their cores)?
the conversion of mass into energy in accord with E = mc2
gravitational contraction
fission from concentrated radioactive elements
Why do disks form around young stars?
Intense winds from nearby massive stars flatten the gas cloud.
Collisions between gas particles flatten the rotating gas cloud.
The rotation of the cloud causes gas to be spun outward from the central star.
Generally speaking, a main-sequence star is __________ than it was during the time it was a protostar
cooler and dimmer
hotter and brighter
hotter and dimmer
Why are main-sequence lifetimes shorter for more massive stars?
They aren't; higher mass stars have longer lifetimes.
Strong stellar winds cause higher mass stars to lose mass quickly.
Higher core temperatures allow fusion to proceed much more rapidly.
Why does a star grow larger in radius immediately after it exhausts its core hydrogen?
Helium fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward.
Helium fusion in the core generates enough thermal pressure to push the upper layers outward.
Hydrogen fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward.
What happens to the core of a star after a planetary nebula occurs?
It contracts from a protostar to a main-sequence star.
It becomes a white dwarf.
It can become either a neutron star or a black hole.
It breaks apart in a violent explosion.
How will the Sun end its life?
as a massive star supernova
as a neutron star or black hole
as a white dwarf
What is a planetary nebula?
gas ejected from a low-mass star in the final stage of its life
interstellar gas from which planets are likely to form in the not-too-distant future
the remains of a high-mass star that has exploded
gas created from the remains of planets that once orbited a dead star
High-mass stars fuse hydrogen into helium through the CNO cycle rather than solely by the proton-proton chain. Which property of the CNO cycle explains why high mass stars have much shorter lives than low mass stars?
The CNO cycle is very fast at converting hydrogen to carbon, nitrogen, oxygen (CNO).
The CNO cycle is only possible when the abundance of CNO elements is very high.
The CNO cycle is only possible when the star has lived for a very long time.
The CNO cycle is very fast at converting hydrogen to helium.
Why don't low-mass stars have the CNO cycle occurring in their cores?
The CNO cycle makes elements heavier than carbon, nitrogen, and oxygen.
They don't have enough carbon, nitrogen, and oxygen.
Their core temperatures are too low.
Which element has the lowest mass per nuclear particle and therefore cannot release energy by either fusion or fission?
iron
uranium
hydrogen
oxygen
Which element is the last to be produced by fusion in the cores of the most massive stars (just before their lives end in supernova explosions)?
hydrogen
oxygen
iron
lead
Carbon fusion occurs in high-mass stars but not in low-mass stars because _________.
the cores of low-mass stars never contain significant amounts of carbon
carbon fusion can occur only in the stars known as carbon stars
the cores of low-mass stars never get hot enough for carbon fusion
What is the CNO cycle?
The process by which helium is fused into carbon, nitrogen, and oxygen
The set of fusion reactions that have produced all the carbon, nitrogen, and oxygen in the universe
The process by which carbon is fused into nitrogen and oxygen
A set of steps by which four hydrogen nuclei fuse into one helium nucleus
What is the ultimate fate of an isolated white dwarf?
As gravity overwhelms the electron degeneracy pressure, it will explode as a supernova.
The electron degeneracy pressure will eventually overwhelm gravity and the white dwarf will slowly evaporate.
As gravity overwhelms the electron degeneracy pressure, it will explode as a nova.
It will cool down and become gradually dimmer.
What kind of pressure prevents a white dwarf from collapsing?
neutron degeneracy pressure
thermal pressure
radiation pressure
electron degeneracy pressure
What is the upper limit to the mass of a white dwarf?
about 1 solar mass
about 1.4 solar masses
about 2.8 solar masses
There is an upper limit, but we do not yet know what it is.
What is an accretion disk?
a stream of gas flowing from one star to its binary companion star
a disk of material found around every white dwarf in the Milky Way Galaxy
a disk of hot gas swirling rapidly around a white dwarf, neutron star, or black hole
Which of the following statements about novae is true?
When a star system undergoes a nova, it brightens as much as a star system undergoing a supernova.
A nova involves fusion taking place on the surface of a neutron star.
A star system that undergoes a nova may have another nova sometime in the future.
You observe a nova. Based on current understanding, you can also conclude that you are observing
the complete destruction of a white dwarf.
a close binary system in which one member is a neutron star.
a close binary system in which one member is a white dwarf.
What causes the pulses of a pulsar?
The pulsar undergoes periodic explosions of nuclear fusion that generate strong emission.
vibrations on the pulsar's surface
The pulses are peaks and valleys of gravitational waves.
As the pulsar rotates, beams of radiation along its magnetic axis sweep through space.
What is the ultimate fate of an isolated pulsar (not in a binary system)?
It will gradually turn into a white dwarf.
It will gradually cool, and it will become too dim to detect.
As gravity overwhelms the neutron degeneracy pressure, it will explode as a supernova.
It will spin ever faster, becoming a millisecond pulsar.
What makes us think that the star system Cygnus X-1 contains a black hole?
The fact that we see strong x-ray emission tells us that the system must contain a black hole.
It emits x-rays as we expect from a system with an accretion disk, but the unseen star in the system is too massive to be a neutron star.
No light is emitted from this star system, so it must contain a black hole.
The Schwarzschild radius of a black hole depends on __________.
the observationally measured radius of the black hole
the way in which the black hole formed
only the mass of the black hole
