Astronomy Test #3
Observational evidence indicates that at least some gamma ray bursts are produced by __________.
the supernovas of very massive stars that leave behind black holes
What is the approximate range of masses that newborn main-sequence stars can have?
0.1 to 150 solar masses
Which of the following stars will live longest?
1 solar-mass star
Approximately what core temperature is required before hydrogen fusion can begin in a star?
10 million K
When did scientists first detect gravitational waves from mergers of compact objects (pairs of neutron stars or black holes)?
2015
Based on current understanding, the minimum mass of a black hole that forms during a massive star supernova is roughly __________.
3 solar masses
What is the approximate chemical composition (by mass) with which all stars are born?
3/4 Hydrogen, 1/4 Helium, no more than about 2% heavier elements.
What is the CNO cycle?
A set of steps by which four hydrogen nuclei fuse into one helium nucleus
How is an x-ray burst (in an x-ray binary system) similar to a nova?
Both involve explosions on the surface of stellar corpse.
Which of the following statements about brown dwarfs is not true?
Brown dwarfs eventually collapse to become white dwarfs.
Assuming that we can measure the apparent brightness of a star, what does the inverse square law for light allow us to do?
Calculate the star's luminosity if we know its distance, or calculate its distance if we know its luminosity.
Suppose our Sun were suddenly replaced by a supergiant star. Which of the following would be true?
Earth would be inside the supergiant.
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.
Our Sun is a star of spectral type _____.
G
Which of the following types of molecule is the most abundant in an interstellar molecular cloud?
H2
Why is iron significant to understanding how a supernova occurs?
Iron cannot release energy either by fission or fusion.
What makes us think that the star system Cygnus X-1 contains 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.
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.
Why is Supernova 1987A particularly important to astronomers?
It is the nearest supernova to have occurred at a time when we were capable of studying it carefully with telescopes.
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.
What is a white dwarf?
It is the remains of a star that ran out of fuel for nuclear fusion
What do we mean by the main-sequence turnoff point of a star cluster, and what does it tell us?
It is the spectral type of the hottest main sequence star in a star cluster, and it tells us the cluster's age.
What is a helium flash?
It is the sudden onset of helium fusion in the core of a low-mass star.
Imagine that our Sun were magically and suddenly replaced by a black hole of the same mass (1 solar mass). How would Earth's orbit change?
It would not change; Earth's orbit would remain the same.
According to the inverse square law of light, how will the apparent brightness of an object change if its distance to us triples?
Its apparent brightness will decrease from a factor of 9.
The total amount of power (in watts, for example) that a star radiates into space is called its?
Luminosity
From hottest to coolest, the order of the spectral types of stars is __________.
OBAFGKM
Star A has an apparent magnitude = 3 and star B has an apparent magnitude = 5. Which star is brighter in our sky?
Star A
Which of the following statements comparing open and globular star clusters is true?
Stars in open clusters are relatively young, while stars in globular clusters are very old.
What happens when a main-sequence star exhausts its core hydrogen fuel supply?
The core shrinks while the rest of the star expands.
Astronomers can measure a star's mass in only certain cases. Which one of the following cases might allow astronomers to measure a star's mass?
The star is a member of a binary star system.
Algol consist of a 3.7 MSun main-sequence star and a 0.8 MSun subgiant. Why does this seem surprising, at least at first?
The two stars should be the same age, so we'd expect the subgiant to be more massive than the main-sequence star.
What would happen if a white dwarf gained enough mass to reach the 1.4 solar-mass white dwarf limit?
The white dwarf would explode completely as a white dwarf supernova.
Suppose two neutron stars or two black holes are closely orbiting one another. What do scientists predict will eventually happen to them, and why?
Their orbits will spiral inward until the two objects merge because of energy lost through gravitational waves.
How are low-mass red giant stars important to our existence?
These stars manufactured most of the carbon atoms in our bodies.
How are magnetic fields thought to affect star formation in molecular clouds?
They can help resist gravity, so that more total mass is needed before the cloud can collapse to form stars.
An H-R diagram for a globular cluster will show a horizontal branch, which is a line of stars above the main-sequence but to the left of the subgiants and red giants. Which of the following statements about these horizontal branch stars is true?
They generate energy through both hydrogen fusion and helium fusion.
What is the common trait of all main-sequence stars?
They generate energy through hydrogen fusion in their core.
Which of the following statements about the first stars that were born in the universe is expected to be true?
They were made only from hydrogen and helium
Which of the following stars spends the longest time in the protostellar phase of life?
a 1 solar-mass star
Which of the following stars will end its life in a supernova?
a 10 solar-mass star
What kind of gas cloud is most likely to give birth to stars?
a cold, dense gas cloud
What is an accretion disk?
a disk of hot gas swirling rapidly around a white dwarf, neutron star, or black hole
Assume the following stars all have the same mass as our Sun. Which one does not have fusion occurring in its central core?
a red giant
A typical neutron star is more massive than our Sun and about the size (radius) of __________.
a small asteroid (10 km in diameter)
What is a protostar?
a star that is still in the process of forming
The maximum mass of a white dwarf is ___________.
about 1.4 times the mass of our Sun
If you had something the size of a sugar cube that was made of neutron star matter, it would weigh __________.
about as much as a large mountain
According to our modern understanding, what is a nova?
an explosion on the surface of a white dwarf in a close binary system
What is the basic definition of a black hole?
an object with gravity so strong that not even light can escape
A typical white dwarf is __________.
as massive as the Sun but only about as large in size as Earth
If you had something the size of a sugar cube that was made of white dwarf matter, it would weigh __________.
as much as a truck
The ultimate fate of our Sun is to __________.
become a white dwarf that will slowly cool with time
On an H-R diagram, stellar masses __________.
can be determined for main sequence stars but not for other types of stars
What law explains why a collapsing cloud usually forms a protostellar disk around a protostar?
conservation of angular momentum
Suppose you look at a star that can be seen through the edge of a dusty interstellar cloud. The star will look __________ than it would if it were outside the cloud.
dimmer and redder
After a supernova explosion, the remains of the stellar core can be __________.
either a neutron star or a black hole
What is a planetary nebula?
gas ejected from a low-mass star in the final stage of its life
Which of the following pairs of atomic nuclei would feel the strongest repulsive electromagnetic force if you tried to push them together?
helium and helium
The primary source of energy for a star as it grows in size to become a red giant is __________.
hydrogen fusion in a shell surrounding the central core
Question 13: On an H-R diagram, stellar radii __________.
increase diagonally from the lower left to the upper right
Which part of the electromagnetic spectrum generally gives us our best views of stars forming in dusty clouds?
infrared
Which of the following phenomena is not commonly associated with the star-formation process?
intense ultraviolet radiation coming from a protostar
To predict whether a star will eventually fuse oxygen into a heavier element, what do you need to know about the star?
its mass
What can we infer, at least roughly, from a star's luminosity class?
its size (radius)
If star A is closer to us than star B, then Star A's parallax angle is __________.
larger than that of Star B
The vast majority of stars in a newly formed star cluster are __________.
less massive than the Sun
The axes on a Hertzsprung-Russell (H-R) diagram represent __________.
luminosity and surface temperature
According to current models, where does the element gold come from?
mergers of neutron stars
Interstellar dust consists mostly of __________.
microscopic particles of carbon and silicon
In the context of understanding stellar lives, "high-mass" stars have masses __________.
more than about 8 times the mass of our Sun
High-mass stars have __________ lifetimes than low-mass stars.
much shorter
The Schwarzschild radius of a black hole depends on __________.
only the mass of the black hole
Which of the following lists the stages of life for a low-mass star in the correct order?
protostar, main-sequence star, red giant, planetary nebula, white dwarf
Pulsars are thought to be __________.
rapidly rotating neutron stars
Which of the following terms is given to a pair of stars that we can determine are orbiting each other only by measuring their periodic Doppler shifts?
spectroscopic binary
The interstellar clouds called molecular clouds are __________.
the cool clouds in which stars form
What do we mean by the interstellar medium?
the gas and dust that lies in between the stars in the Milky Way galaxy
A neutron star is __________.
the remains of a star that died in a massive star supernova (if no black hole was created)
What can we learn about a star from a life track on an H-R diagram?
the surface temperature and luminosity the star will have at each stage of its life
Most interstellar clouds remain stable in size because the force of gravity is opposed by __________ within the cloud.
thermal pressure
The overall helium fusion reaction is __________.
three helium nuclei fusing to form one carbon nucleus
On an H-R diagram, a red supergiant would be located in the __________.
upper right
A white dwarf is __________.
what most stars become when they die
When does a protostar become a main-sequence star?
when the rate of hydrogen fusion becomes high enough to balance the rate at which the star radiates energy into space