Exam #2 Flashcards
What is the Sun made of?
73 percent hydrogen, 25 percent helium, 2 percent other elements
What is the average temperature of the surface of the Sun?
6,000 K
Which is closest to the temperature of the core of the Sun?
10 million K
From the center outward, which of the following lists the “layers” of the Sun in the correct order?
Core, radiation zone, convection zone, photosphere, chromosphere, corona
Which layer of the Sun do we normally see?
Photosphere
The core of the Sun is …
hotter and denser than the surface.
Why do sunspots appear dark in pictures of the Sun?
They actually are fairly bright but appear dark against the even brighter background of the surrounding Sun.
How does the Sun generate energy today?
Nuclear fusion
At the center of the Sun, fusion converts hydrogen into …
helium, energy, and neutrinos.
The light radiated from the Sun’s surface reaches Earth in about 8 minutes, but the energy of that light was released by fusion in the solar core about …
500,000 years ago.
What happens to energy in the convection zone of the Sun?
Energy is transported outward by the rising of hot plasma and the sinking of cooler plasma.
What is granulation in the Sun?
The bubbling pattern on the photosphere produced by the underlying convection.
What processes are involved in the sunspot cycle?
The winding of magnetic field lines due to differential rotation.
What observations characterize solar maximum?
We see many sunspots on the surface of the Sun.
According to modern science, approximately how old is the Sun?
4.5 billion years
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.
What is the solar wind?
A stream of charged particles flowing outward from the surface of the Sun.
The Sun is in hydrostatic equilibrium. This means that in the Sun…
Pressure and gravity are in balance throughout the Sun.
A neutrino produced in the Sun takes time to reach Earth as a photon produced at the same time.
less
The energy released by fusion in the core of the Sun is in the form of gamma rays. What happens to these gamma rays after fusion?
Over a long period of time the gamma rays collide with many particles of matter and loses some of their energy in these collisions.
Two stars are of equal luminosity, but one is three times farther away than the other. The more distant star’s brightness is that of the more nearby star.
1/9
The apparent brightness of a star is dependent on its and .
luminosity, distance
Two stars have the SAME surface temperature, but one is twice as far away as the other. The more distant star will be the more nearby star.
the same color as
Stars with a higher surface temperature will be stars with a lower surface temperature.
bluer than
The spectral sequence, from hottest to coldest is .
OBAFGKM
The spectral sequence is an ordering of stars based on
decreasing temperature.
The Sun is a type star.
G
Within about 21 light years of the Sun, most stars tend to be .
smaller and less luminous than the Sun
The H-R diagram plots stars according to their
luminosity and surface temperature
Stars on the main sequence are all .
fusing hydrogen to helium
Massive stars on the main sequence have lives than smaller stars on the main sequence.
shorter
White dwarfs are found in the of the H-R diagram.
lower left
The radius of a white dwarf is about the same as .
Earth
White dwarfs are stars near the of their life.
end
Cool supergiants are found in the corner of the H-R diagram.
upper right
As we go down the main sequence from left to right, we will find stars that have .
smaller and smaller masses
of all stars are found on the main sequence.
90%
This method of determining distance to stars relies on an apparent displacement of a nearby star that results from the motion of Earth around the Sun
parallax
White dwarfs are the Sun.
more dense than
A star’s luminosity is the
total amount of light that the star radiates each second.
The interstellar medium (ISM) is .
the gas and dust found between the stars in a galaxy
The temperature range for molecular clouds is closest to .
10-30 K
The “neutral” in neutral hydrogen clouds means that these clouds are .
not ionized, that is their protons are each paired with an electron
The mass of the interstellar medium in our Galaxy is about .
10 billion times the mass of the Sun
About of the interstellar medium is in the form of gas, while is in the form of dust.
99%, 1%
A typical dust grain in the interstellar medium consists of a or core beneath an icy mantle.
A typical dust grain in the interstellar medium consists of a or core beneath an icy mantle.
Debris disks, such as that found around HL Tau, are formed because
the newly formed planets can concentrate the remaining gas into clumps and arcs.
As a star and its planet orbit their common center of mass, the star at times moves toward us and at times moves away. When it moves toward us, the star’s light is than usual, and when it moves away, it is .
bluer, redder
The most abundant elements in the interstellar medium are .
hydrogen and helium
The gas in molecular clouds has a density of .
hundreds to thousands of atoms per cm3
Interstellar dust usually blocks light and emits light.
visible, infrared
New stars are formed in this component of the interstellar medium:
molecular clouds
When a star formation event occurs, about of the available gas is transformed into stars.
1%
The key to triggered star formation is to
compress the gas and dust so that gravitation will overcome the gas pressure.
The protoplanetary disks found around young stars typically have masses of the mass of our own Sun, which is than the mass of all the planets in our solar system
1-10%, more
The average density of the interstellar medium is .
about one atom of gas per cm3
The temperature in the core of main sequence stars gradually rises. How does this affect the hydrogen fusion rate of the star?
The rate of fusion increases.
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.
What is happening inside a star while it expands into a subgiant?
It is fusing hydrogen into helium in a shell outside the core.
Compared to the star it evolved from, a red giant is
cooler and brighter.
At approximately what temperature can helium fusion occur?
100 million K
Why does a star grow larger after it exhausts its core hydrogen?
Hydrogen fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward.
How many helium nuclei fuse together when making carbon?
3
What is a planetary nebula?
the expanding shell of gas that is no longer gravitationally held to the remnant of a low-mass star
What happens to the core of a star after a planetary nebula occurs?
It becomes a white dwarf.
Which of the following sequences correctly describes the stages of life for a low-mass star?
protostar, main-sequence, red giant, white dwarf
Compared to the star it evolved from, a white dwarf is
hotter and dimmer.
Stars spend about of their lives on the main sequence.
90%
Why do more massive stars have shorter main-sequence lives than smaller stars?
Their greater mass means they have a greater core temperature, which in turns produces a greater fusion rate.
Rank the three types of clusters from youngest to oldest.
stellar associations, open clusters, globular clusters
Cluster ages can be determined from
the main sequence turnoff.
Which of the following statements about a globular cluster is true?
Most stars in the cluster are yellow or reddish in color.
Which of the following statements about an open cluster is true?
All stars in the cluster are approximately the same age.
Why is iron the endpoint of fusion in massive stars?
The fusion of iron requires energy instead of producing it.
What types of stars end their lives with supernovae?
stars that are at least several times the mass of the Sun
During which stage does the star have an inert (nonburning) helium core?
E
During which stage does the star have an inert (nonburning) carbon core?
E
Which stage lasts the longest?
A
What will happen to the star after stage E?
It will eject a planetary nebula.
In the end, the remaining core of this star will be left behind as
a white dwarf made primarily of carbon and oxygen.
Which of the following is closest in mass to a white dwarf?
The Sun
If we were to crush the Sun such that it occupied a smaller volume, the escape velocity from the surface of the Sun would
increase
White dwarfs no longer undergo core fusion, and their outward pressure is provided by .
degenerate electrons
After a massive-star supernova, what is left behind?
either a neutron star or a black hole
What is the upper limit to the mass of a neutron star?
The upper limit may be around 3 solar masses, but we do not yet know precisely what it is.
According to the general theory of relativity light cannot leave a black hole because
spacetime is so curved that light cannot travel in a straight line outward, and instead curves back in.
The interior of a neutron star is composed of
about 95% neutrons, with a small number of electrons and protons.
Which of the following is closest in size (radius) to a neutron star?
a city
From an observational standpoint, what is a pulsar?
an object that emits flashes of light several times per second or more, with near perfect regularity
What causes the radio pulses of a pulsar?
As the star spins, beams of radio radiation sweep through space. If one of the beams crosses Earth, we observe a pulse.
How do we know that pulsars are neutron stars?
No massive object, other than a neutron star, could spin as fast as we observe pulsars spin.
How does a 1.2-solar-mass white dwarf compare to a 1.0-solar-mass white dwarf?
It has a smaller radius.
from SN 1987A was observed the day before the light was observed.
a burst of neutrinos
Which of the following is YOUNGEST (i.e. has had the least time since its formation)
A star of uniform composition from center to surface; it contains hydrogen but has no nuclear reactions going on in the core.
If most stars become white dwarfs at the ends of their lives and the formation of white dwarfs is accompanied by the production of a planetary nebula, why are there more white dwarfs than planetary nebulae in the Galaxy?
After about 10,000 years or so, the gas shell that is ejected in the planetary nebula phase expands and thins out to such an extent that it becomes unobservable.
Based upon the evolution of stars, which of the following elements should be LEAST common in the Galaxy?
gold
From a theoretical standpoint, what is a pulsar?
a rapidly rotating neutron star
White dwarfs are so called because
they are both very hot and very small.
What is the basic definition of a black hole?
any object from which the escape velocity exceeds the speed of light
Black holes at the centers of galaxies can grow to millions or billions of times the mass of the Sun.
True
How does a black hole form from a massive star?
During a supernova, if a star is massive enough for its gravity to overcome neutron degeneracy of the core, the core will be compressed until it becomes a black hole.
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.
False
If we watch a clock fall toward a black hole, we will see it tick slower and slower as it falls nearer to the black hole.
True
If you watch someone else fall into a black hole, you will never see him or her cross the event horizon. However, he or she will fade from view as the light he or she emits (or reflects) becomes more and more redshifted.
True
If you fell into a black hole, you would experience time to be running normally as you plunged rapidly across the event horizon.
True
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.
The equivalence principle states that
gravity and acceleration are the same within a local environment.
The Schwarzschild radius of a black hole depends on ________.
only the mass of the black hole
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.
from the merger of have been detected.
Gravitational waves, two black holes
What is the upper limit to the mass of a white dwarf?
1.4 solar masses
What is the ultimate fate of a pulsar?
It will slow down, the magnetic field will weaken, and it will become invisible.
Black holes in the centers of galaxies can grow via swallowing stars and gas that get too close to their event horizons.
True
