ASTRONOMY 2 (12-14)
What is the observed lower mass limit to the main sequence (in units of M)?
0.08 M
What is the Schwarzschild radius for a 5 solar mass star?
14.8 km
If the event horizon is at a radius of 100,000 m, what is the mass of the black hole?
34 solar masses
Roughly how long would it take a protostar with moderately small mass to reach the main sequence?
5 million years
What happens when a star expands to fill its Roche lobe?
Matter from the star overflows forming an accretion disk around its companion.
How do you know which one is more massive?
More luminous stars on the main sequence have more mass.
Why does a star's life expectancy depend on mass?
More massive stars have shorter lifetimes because their larger masses cause them to consume their fuel faster.
Why would astronomers naturally assume that the first discovered millisecond pulsar was relatively young?
When a pulsar first forms, it is spinning fast, and its rotation begins to slow as it radiates energy into space.
Why do you expect neutron stars to have a powerful magnetic field?
When a star collapses into a neutron star, its magnetic field is squeezed into a small volume, which can make the field as much as a billion times stronger.
How do they differ?
White dwarfs are produced by the final gravitational contraction of the cores of low- to medium-mass stars. Neutron stars are produced from the collapsing core of a massive star as it undergoes a supernova explosion.
Rank the objects in order of increasing size. (Assume the objects are of equal mass.)
smallest 1. black hole 2. neutron star 3. white dwarf largest
If neutron stars have hot surface temperatures, why aren't they very luminous?
Although neutron stars are hot, they are very small and have little surface area from which to radiate, so their luminosity is low.
Why do expanding, aging stars become cooler and more luminous?
An overproduction of energy causes the outer layers of gas to expand. When this happens, the surface area increases rapidly while the temperature decreases rather slowly, and therefore the luminosity increases.
What is a reason for the answer to the previous question?
As long as more mass is transferred, a nova explosion can occur again. In supernovae, stars lose their outer layers. The core of the star collapses in a supernova. (All of the above are reasons.)
How are neutron stars and white dwarfs similar?
Both are compact objects. Both eventually cease to shine. Both are produced by dying stars. Both are faint and difficult to detect. (all of the above)
What is the reason for the answer to the previous question?
Degenerate matter cannot contract, and a cooler star is dimmer.
What is a prominence in the Solar surface?
Eruption on the solar surface; visible during total solar eclipses.
How can mass transfer into a compact object produce jets of high-speed gas?
Gas in an accretion disk is accelerated to high speeds and can interact with the compact object's magnetic field creating powerful jets of excited gas.
How can mass transfer into a compact object produce X-ray bursts?
Gas that flows from the accretion disk of a neutron star down to the surface of the neutron star accumulates in a dense layer that becomes degenerate until it ignites helium fusion to produce a burst of X-rays. The bursts repeat every time a large enough layer of degenerate fuel accumulates.
Why does a helium flash make understanding the later stages of stellar evolution more difficult?
Helium flash occurs too rapidly and violently that computers cannot sufficiently model it without making certain assumptions. This uncertainty during a helium flash means that there is an uncertainty in the structure of the star at the end of the helium flash, which makes the rest of the models uncertain.
A star moves off the ZAMS after burning up what element?
Hydrogen
Why is it that can some stars cannot fuse carbon or heavier elements?
If they are not massive enough, there is not enough pressure on the core to achieve the necessary temperatures.
In what sense is a black hole actually black?
It absorbs all light within the Schwarzschild radius and emits no light itself
How can mass transfer into a compact object produce gamma-ray bursts?
Mass transferred to a neutron star as the result of a merger between it and another neutron star or a black hole can cause a violent explosion resulting in a gamma-ray burst.
How can mathematical models allow scientists to study processes that are hidden from human eyes or happen too fast or too slowly for humans to experience?
Mathematical models use physical laws to explain and predict phenomena that we cannot directly observe. However, models are only as good as the assumptions they are built on, and we therefore must compare them to the real world at every opportunity to see how accurate they are.
Why do you expect neutron stars to spin rapidly?
Neutron stars are formed by the collapse of massive stars. Since all stars rotate, the principle of conservation of angular momentum predicts that as a massive star collapses it must rotate faster to conserve angular momentum.
How did astronomers conclude that pulsars actually could not be pulsating stars?
Normal stars are much too large to pulse with a period as small as the period of a pulsar.
If the Sun has a Schwarzschild radius, why isn't it a black hole?
Not all of the Sun's mass is inside its Schwarzschild radius.
Why is there a lower mass limit?
Objects with lower mass do not have high enough temperatures in their cores to ignite hydrogen fusion
Star A and Star B are both on the main sequence. Star A is 71 times more luminous than Star B. Which star is more massive?
Star A is more massive.
Why?
The Sun's mass is too small
What is parallax? Give one example
The apparent change in the position of an object as a result of a change in the location of the observer. Astronomical parallax is measured in seconds of arc; driving down the highway
Which is a way that some stars avoid the helium flash?
The core doesn't get hot enough for helium fusion.
Which star has a longer life? Why?
The less massive star will have a longer life because it will consume fuel more slowly.
Why is there a mass-luminosity relation for main sequence stars?
The more massive a star is, the higher the rate of fusion in the core is due to the larger weight pressing down on the inner layers. When the energy from the fusion reaction reaches the surface it radiates into space with a characteristic luminosity.
Which of the following statements is true of more massive stars?
The more massive the star, the faster it contracts to the main sequence. The more massive the star, the more luminous it is. The more massive the star, the faster it uses up its nuclear fuels. (all of these)
What is the reason for the answer to the above question?
There has not been enough time for these objects to form.
Why do neutron stars have an upper mass limit?
There is a maximum mass that can be supported against gravity by the outward pressure of degenerate neutrons.
Why are stars that contract toward the main sequence not detectable at visible wavelengths?
They are hidden inside clouds of gas and dust.
As white dwarfs cool, which of the following occurs?
They stay the same size and get dimmer.
Where is mass transfer likely to occur?
a binary system
Which event can repeat?
a nova explosion
Which is the result of the accumulation of mass in a close binary system igniting the core of a white dwarf?
a nova explosion
The closer an object approaches to a singularity, the more violent the tidal forces it experiences. For which black hole are the tidal forces a greater concern?
a small mass black hole
Which is the result of a collapsing iron core in a massive star?
a type II supernova
Which is the result of mass transfer pushing a white dwarf over the Chandrasekhar limit and its core collapsing, causing all its carbon to fuse at once?
a type Ia supernova
Which is the result of massive star losing its outer layers in a binary system?
a type Ib supernova
The stars in a cluster move off the main sequence _____.
at an ever slower pace
The Roche lobes of a binary system meet at what point?
at the Lagrange point between the two stars
What is the probable next step in a star's evolution after the white dwarf stage?
black dwarf
How do star clusters confirm that stars evolve?
by the more massive, brighter stars leaving the main sequence
When a star becomes a red giant, its surface temperature _____.
decreases
Describe the three kinds of nebula in the Interstellar medium (ISM)
emission nebula: A cloud of glowing gas excited by ultraviolet radiation from hot stars. reflection nebula: A nebula produced by starlight reflecting off dust particles in the interstellar medium. dark nebula: A nonluminous cloud of gas and dust visible because it blocks light from more distant stars and nebulae.
High mass stars live longer than low mass stars.
false
Low mass stars explode as supernovae.
false
All main-sequence stars do which of the following?
fuse hydrogen into helium
Rank the objects in order of decreasing density.
greatest 1. black hole 2. neutron star 3. white dwarf least
What is required for carbon fusion to occur in the core of a star?
high mass, high temperature, high pressure (all of the above)
Which is not true about a type I supernova?
it always involves a massive star
What happens when a star evolves through the instability strip?
it begins to pulsate
How does a star that is burning helium differ from one burning hydrogen?
it gets brighter
If a star eventually produces an iron core, which of the following must be true?
it must be very massive
Which is not true about a type II supernova?
it occurs in a binary system
Organize form larger to smaller stars if they died as: White dwarf Black hole Neutron star
largest white dwarf neutron star black hole smallest
Rank the objects in order of increasing mass. (Assume the objects are equal in diameter.)
least 1. white dwarf 2. neutron star 3. black hole greatest
As the animation shows, as stars age and move off the main sequence, they initially become _____.
more luminous and cooler
Why does mass determine the evolution of a star?
more massive states have more weight to support
A cluster whose stars fill the entire s-curve of the main sequence _____.
must be young
Can the Sun ever become a black hole?
no
How many of the objects from the previous question have been observed?
none
Which is evidence that stars lose mass?
planetary nebulae, nova explosions supernovae (all of the above)
What causes the helium flash?
the beginning of helium fusion in the core of a star in a certain mass range, the gas in the core of a star in a certain mass range becoming degenerate, gas pressure in a star in a certain mass range becoming independent of temperature (all of the above)
Why can't the lowest-mass main-sequence stars become giants?
they are not massive enough to fuse helium
How can white dwarfs be both very hot and have very low luminosity?
they have small surface area
The more massive the star, the more dense its end state.
true
Where is the center of mass in the binary system illustrated?
within the Roche lobe of the larger star
