No nuclear reactions occur in the interior of a white dwarf. Its brilliance comes from stored heat. Therefore, the fate of an isolated white dwarf is to slowly lose its stored energy and dim over a long period of time. However, if a white dwarf is a member of a binary-star system with specific properties, the white dwarf can become explosively active. Label the components of such a binary-star system capable of producing a nova event.
Drag the appropriate labels to their respective targets.
answer
See picture at: http://www2.astro.psu.edu/users/rbc/a1/lec15_f3.jpg
Feedback: Correct
For a white dwarf to become explosively active, the distance between the dwarf and the companion must be small enough that the white dwarf's gravitational field can pull matter away from the surface of the companion. Due to the rotation of the binary system, the matter flowing through the mass-transfer stream from the companion star forms a flattened disk, called an accretion disk, which orbits around the white dwarf.
question
In a binary-star system that produces a nova, the white dwarf pulls matter from the companion star. The matter forms an accretion disk that orbits the white dwarf. Then a specific sequence of events must take place for a nova event to occur.
Rank the steps leading up to the observed nova event in chronological order from first to last.
answer
1. Material accumulates onto
the white dwarf's surface,
increasing in temperature
and density.
2. At a temperature of 10
million K, the accumulated
surface hydrogen begins
nuclear fusion.
3. Nuclear fusion reactions cause
an enormous but temporary
increase in luminosity.
4. As nuclear fuel is burned up
or blown into space, fusion
ceases and the star dims.
Feedback: Correct
Once nuclear fusion starts for the material on the surface, it proceeds at a furious rate. The white dwarf flares up, then fades away as some of the nuclear fuel is exhausted. The rest of it is blown into space. Neither the companion nor the white dwarf is destroyed in the nova process, so once fusion ceases, they return to their original states and repeat the process. Astronomers have observed many such scenarios, called recurrent novae.
question
A nova produces a characteristic light curve, which provides astronomers with details about the nova event. Select the appropriate light curve that shows how a nova's brightness changes over time.
Select the correct light curve.
answer
http://staff.on.br/jlkm/astron2e/AT_MEDIA/CH21/CHAP21AT/AT21FG04.JPG
Feedback: Correct
A light curve of a nova event shows a sudden increase in luminosity in a matter of days, followed by a gradual fade over several months, until the white dwarf's luminosity returns to normal.
question
A white dwarf can dramatically increase in brightness only if it:
a. is descended from a very massive star.
b. is spinning very rapidly.
c. can avoid nuclear fusion in its core.
d. has another star nearby.
answer
d. has another star nearby
question
Which of the following stars will become hot enough to form elements heavier than oxygen?
Check all that apply.
- A star that is half the mass of the Sun.
- A star having the same mass as the Sun.
- A star that is twice as massive as the Sun.
- A star that is eight times more massive than the Sun
answer
A star that is eight times more massive than the Sun
question
Some of the most energetic events in the universe occur when stars explode in an incredible release of energy known as a supernova. Although every supernova is characterized by a sudden, intense brightening, the differences in types of progenitor stars and the mechanisms of detonation produce two distinct supernova types: Type I and Type II.
Rank the following steps that lead to a Type I supernova event in order of when they occur from first to last.
Rank the steps of the supernova mechanism from first to last.
answer
Initial State
-accretion disk on growing WD
-WD at Chandrasekhar limit
-carbon fusion begins
-detonation
Final State
Feedback: Correct
A Type I supernova occurs in a binary system with a white dwarf and a companion star. The white dwarf gains mass by accreting matter from the companion, until enough mass accumulates and it cannot support its own weight. The white dwarf begins to collapse, causing carbon fusion to begin throughout the star, which leads to its detonation.
question
Rank the following steps that lead to a Type II supernova event in order of when they occur from first to last.
Rank the steps of the supernova mechanism from first to last.
answer
Initial State
- Fusion Ceases
- Photodisintegration of Core Atoms
- Neutronization Begins
- Core Rebound
- Neutron Core
Final State
Feedback: Correct
A Type II supernova occurs when a high-mass star's core becomes dominated by iron, halting the nuclear fusion process. The star's core can no longer maintain equilibrium, and the core begins to contract. The gravitational compression generates heat, which initiates photodisintegration. During photodisintegration, high-energy photons split large atoms into smaller and smaller atoms until only protons, neutrons, and electrons are left. Protons and electrons combine to form neutrons and neutrinos. The neutrinos are able to escape the star, carrying off energy, causing further contraction of the core. When the core contracts to the point where neutrons come into contact with each other, the core rebounds, sending out an energetic shockwave. This shockwave blasts the outer layers of the star into space, leaving behind a neutron core.
question
Each supernova type is distinct in initial components, process, and observational properties. Sort the following characteristics as to whether they describe a Type I or Type II supernova.
Drag the appropriate items to their respective bins.
answer
Type I:
- low-mass star
- hydrogen-poor
- carbon-detonation supernova
- graph with green line (initial increase followed by steady decrease)
Type II:
- high-mass star
- hydrogen-rich
- core-collapse supernova
- graph with red line (higher initial luminosity but luminosity constant from 25 to 100 days before decrease)
Feedback: Correct
Although both Type I and Type II supernovae are similar in their impressive release of energy, the two events occur under vastly different circumstances and are produced by different mechanisms.
question
The radius of a white dwarf is determined by a balance between the inward force of gravity and the outward push of _________________
answer
electron degeneracy pressure
question
A(n) __________________ occurs when hydrogen fusion ignites on the surface of a white dwarf in a binary system.
answer
nova
question
A(n) __________________ occurs when fusion creates iron in the core of a star.
answer
massive star supernova
question
A white dwarf in a close binary system will explode as a supernova if it gains enough mass to exceed the __________________.
answer
white dwarf limit (1.4 solar masses)
question
A(n) _________________ consists of hot, swirling gas captured by a white dwarf (or neutron star or black hole) from a binary companion star.
answer
accretion disk
question
A(n) __________________ can occur only in a binary system, and all such events are thought to have the same luminosity.
answer
white dwarf supernova
question
A nova differs from a supernova in that the nova
a. is much less luminous.
b. is much more luminous.
c. involves only high-mass stars.
d. can occur only once.
answer
a. is much less luminous.
question
Figure 21.8 in the textbook ("Supernova Light Curves") indicates that a supernova whose luminosity declines steadily in time is most likely associated with a star that is
a. on the main sequence.
b. more than eight times the mass of the Sun.
c. without a binary companion.
d. comparable in mass to the Sun.
answer
d. comparable in mass to the Sun.
question
Suppose that stars form in our Galaxy at an average rate of 10 per year. Suppose also that all stars greater than 8 solar masses explode as supernovae.
Use Figure 17.23 in the textbook to estimate the rate of Type II supernovae in our Galaxy.
Express your answer using one significant figure.
answer
4×10^−2 supernovae per year
question
The Hubble Space Telescope is observing a distant Type I supernova with peak apparent magnitude 25.
Using the light curve in Figure 21.8 in the textbook, estimate how long after the peak brightness the supernova will become too faint to be seen.
answer
about 70 days
question
Nuclear fusion in the Sun will
a. never create elements heavier than helium.
b. create elements up to and including oxygen.
c. create all elements up to and including iron.
d. create some elements heavier than iron.
answer
b. create elements up to and including oxygen.
question
The silver atoms found in jewelry originated in
a. nearby galaxy.
b. the core of a red-giant star.
c. supernova.
d. the core of the Sun.
answer
c. supernova.
question
A massive star becomes a supernova when it
a. suddenly increases in surface temperature.
b. forms iron in its core.
c. suddenly increases in mass.
d. collides with a stellar companion.
answer
b. forms iron in its core.
question
An observable supernova should occur in our Galaxy about once every
a. year.
b. decade.
c. century.
d. millennium.
answer
c. century
question
Which one of the following does not provide evidence that supernovae have occurred in our Galaxy?
a. The existence of binary stars in our Galaxy.
b. Historical records from China and Europe.
c. The rapid expansion and filamentary structure of the Crab nebula.
d. The existence of iron on Earth.
answer
a. The existence of binary stars in our Galaxy.
question
Most of the carbon in our bodies was formed in
a. the core of a red-giant star.
b. nearby galaxy.
c. supernova.
d. the core of the Sun.
answer
a. the core of a red-giant star.
question
A surface explosion on a white dwarf, caused by falling matter from the atmosphere of its binary companion, creates what kind of object?
a. hypernova
b. nova
c. gamma ray burstar
d. type I supernova
e. type II supernova
answer
b. nova
question
The supernova that formed M-1, the Crab Nebula, was observed in:
a. 1054 AD by Chinese and Middle Eastern astronomers.
b. about 9,000 BC by all our ancestors.
c. 1572 AD by Tycho Brahe.
d. 1006 by observers in the southern hemisphere.
e. 1604 AD by Johannes Kepler.
answer
a. 1054 AD by Chinese and Middle Eastern astronomers.
question
What produces a type-I supernova?
a. a nova igniting a helium flash in its red giant companion
b. mass transfer onto a white dwarf pushing it over 1.4 solar masses
c. the helium flash blows apart a giant's core
d.the collapse of the core of a massive star
e. the radioactive decay of nickel 56 into cobalt 56 into iron 56
answer
b. mass transfer onto a white dwarf pushing it over 1.4 solar masses
question
Type II supernovae and star forming regions are related to one another because:
a. as a result of both processes, lighter elements are transformed into heavier elements.
b. they both involve high mass ionizing stars.
c. the shock waves of a supernova can trigger star formation.
d. they both contain ionized hydrogen.
e. All of the above.
answer
e. All of the above.
question
For a white dwarf to explode entirely as a Type I supernova, its mass must be:
a. at least 0.08 solar masses.
b. 1.4 solar masses, the Chandrasekhar Limit.
c. 3 solar masses, the Schwartzschild Limit.
d. 20 solar masses, the Hubble Limit.
e. 100 solar masses, the most massive known stars.
answer
b. 1.4 solar masses, the Chandrasekhar Limit.
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