As their names imply, red giant stars and asymptotic giant branch (AGB) stars are both giant and red. Which one statement below correctly describes the fundamental difference between them?
Red giants have a hydrogen-burning shell, whereas AGB stars have both a hydrogen- and a helium-burning shell.
Yes. Red giants go on to ignite core helium burning and become horizontal branch stars, then after core helium burning ends and a helium shell is ignited (with hydrogen shell burning around it), they become AGB stars. Section 20-1
A star moving up the red giant branch for the second time will have
no nuclear fusion occurring in the core, but helium fusion reactions taking place in a shell around the core and a quiescent layer of hydrogen surrounding this region.
1 out of 1
Yes. The hydrogen and helium in the core are depleted, and gravitational energy has compressed this region and increased its temperature to the point where helium reactions can occur in a shell surrounding the core. Section 20-1
As massive stars evolve, they burn successively heavier elements (helium, carbon, oxygen, silicon) in their cores. Why is a higher temperature needed in each succeeding stage?
c) Heavier nuclei have higher electric charges, so higher speeds are needed for the nuclei to overcome the electrostatic repulsion and undergo nuclear fusion.
1 out of 1
Yes. Fully ionized nuclei of heavier elements have progressively higher electrical charges and hence greater mutual electrostatic repulsion. This repulsion can be overcome only by increased particle velocities and therefore higher temperatures. Section 20-5
In which of the following atomic species will energy generation not occur when their nuclei undergo nuclear fusion?
1 out of 1
Yes. Iron nuclei do not produce energy when they fuse together. In fact, they need extra energy to undergo fusion. Section 20-5
The progenitor star of supernova 1987A was a blue supergiant, whereas most progenitors of massive-star supernovae are red supergiants. What major observational effect did this have on the supernova?
c) It was not as bright as most other core-collapse supernovae because it is harder to eject the outer layers from a blue supergiant than from a red supergiant.
1 out of 1
Yes. Blue and red supergiants have similar masses, but the blue supergiant is smaller. Consequently, the blue supergiant exerts a stronger gravitational pull on its outer layers, and it takes more energy to eject these layers into space. Section 20-7
What causes a Type Ia supernova?
c) In a binary star system, a giant star filling its Roche lobe dumps gas onto a white dwarf, putting the white dwarf over the Chandrasekhar mass limit.
1 out of 1
Yes. Type Ia supernovae involving a white dwarf exploding can occur only in a binary star system where mass transfer increases the mass of the white dwarf. Section 20-9
How does a planetary nebula form?
Episodes of thermal runaway in the helium-burning shell of a low-mass star push the envelope of the star off into space. The expelled envelope forms a planetary nebula.
Yes. This happens at the end of the life of a low-mass star like the Sun, and is a slow expansion, not a violent explosion. Section 20-3
A planetary nebula appears as a glowing shell or cloud around a very hot, white dwarf star. What are the physics and dynamics of this radiating region?
The shell is the expanding atmosphere of the star. The intense UV radiation from the remaining core of this star excites and ionizes the gas, causing atoms to emit atomic line radiation.
Yes. The expanding atmosphere of the star makes up the shell of the planetary nebula. This represents a late phase in the evolution of a low-mass star. Section 20-3
As a white dwarf star slowly evolves over billions of years,
its temperature and luminosity both decrease, but its size remains constant.
Yes. Electron degeneracy prevents further reduction in size but radiation cools it, leading to decreased luminosity. Section 20-4
Which of the following will a high-mass star (say, 25 solar masses) not do at or near the end of its life?
Eject its outer layers and become a white dwarf.
Yes. High-mass stars explode as supernovae, leaving a cloud of debris with a neutron star at its center. Section 20-5
What causes a massive star to explode as a supernova?
Fusion reactions produce an iron-rich core in the star, and this core collapses because its temperature is so high.
Yes. The high temperature results in large energy losses due to neutrino emission, but iron cannot replace this energy by fusion reactions, and the core collapses. Section 20-6
What is believed to be the usual process that causes the actual explosion in a type II supernova (massive star exploding)?
The core collapses inward gravitationally until it has the density of an atomic nucleus, then stops. The envelope collapsing against this hard core then explodes outwards.
Yes. The envelope collapsing against this hard core generates a shock wave that disrupts and ejects the envelope in a supernova explosion, leaving the core behind as a neutron star. Section 20-6
Cerenkov light is produced whenever a charged particle
travels in a transparent medium faster than the speed of light in that medium.
Yes. While no particle can travel faster than the speed of light in a vacuum, it can move through a transparent medium faster than the speed of light in that medium, and will then produce Cerenkov radiation. Section 20-8
Carbon stars are
asymptotic giant branch stars with hydrogen-rich envelopes, in which carbon has been "dredged up" from the carbon-rich core by convection
Yes. During the AGB phase, convection in the hydrogen-rich envelope can reach down into the helium-exhausted (carbon-rich) core. The convection then mixes this carbon throughout the envelope. Carbon-rich molecules form at the star's surface, creating prominent absorption bands in the star's spectrum. Section 20-2
Where do white dwarf stars come from?
They are the former cores of asymptotic giant branch stars that have pushed off their outer layers, leaving the core behind.
Yes. The envelope is pushed off quite smoothly (not explosively), producing a planetary nebula around a white dwarf star. Section 20-4
Which of the following household items is the closest analogy to a white dwarf star?
An incandescent bulb.
No. While the bulb has a continuous spectrum and its size remains constant (like a white dwarf), it has an internal energy source and its "luminosity" and temperature are constant. Section 20-4
An important quantity in astronomy is the Chandrasekhar limit, which is equal to
1.4 solar masses, the maximum mass of a white dwarf star.
Yes. If the mass of a white dwarf star exceeds the Chandrasekhar limit, then electron degeneracy cannot prevent core contraction. The star then proceeds to the supernova stage (immediately, for a white dwarf, or after some intervening stages for a more massive star). Sections 20-4 and 20-5
During a supernova explosion involving a high-mass star, what happens to the iron in the core of the star?
The iron nuclei are broken up by high-energy photons into their individual protons and neutrons (photo disintegration).
Yes. The temperature reaches about 5 billion K during the core collapse, which initiates the supernova explosion; at this temperature the gamma ray photons are energetic enough to photodisintegrate the iron nuclei into protons and neutrons. Section 20-6
The brightness of a supernova just after the explosion is equivalent to the total output of
all the stars in a typical spiral galaxy, up to 109 Lυ.
Yes. A supernova can easily outshine the galaxy in which it resides, for a few days. Section 20-7
Observations of supernovae in distant galaxies similar to our own Milky Way suggest that they occur at the rate of 1 every 20 years per galaxy. The last recorded supernova in our galaxy was by Kepler in 1604, almost 400 years ago. Why is this?
Most supernovae have occurred in the disk of our galaxy, and our view of this disk is obscured by dense dust and gas clouds; hence we have not seen the majority of supernova explosions that have occurred in the Milky Way.
Yes. Our view within the disk of our galaxy is limited to 10,000 light years or so by dense dust clouds. Section 20-10
What is the last nuclear burning stage in the life of a low-mass star like the Sun?
fusion of helium nuclei to form carbon and oxygen
A "carbon star" has more carbon on its surface than does the Sun. This is the result of
dredge-up, in which the convective envelope transports material from a star's core to its surface
All of the 12C in the universe, including that in our bodies, is believed to come from
the triple alpha process in helium burning in red giants.
A planetary nebula is
a disk-shaped nebula of dust and gas from which planets will eventually form, easily
photographed around relatively young stars.
The physical process believed to provide the energy for the ejection of a planetary nebula from a star is
a series of thermal pulses in a helium-burning shell.
The shell of a planetary nebula is measured by the Doppler shift of emission lines to be expanding outward at a speed of 10 km/s and its diameter is measured to be 1 light-year. How long has the shell been expanding?
Our Sun will end its life by becoming a
In which order will a single star of about 1 solar mass progress through its various stages of evolution?
T Tauri, main sequence, planetary nebula, white dwarf
Which physical phenomenon keeps a white dwarf star from collapsing inward on itself?
electron degeneracy or "quantum crowding"
What is the mass limit above which the self-gravity of stars can overcome electron degeneracy pressure?
1.4 solar masses
A sequence of thermonuclear fusion processes inside massive stars can continue to transform the nuclei of elements such as carbon, oxygen, etc. into heavier nuclei and also generate excess energy, up to a limit beyond which no further energy-producing reactions can occur. The element that is produced when this limit is reached is
It is now thought that most elements in the universe heavier than iron in the periodic table
are produced by nuclear reactions in the shock wave regions surrounding supernova
One technique for the detection of neutrinos from outer space involves the reaction of neutrinos with protons in a water tank, with the resulting high-speed positrons then generating Cerenkov radiation in the water. How is this Cerenkov radiation produced?
by the charged positron moving in the water faster than the speed of light in water
The detection of neutrinos from supernova SN 1987A occurred three hours before the detection of the burst of visible light. What caused this time lag?
The neutrinos were produced earlier (when the core collapsed) and the light was produced three hours later (when the shock wave reached the outer layers).
A Type Ia supernova is the
explosion of a white dwarf in a binary star system after mass has been transferred onto it from its companion.