A nova is a star that suddenly increases greatly in brightness, then slowly fades back to its normal appearance over a period of months. It results when a white dwarf in a binary system draws hydrogen-rich material from its companion. The gas builds up on the white dwarf's surface, eventually becoming hot and dense enough for the hydrogen to burn explosively, temporarily causing a large increase in the dwarf's luminosity.
The matter flowing from the companion star does not fall directly onto the surface of the dwarf. Instead it goes into orbit around it, forming an accretion disk. Friction within the disk causes the gas to spiral slowly inward, heating up and glowing brightly as it nears the dwarf's surface.
Stars more massive than about 8 solar masses are able to attain high enough central temperatures to burn carbon and heavier nuclei. As they burn, their cores form a layered structure consisting of burning shells of successively heavier elements. A nonburning core of iron builds up at the center.
Iron is special in that its nuclei can neither be fused together nor split apart to produce energy. As a result, stellar nuclear burning stops at iron. As a star's iron core grows in mass it eventually becomes unable to support itself against gravity and begins to collapse. At the enormous densities and temperatures produced during the collapse, iron nuclei are broken down into their constituent particles—protons and neutrons. The protons combine with electrons to form more neutrons. Eventually, when the core has become so dense that the neutrons are effectively brought into physical contact with one another, their resistance to further squeezing stops the collapse and the core rebounds, sending a violent shock wave out through the rest of the star. The star is blown to pieces in a core-collapse supernova.
Astronomers classify supernovae into two broad categories: Type I and Type II. These classes differ by their light curves and by their composition. Type I supernovae are hydrogen poor and have a light curve similar in shape to that of a nova. Type II supernovae are hydrogen rich and have a characteristic bump in the light curve a few months after maximum. A Type II supernova is a core-collapse supernova. A Type I supernova occurs when a carbon—oxygen white dwarf in a binary system exceeds about 1.4 solar masses (the Chandrasekhar)—the maximum mass that can be supported against gravity by electron degeneracy pressure. The star collapses and explodes as its carbon ignites. This type of supernova is called a carbon-detonation supernova.
Theory predicts that a supernova visible from Earth should occur within our Galaxy about once a century, although none has been observed in the last 400 years. We can see evidence of a past supernova in the form of a supernova remnant, a shell of exploded debris surrounding the site of the explosion and expanding into space at thousands of kilometers per second.
All elements heavier than helium formed by stellar nucleosynthesis—the production of new elements by nuclear reactions in the cores of evolved stars. Elements beyond carbon tend to form by helium capture rather than by the fusion of two heavy nuclei. Therefore, nuclei whose masses are a multiple of the mass of a helium nucleus tend to be more common than others.
At high enough core temperatures, photodisintegration breaks apart some heavy nuclei, providing helium-4 nuclei for the synthesis of even more massive elements, leading to a buildup of iron-56 in the core. Elements beyond iron form by neutron capture in the cores of evolved stars. With no repulsive electromagnetic barrier to overcome, neutrons can easily combine with nuclei. During a supernova explosion, rapid neutron capture occurs, producing the heaviest nuclei of all.
Comparisons between theoretical predictions of element production and observations of element abundances in stars and supernovae provide strong support for the theory of stellar nucleosynthesis.
The processes of star formation, evolution, and explosion form a cycle that constantly enriches the interstellar medium with heavy elements and sows the seeds of new generations of stars. Without the elements produced in supernovae, life on Earth would be impossible.
1. A nova is a sudden outburst of light coming from an old main-sequence star. (Hint)
2. Some, but not all, novae occur in binary star systems. (Hint)
3. It takes less and less time to fuse heavier and heavier elements inside a high-mass star. (Hint)
4. In a core-collapse supernova, the outer part of the core rebounds from the inner, high-density core, destroying the entire outer part of the star. (Hint)
5. Most of the energy released during a supernova is emitted in the form of neutrinos. (Hint)
6. A Type I supernova is the same as a nova, but it appears much brighter because it occurs closer to us. (Hint)
7. The spectrum of a Type II supernova shows the presence of lots of hydrogen. (Hint)
8. It is possible that a recurrent nova will eventually result in a core-collapse supernova. (Hint)
9. Once the process gets underway, the core of a massive star collapses in about 1 second. (Hint)
10. Different isotopic forms of an element differ only in the number of protons in the nucleus. (Hint)
11. Carbon can fuse with helium more easily than it can fuse with another carbon atom because there is more helium than carbon in stars. (Hint)
12. The s-process is one in which heavy elements are formed from silicon. (Hint)
13. The r-process occurs only during the first few minutes of a supernova explosion. (Hint)
14. Stellar nucleosynthesis can account for the existence of all elements except hydrogen and helium. (Hint)
15. Stellar nucleosynthesis causes the abundance of heavy elements in interstellar gas and newborn stars to increase with time. (Hint)
1. In a semidetached binary consisting of a white dwarf and a main-sequence or giant companion, matter leaving the companion forms an _____ disk around the dwarf. (Hint)
2. A nova explosion is due to _____ fusion on the _____ of a white dwarf. (Hint)
3. In the collapsing core of a massive star, photodisintegration is caused by the very high _____ in the core. (Hint)
4. When a proton and an electron are forced together, they combine to form a _____ and a _____ . (Hint)
5. Core collapse in a massive star is eventually stopped by _____ degeneracy pressure. (Hint)
6. A Type _____ supernova occurs when the core of a massive star implodes and rebounds. (Hint)
7. A Type _____ supernova occurs when a white dwarf exceeds the Chandrasekhar mass. (Hint)
8. The maximum mass for a white dwarf is roughly _____ solar masses. (Hint)
9. The two types of supernovae can be distinguished observationally by their spectra and by their _____. (Hint)
10. The first evidence of supernova 1987A was a burst of _____. (Hint)
11. Supernova 1987A surprised astronomers because its progenitor star was a _____ instead of a _____ at the moment of the explosion. (Hint)
12. Elements differing by 4 mass units, such as carbon, oxygen, neon, and silicon, are produced in stars by _____. (Hint)
13. By the time silicon appears in the core of a massive star, the temperature has reached _____ , sufficient to break apart heavy nuclei. (Hint)
14. Neutron capture is responsible for the formation of elements heavier than _____. (Hint)
15. Because of stellar nucleosynthesis, the spectra of old stars show _____ heavy elements than those of young stars. (Hint)
1. Under what circumstances will a binary star produce a nova? (Hint)
2. What is a light curve? How can it be used to identify a nova or a supernova? (Hint)
3. What occurs in a massive star to cause it to explode? (Hint)
4. What are the observational differences between Type I and Type II supernovae? (Hint)
5. How do the mechanisms that cause Type I and Type II supernovae explain their observed differences? (Hint)
6. Roughly how often would we expect a supernova to occur in our own Galaxy? How often would we expect to see a Galactic supernova? (Hint)
7. What evidence is there that many supernovae have occurred in our Galaxy? (Hint)
8. How can astronomers estimate the age of an isolated star? (Hint)
9. What proof do astronomers have that heavy elements are formed in stars? (Hint)
10. As a star evolves, why do heavier elements tend to form by helium capture rather than by fusion of like nuclei? (Hint)
11. Why do the cores of massive stars evolve into iron, not heavier elements?
12. How are nuclei heavier than iron formed? (Hint)
13. What is the r-process? When and where does it occur? (Hint)
14. Why was supernova 1987A so important? (Hint)
15. Why are neutrino detectors important to the study of supernovae? (Hint)
1. Estimate how close an 0.5-solar mass white dwarf must come to the center of a 2-solar mass subgiant of radius 10 times that of the Sun in order for the white dwarf's tidal field to strip matter from the companion's surface. (Hint)
2. A certain telescope could just detect the Sun at a distance of 10,000 pc. What is the maximum distance at which it could detect a nova having a peak luminosity of 105 solar luminosities? (Hint)
3. Repeat the previous calculation for a supernova having a peak luminosity 1010 times that of the Sun. (Hint)
4. At what distance would a supernova of absolute magnitude -20 look as bright as the Sun? Would you expect a supernova to occur that close to us? (Hint)
5. A (hypothetical) supernova at a distance of 20 pc has an absolute magnitude of -20. Compare its apparent magnitude with that of (a) the full Moon. (b) Venus at its brightest. (See Figure 17.9.) (Hint)
6. A supernova's energy is often compared to the total energy output of the Sun over its lifetime. Using the Sun's current energy output, calculate its total energy output, assuming it has a 1010 year main-sequence lifetime. How does this compare with the energy released by a supernova? (Hint)
7. The Crab Nebula is now about 1 pc in radius. If it was observed to explode in A.D. 1054, roughly how fast is it expanding? (Assume a constant expansion rate. Is that a reasonable assumption?) (Hint)
8. As we will see in Chapter 23, the star-forming portion of our Galaxy consists of a highly flattened circular disk about 30 kpc (30,000 pc) in diameter. Interstellar extinction limits our view to within a radius of about 3 kpc of the Sun. If supernovae occur in the Galaxy roughly once every 30 years, on average, and are uniformly spread throughout the disk, calculate how often we should expect to see a supernova. (Hint)
9. Assuming the data in the previous question and taking all supernovae, for simplicity, to have absolute magnitude -20, calculate how often we should expect to observe a supernova brighter than the full Moon (apparent magnatude
12.5). (Hint)
10. Based on the data in Table 21.1, estimate the total masses of "iron-group" and "heaviest-weight group" elements in the Sun. Compare these with Earth's mass. (Hint)
1. In 1758 the French comet hunter Charles Messier discovered the sky's most legendary supernova remnant, now called M1, or the Crab Nebula. An 8-inch telescope reveals the Crab's oval shape, but it will appear faint. It is located northwest of Zeta Tauri, the star that marks the southern tip of the horns of Taurus the Bull. A 10-inch or larger telescope reveals some of its famous filamentary structure.
2. In the Handbook of Chemistry and Physics, available in any library reference section, look up the table of isotopes. Pick one or more isotopes and follow their decay into a final stable isotope. For example, choose cobalt-59, formed in the s-process. Note how the isotope decays, what is emitted, and the half-life of the decays. Try this exercise for uranium-235, uranium-238, and plutonium-239.
