Supernova searches, organized on a regular basis since 1936, have discovered more than 800 extragalactic supernovae. Supernova 1987A was a recent naked-eye supernova. In addition, past supernovae in our Galaxy have been recorded in Western Europe (Tycho's star of 1572 and Kepler's star of 1604) and in China and Korea (ad 185, 386?, 393, 1006, 1054, and 1181). The total supernova production rate in the Galaxy is currently estimated as two or three per century.
Theoretically, two different explosion mechanisms can be distinguished: thermonuclear explosions and core collapse supernovae. Thermonuclear explosions probably occur when a white dwarf or the degenerate core of a moderately massive star is pushed over the Chandrasekhar limit. The degenerate matter containing mainly carbon and oxygen nuclei then ignites explosively, leading to a thermonuclear runaway and probably the complete disruption of the star. The ejecta will contain a large amount of radioactive nickel, produced in the nuclear runaway, whose decay to cobalt and then iron may explain the exponential decay observed in many supernova light curves. Supernovae of this type are probably a major source of iron in the Galaxy.
Core collapse supernovae occur when all the material in the core of a massive star has been completely burned to iron. Because an iron core is incapable of generating any more energy by nuclear fusion, it has to collapse (if its mass exceeds the Chandrasekhar mass) to become a neutron star; if the star is extremely massive it may collapse completely to a black hole. The link between the core's implosion and the explosion of the outer layers is still uncertain. One possibility is that, when the core reaches neutron-star densities, the newly formed neutron-star core suddenly presents a rigid surface to the infalling gas and the collapse stops, driving a shock wave into the overlying material that may blow off the star's outer layers. The high pressures and temperatures suddenly created in the layer just above the neutron star cause a spate of nuclear reactions; neutrons from these reactions build up very heavy nuclei by the r-process.
Observationally, supernovae have traditionally been divided into two main types, based on the absence (type I) or presence (type II) of hydrogen in their spectra. Recent observations have revealed a large diversity of supernova types and the need for further subclassification (e.g. type Ia, Ib, Ic; type II-P, II-L, IIb). This classification is further complicated by the fact that there is no simple one-to-one relationship between the theoretical explosion mechanisms and the two main observational types. Recently a more energetic type of supernova, called a hypernova, has been observed, possibly related to gamma-ray bursts.
Type Ia supernovae occur in both elliptical and spiral galaxies (but without notable preference for spiral arms); most of them have very similar light curves and absolute magnitudes (–19) at maximum. Contrary to earlier beliefs, type Ia supernovae are not perfect standard candles. However, observationally there appears to be a tight correlation between the shape of the supernova distances to fairly large redshifts (z ˜ 1). In recent years, high-z supernova searches have found a large number of distant type Ia supernovae. These can be used to measure the expansion rate of the Universe when it was much younger. At face value, the results suggest that the expansion of the Universe if accelerating rather than decelerating – as it would in the standard cosmology. However, an alternative explanation might be that type Ia supernovae were systematically different at an earlier cosmological epoch than the present one. Their spectra reveal very broad absorption lines (for example, of silicon), but generally show no evidence for hydrogen in the ejected gas, which has a mass of roughly a solar mass and a velocity around 10 000 km s–1; the total energy is about 1044 joules. The pre-supernova of a type Ia supernova is still uncertain, but a popular theory is that it is a white dwarf that accretes enough matter from a companion star or that merges with another lighter white dwarf to be pushed above the Chandrasekhar mass, leading to a thermonuclear explosion.
Type Ib/Ic supernovae, which unlike type Ia supernovae show no evidence for silicon in their spectra, probably make up the bulk of type I supernovae. They are less luminous than type Ia supernovae, indicating that they produce less radioactive material, and they are probably intrinsically related to type II supernovae. Their progenitors may be massive stars that have lost all of their hydrogen-rich envelopes in a stellar wind (see Wolf–Rayet stars) or by mass transfer in a binary (see helium star). Unlike type Ia supernovae, they probably experience core collapse and leave neutron-star or black-hole remnants.
Type II supernovae occur mainly in the spiral arms of spiral galaxies. They show more diversity in light curves and absolute magnitude than type I, although many of them reach a maximum of around –17. Their early spectra show a normal abundance of the elements and indicate that several solar masses are ejected at around 5000 km s–1. The pre-supernova, in most cases (but not in the case of supernova 1987A), is a red supergiant with a diameter of about 10 AU. The mass in its hydrogen-rich envelope may vary from a few tenths of a solar mass to several tens of solar masses. This large range in envelope masses is probably the main reason for the large diversity of type II light curves. The explosion mechanism in most cases is believed to be core collapse, but some type II supernovae may be caused by thermonuclear explosions.
a star that has undergone a catastrophic explosion followed by an enormous increase in brightness. At maximum brightness, the luminosity of a supernova is a billion times greater than the luminosity of such stars as the sun, sometimes exceeding the luminosity of the entire galaxy in which the supernova is located. The maximum brightness of a supernova occurs approximately two to three weeks after the explosion. The brightness subsequently decreases gradually, diminishing by a factor of 25 to 50 during the following 100 days. In a galaxy such as ours, there is an average of one or two supernovae per century. The last supernovae in our galaxy were observed by Tycho Brahe in 1572 and by J. Kepler in 1604. It is possible that in our galaxy during the last three centuries there have been several more supernovae that were not observed because their light was strongly absorbed by interstellar dust. In observing a large number of galaxies simultaneously, modern astronomers discover 15 to 20 extragalactic supernovae each year. The term “supernova” is applied to these objects by analogy with the term “nova”; it stresses the much greater power of these outbursts.
Supernovae are divided into two types according to the nature of the change in brightness over time and to the supernova spectrum. Type I supernovae are usually three to five times brighter than type II supernovae, and their brightness diminishes more slowly after reaching maximum. The spectra of type II supernovae have wide emission lines—their most characteristic feature; type I supernovae have very broad absorption lines. Type II supernovae also differ in having spectra with wide hydrogen lines, which are virtually absent in the spectra of type I supernovae.
The discovery of the products of supernova explosions in the Milky Way Galaxy was of great significance in the study of supernovae. These products consist of gas envelopes, called supernova remnants, which are expanding at great speed, and starlike objects, called pulsars. The latter are rapidly revolving neutron stars, characterized by radio emission, which pulsate with a period equal to their period of rotation. Supernova remnants are sources of synchrotron radiation, which occurs when high-energy electrons are braked in the magnetic fields of the gas envelopes. Some supernova remnants are also sources of thermal X-ray emission with temperatures of 106°–107°K. The Crab Nebula, which is located at the position where Chinese and Japanese chronicles record a bright supernova in 1054, may be considered the most impressive of all supernova remnants in our galaxy. In addition to the odd, filamentous nebula, which is expanding at a velocity of approximately 1,500 km/ sec, this remnant has a pulsar with an emission period of 0.033 sec in the radio-optical, X-ray, and gamma-ray bands. In some respects, the supernova of 1054 cannot be considered either type I or type II.
An analysis of available observation data on supernovae and remnants enables us to give a general outline of the evolution of a supernova (typical parameters are shown in Table 1). When the supernova explodes, a significant share of the mass of the star (in some cases, possibly the entire mass) is converted into an envelope, which expands at velocities up to 20,000 km/sec. The increase in brightness is related to a large degree to an increase in the radius of the radiating surface. At maximum brightness, the supernova has a colossal radius, exceeding that
| Table 1. Characteristics of supernovae | ||
|---|---|---|
| Parameters | Type I supernova | Type II supernova |
| Mass of the ejected envelope (in units of solar mass).......... | 0.1–0.5 | approximately 1 |
| Expansion velocity at maximum brightness (km/sec) ......... | 10,000–20,000 | 5,000–15,000 |
| Temperature at maximum brightness (°K) ................ | 15,000–20,000 | 10,000–15,000 |
| Total energy of emission (ergs)....................... | 1049–1050 | 3 × 1048-3 × 1049 |
| Kinetic energy of the envelope (ergs) ................... | 1050-1051 | 2 × 1050-2 × 1051 |
of the sun by 20,000–40,000 times. As the envelope expands, its density decreases. With continuing expansion in the interstellar medium, the envelope begins interacting with interstellar gas, leading to the formation of a shock wave. This causes the envelope to heat up and lose velocity. In tens of thousands of years, the supernova remnant engulfs a volume of space with a radius of more than 10 parsecs; the space is filled with hot plasma at a temperature of approximately 106°K. On the boundary of this space is a layer of cooler and denser interstellar gas entrained during the envelope’s expansion; the mass of this gas reaches several hundred times that of the sun (a typical example of such a supernova remnant is the Veil Nebula in the constellation Cygnus). After hundreds of thousands of years, the envelope’s expansion velocity decreases to a magnitude of the order of 10 km/sec, and the envelope can no longer be identified against the background of chaotically moving clouds of interstellar gas.
As of the 1970’s, astronomical theory is unable to give a definite explanation of the mechanism of supernova outbursts. It appears, however, that the supernova explosion may result from the instability that arises in the later stages of the evolution of a star. The two most probable mechanisms of the outbursts are the thermonuclear explosion of a degenerated carbon nucleus and gravitational collapse, that is, a catastrophic collapse of stellar matter toward the center of the star when the star’s thermonuclear energy is completely exhausted. In the latter case, it is assumed that under certain conditions the intense liberation of gravitational energy causes the exterior layers of the star to fly apart.
One of the most interesting aspects of supernova physics is the role supernovae play in the thermonuclear fusion of chemical elements and the transformation of the chemical composition of the Milky Way Galaxy. At the moment of explosion, a significant share of a supernova’s mass in the form of hydrogen and helium is converted by thermonuclear reactions into elements with greater atomic weights. In the explosion, conditions arise for the fusion of even heavier elements, including those of the iron group. As a result, the matter released by supernovae into the interstellar medium is enriched with heavy elements. During the early history of our galaxy, quite a large number of supernovae exploded, which substantially changed the Galaxy’s initial chemical composition. Observations show that the oldest stars in our galaxy contain 100–1,000 times less matter composed of heavy elements than the sun and other stars that formed later.
The origin of cosmic rays in the Milky Way Galaxy is also significantly linked to supernovae. It is assumed that the acceleration of cosmic rays occurs in the electromagnetic fields of pulsars and, partially, in the shock waves of expanding supernova envelopes.
E. R. MUSTEL’ and N. N. CHUGAI