The core collapse supernovae are thought to appear as type Ib/c and type II supernovae, and are associated with young stellar populations. In contrast, the thermonuclear detonation of a carbon-oxygen white dwarf, whose mass approaches the Chandrasekhar limit, is thought to produce type Ia supernovae (2,3). Such supernovae are observed in both young and old stellar environments (4). White dwarfs represent an intriguing state of matter; another reason is that most stars, including our Sun, will become white dwarfs when they reach their final, burnt-out collapsed state. White dwarfsare among the dimmest stars in the universe. Even so, they have commanded the attention of astronomers ever since the first white dwarf was observed by optical telescopes in the middle of the 19th century (1).
Perets et al.(4) discovered a faint type Ib supernova, SN 2005E, in the halo of the nearby isolated galaxy, NGC 1032. The unusual helium-rich (type Ib) supernova SN 2005E is distinguished from all supernovae hitherto observed by its faint and rapidly fading light curve, prominent calcium lines in late-phase spectra and lack of any mark of recent star formation near the supernova location. These properties claimed by Perets(4) to be explained by a helium detonation in a thin surface layer of an accreting white dwarf(5). Kawabata et al (5) reported that the observed properties of SN 2005cz, which appeared in an elliptical galaxy, resemble those of SN 2005E. An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth nearly featureless brightness profile. They argued that these properties are best explained by a core-collapse supernova at the low-mass end (8-12 solar masses) of the range of massive stars that explode (6). Such a low-mass progenitor lost its hydrogen-rich envelope through binary interaction, had very thin oxygen-rich and silicon-rich layers above the collapsing core, and accordingly ejected a very small amount of radioactive 56Ni and oxygen. Although the host galaxy NGC 4589 is an elliptical, some studies have revealed evidence of recent star-formation activity (7), consistent with core-collapse model.
Type II supernovas show conspicuous evidence for hydrogen in the expanding debris ejected in the explosion; Type Ia explosions do not. Recent research has led to a refinement of these types, and a classification in terms of the types of stars that give rise to supernovas. A Type II, as well as Type Ib and Type Ic explosions, is produced by the catastrophic collapse of the core of a massive star. A Type Ia supernova is produced by a sudden thermonuclear explosion that disintegrates a white dwarf star. Type II supernovas occur in regions with lots of bright, young stars, such as the spiral arms of galaxies. They apparently do not occur in elliptical galaxies, which are dominated by old, low-mass stars. Since bright young stars are typically stars with masses greater than about 10 times the mass of the sun, this and other evidence led to the conclusion that Type II supernovae are produced by massive stars.
Some Type I supernovas show many of the characteristics of Type II supernovas. These supernovas, called Type Ib and Type Ic, apparently differ from Type II because they lost their outer hydrogen envelope prior to the explosion. The hydrogen envelope could has been lost by a vigorous outflow of matter prior to the explosion, or because it was pulled away by a companion star.
Supernovas -core-collapse type
The core of a star collapses when the nuclear power source at the center or core of a star is exhausted. In less than a second, a neutron star (or a black hole, if the star is extremely massive) is formed. Such extreme forces occur in nature when the central part of a massive star collapses to form a neutron star. The atoms are crushed completely, and the electrons are jammed inside the protons to form a star composed almost entirely of neutrons. The result is a tiny star that is like a gigantic nucleus and has no empty space. Neutron stars are strange and fascinating objects. They represent an extreme state of matter that physicists are eager to know more about. All but the central neutron star is blown away at speeds in excess of 50 million kilometers per hour as a thermonuclear shock wave races through the now expanding stellar debris, fusing lighter elements into heavier ones and producing a brilliant visual outburst that can be as intense as the light of several billion Suns. The core-collapse supernovas are generally Type II, Type Ib and Type Ic
Supernovas –thermonuclear type
Type Ia supernovae produce white dwarf stars which are the condensed remnant of what used to be sun-like stars. A white dwarf star, a dense ball primarily composed of carbon and oxygen atoms, is intrinsically the most stable of stars, as long as its mass remains below the so-called Chandrasekhar limit of 1.4 solar masses. While still in his twenties Subrahmanyan Chandrasekhar, the Chandra X-ray Observatory's namesake, used relativity theory and quantum mechanics to show that degenerate electron pressure can do only so much. If the mass of the white dwarf becomes greater than about 1.4 times the mass of the Sun—called the Chandrasekhar limit—it will collapse. In a binary star system this could happen if a nearby companion star dumps enough material onto a white dwarf to push it over the Chandrasekhar limit.
The resulting collapse and explosion of the white dwarf is believed to be responsible for the so-called Type Ia supernovas. If, however, accretion of matter from a companion star or the merger with another white dwarf, push a white dwarf star over the Chandrasekhar limit of 1.4 solar masses, the temperature in the core of the white dwarf will rise, triggering explosive nuclear fusion reactions that release an enormous amount of energy. The star explodes in about ten seconds, leaving no remnant. The expanding cloud of ejecta the explosion decays into cobalt and then iron. Because Type Ia supernovas all occur in a star that has a mass of about 1.4 solar masses, they produce about the same amount of light. This property makes them extremely useful as a distance indicator - if one Type Ia supernova is dimmer than another one, it must be further away by an amount that can be calculated. In recent years Type Ia supernova have been used in this way to determine the rate of expansion of the universe. This research has led to the astounding discovery that the expansion of the universe is accelerating, possibly because the universe is filled with a mysterious substance called dark energy (1).
Supernovas -pair-instability type
The massive star is the source for formation of more violent type pair- instability supernovas. According to stellar evolution theory, temperatures rise to several billion degrees in the central regions of stars with masses between 140 and 260 suns. At these temperatures the usual process of converting mass into energy (E = mc2) by nuclear reactions is reversed, and energy is converted into mass in the form of pairs of electrons and anti-electrons, or positrons. The production of electron-positron pairs saps energy from the core of the star, disturbing the equilibrium between the outward push of pressure and the inward crush of gravity. This so-called "pair instability" cause violent pulsation that eject a large fraction of the outer layers of the star, and eventually disrupt the star completely in a thermonuclear explosion. Pair-instability supernovas, if they exist, would be the most energetic thermonuclear explosions in the universe. In stars with masses greater than about 260 suns, the pulsations would be overwhelmed by gravity and the star would collapse to form a black hole without an explosion (1). A black hole, according to the general theory of relativity, is a region of space from which nothing, including light, can escape. It is the result of the deformation of space-time caused by a very compact mass. Around a black hole there is an undetectable surface which marks the point of no return, called an event horizon. It is called "black" because it absorbs all the light that hits it, reflecting nothing, just like a perfect black body in thermodynamics. Under the theory of quantum mechanics, black holes possess a temperature and emit Hawking radiation, but for black holes of stellar mass or larger this temperature is much lower than that of the cosmic background radiation (8). Observations with Chandra (1) and optical telescopes indicate that supernova 2006gy, the most luminous supernova ever recorded, may be a long sought (40 years) pair-instability supernova.
Remnants of supernova
The intense radiation emitted by a supernova lasts from several months to a few years before fading away. In the meantime, the rapidly expanding (millions of miles per hour) matter from the explosion eventually crashes into circumstellar gas. This collision creates a supernova remnant consisting of hot gas and high-energy particles that glow in radio through X-ray wavelengths for thousands of years. The process of forming the remnant is somewhat like an extreme version of sonic booms produced by the supersonic motion of an airplane. Expanding stellar debris creates a shock wave that races ahead of the debris. This forward shock wave produces sudden, large changes in pressure and temperature behind the shock wave. The forward shock wave also accelerates electrons and other charged particles to extremely high energies. Electrons spiraling around the magnetic field behind the shock wave produce radiation over a wide range of wavelengths. Radiation from supernova remnants is especially conspicuous at radio wavelengths, and radio telescopes have traditionally been the primary tools for discovering these objects. In recent years, supernova remnants have also been discovered with focusing X-ray telescopes. The X-rays are produced by the forward shock wave and by a reverse shock wave that heats the debris, or ejecta, of the exploded star. The reverse shock is formed as the high pressure gas behind the forward shock wave expands and pushes back on the stellar ejecta. A Chandra observation of the supernova remnant Cassiopeia A (Cas A) clearly shows both the outer shock wave and the debris heated by the reverse shock wave. The study of supernova remnants with radio, infrared, optical and X-ray telescopes enables astronomers to trace the progress of the shock waves and distribution of elements ejected in the explosion.
References :-
1. http://chandra.harvard.edu/xray_sources/supernovas.html 02/08/2010)
2. Filippenko, A.V.Optical spectre of supernovae. Annu.Rev.Astrophys, 35.309-355 (1997).
3. Mazzali,P.A et al. A common explosion mechanism for type Ia supernovae. Science. 315, 825-828 (2007).
4. Perets, H.B et al. A faint type supernova from a white dwarf with a helium-rich companion. Nature (Letters). 465, 322-325 (2010).
5. Kawabata, K.S et al. A massive star origin for an unusual helium-rich supernova in an elliptical galaxy. Nature (Letters). 465, 326-328 (2010).
6. Smartt, S.J. Progenitors of core-collapse supernovae. Annu. Rev. Astron. Astrophys. 47, 63-106 (2009).
7. Zhang, Y, Gu, Q-S and Ho, L.C. Stellar and dust properties of local elliptical galaxies: clues to the onset of nuclear activity. Astron. Astrophys, 487, 177-183 (2008).
8. http://en.wikipedia.org/wiki/Black_hole
