A supernova is an exploding star. We see the phenomenon as the brightening of a star by as much as 21 magnitudes within a matter of hours followed by a characteristic fading over a period of months. At peak brightness a supernova might outshine its parent galaxy.
There are two types of supernovae, Type I and Type II. The difference between the two is simply the presence or absence of the Balmer Hydrogen lines in the spectrum of the supernova.
Within the Type I supernovae there is a further subdivision into Type Ia, Ib, and Ic. Supernovae that show a strong ionized silicon line at 6150 Angstroms are classified Ia. Type I supernovae lacking this Si II feature are classified as Type Ib if they show substantial evidence of neutral Helium, especially the neutral Helium line at 5876 Angstroms. If the neutral Helium features are weak or missing entirely then the supernova is classified as Type Ic.
Supernovae of Type II are further subdivided by the way their brightness fades. In many cases the type II will reach maximum brightness, dim slightly, and then stay at almost the same brightness "plateau" for many days before fading at a fairly regular rate and are designated Type IIP (II-Plateau). Other type II supernovae quickly reach maximum brightness and then dim in a linear fashion and are classified as Type IIL (II-Linear).
Some type II supernovae exhibit stranger behavior, dimming and brightening again, as in the case of the recent SuperNova SN1993J (NASA newsnoteSept. 9, 1998: Most objects in the sky can be pigeonholed into a few of the hundreds of categories that classify stars, galaxies, and other bodies. Every now and then, you get one that changes its colors - literally - and seems to beg for closer examination. That's the case with a five-year-old supernova known as SN 1993J. "It started out as a classic Type II supernova," said Dr. Doug Swartz of NASA's Marshall Space Flight Center, "with hydrogen lines in its spectrum. These weakened in a few weeks and helium lines appeared, more like a Type Ib supernova." In effect, SN 1993J changed its appearance from a supernova caused by an ordinary massive star to a supernova caused by a dense helium stellar core).
Supernova pictures here (40k)
Current models of supernovae suggest that all types of supernovae other than Type Ia are caused by the catastrophic death of stars far more massive than the Sun. These progenitors are thought to be stars which started their main sequence lives with at least 12 times the Sun's mass - and possibly as much as 50 solar masses. After their brief main sequence hydrogen burning phase these massive stars rapidly evolve into giant stars while successive stages of nuclear burning within their interiors build an onion-like structure of shells - each consisting of successively heavier groups of elements. When at last the star's core has been transmuted to iron and nickel it reaches a dead end, for fusion of these elements into heavier species takes energy out of the surrounding star rather than pumping energy into it as did earlier reactions. Now gravity wins. The core must contract and the burning shells rain more and more iron "ash" on the core hastening the contraction. at some point in the iron core begins to collapse and the gravitational energy releases a vast number of high energy photons that photodisintegrate complex atoms.
In a few minutes the collapsing core is converted from nickel and iron nuclei to mostly alpha particles (helium nuclei). Deprived of support from the core, the overlying mass of the star freefalls. As this mass impacts onto the now largely Helium core it is further compressed and heated. The Helium is then dissociated into the fundamental subatomic particles - protons, neutrons, and electrons; and for a brief time the electrostatic force of the electrons resists the pressure of the star's overlying weight. But this resistance, known as electron degeneracy pressure, is not enough to resist the force of gravity given the tremendous mass of the star. In a white dwarf star electron degeneracy pressure is able overcome by gravity only if the mass is under a limit close to the Chandrasekhar limit. When the core approaches this limit the proton finds itself in a sea of electrons that cause the proton to be unstable against electron capture or "inverse beta decay". Electrons are absorbed into protons transmuting each electron-proton pair into a single neutron and releasing a neutrino in the process. Within fractions of a second the core is converted to a mass of neutrons at near nuclear density -- the core is literally a giant neutron-rich atom!
The newly formed neutron core, compressed still more by the infalling stellar matter, rebounds from its maximum compression(?). The shock wave(?) of the rebound travels outward through the star - tearing it apart and triggering a number of complex "neutron-activation" reactions which produce elements spanning the periodic table.
The current understanding of this process is far from complete. How elastic is the neutron core? The core must have a tremendous rotation and rotational kinetic energy. When the rebound occurs it will not be a pretty sight, most likely the star will fragment rather than explode in the symmetric manner that is so easy to program but so unrealistic in nature.
The explanation for how this basic model can produce the various types observed, Ib, Ic, IIP and IIL, is that by the time a large star reaches the iron fusion stage its outer layers are mostly out of touch with the core. In the cases where no Hydrogen is observed (Ib and Ic) the accepted explanation is that a strong stellar wind or a nearby binary companion has swept away the star's outer layer of Hydrogen prior to the core collapse. In the case of SN Ic the Helium layer beneath the Hydrogen layer has been largely depleted as well. Conversely, the type II have retained a large portion of their Hydrogen envelopes, and we see this Hydrogen in the spectrum of the supernova.
The Type IIL supernovae show a linear decline in their light curves, similar to the light curves of Type I. The IIP plateau supernovae, are thought to have this property because the exploding star is expanding just fast enough for the increase in radius to compensate for decreasing surface temperature. (Stars emit light proportional to the square of their radii and the fourth power of temperature.) The few supernovae that have been observed to dim and then re-brighten before entering the plateau phase are postulated to have experienced an accelerated expansion after the initial brightening began to decrease.
What about SN1993J?
Its change from Type II to Type Ib emissions is strange because the two supernova types are distinctly different. A Type II supernova happens when a lone, massive star has burned everything in its core. The furnace inside turns off and the star collapses to become a massive piston that blows most of its mass into space and compresses the core into a neutron star. A Type Ib is believed to result from a star that has somehow lost its entire hydrogen envelope, probably as a result of mass transfer in a binary system, before collapse. Because the supernovae have different origins, they emit light differently as they explode. But SN 1993J is a transition object which had lost most, but not all, of its hydrogen envelope. "The idea is that supernovae, when they go off, are surrounded by whatever materials the star emitted in the last 10,000 to 100,000 years of its life," Swartz explained. All stars have stellar winds - just as our sun has a solar wind - that carry away parts of the star's mass. Stars in binary systems can lose material to a companion. A star destined to become a Type IIb supernova will start off with perhaps 15 times as much mass as our Sun, and lose about 9 solar masses over the course of its life. When it finally explodes, the outrushing blast wave - about 2.5 solar masses' worth in the case of SN 1993J - will run into and energize the hydrogen cloak, or circumstellar medium, around the star and emit x-rays. "Supernova 1993J is one of the few that has made the transition from one type to another," Swartz explained. Only one other supernova, SN 1987K, has been seen making such a change.
The Type Ia supernovae can not be caused by the deaths of massive stars. First, no compact neutron stars are found in the remnants of type Ia supernovae. Second, no physical process, no matter how violent, can produce the luminosity observed from Type Ia by core collapse of even a supermassive star. The outer layers of a giant star would absorb too much of the energy. Rather, it is necessary that a mass of something more than the Sun's mass be completely destroyed while unshielded by overlaying material for anything as bright as a Type Ia SN to be possible. Rather, it is thought this special class consists of a white dwarf star accreting matter from a nearby evolving companion, a contact binary system.
White Dwarf stars have the curious property that the more massive White Dwarves are smaller in radius. The larger the mass the more the central pressure crushes the matter and at one solar mass the increase in mean density more than makes up for additional mass. (Look at Jupiter and Saturn-- Jupiter is almost the largest body that can exist in the absence of energy sources other than gravitational.) At what point will the mass of a White Dwarf become so much that it is compressed into a point mass of zero radius? S. Chandrasekhar has shown that the limit at which a star supported only by electron degeneracy compresses to a point is 1.44 solar masses. Other considerations related to the internal physics of real stars give the upper limit for the mass of a White Dwarf star to be about 1.2 solar masses. So a white dwarf star somewhat greater than the Sun's mass in a contact binary system could be "pushed over the limit" as the companion evolves. (A similar scenario is used to explain novae, and perhaps novae presage type Ia supernovae.)
|Supernova type criteria|
|No Balmer Lines
|Si II 6150||No Si||
|He I 5876||Weak He||then linear|