Supernovae

Definition, Classification

Supernovae are explosion events occurring at a moment in the life of certain stars. At the moment of the explosion a supernova may illuminate an entire galaxy as those seen in our own Galaxy are rivalling the bright planets. Within months the supernova is fading back to invisibility as the expelled shell, called a "supernova remnant" (SNR), is remaining visible during

Old White Dwarfs and Young, Massive Stars A Type Ia supernova -which is triggered by the infall of matter from a companion star onto an old (some billions years old) white dwarf- is leaving a gas shell which contains a lot of iron. In contrast, a Type II supernova -which is yielded by the explosion of a massive, young (some million years old) star contains a much lower quantity of iron

for millenia. In some cases, the star's core survives like a rapidly rotating neutron star or a pulsar. The frequency of supernovae in our own Galaxy is about one each 40 years. Some are remaining unseen however as obscured or hidden by gas clouds

A classification of supernovae was devised by the Caltech astronomer Fritz Zwicky during the 1960s as 3 sub-types were added to Type I in the 1980s. All supernovae except type Ia are caused by a star's core collapse. The star needs to be above 8 solar masses as it continues to burn heavier and heavier elements beyond hydrogen. Once the core transformed into iron it increases in size. At a moment, the pressure coming from the star's center becomes so high that electrons merge with protons into neutrons leading to the pressure to vanish. Neutrinos are produced too. The star just collapses on itself. Type II are just caused by a core collapse. Type Ib are caused in the same way at the exception that the star has already shed their hydrogen envelope. Type Ic are stars which have further shed their helium enveloppe. Type Ia supernovae occurs in binary systems only where the material of a companion star is infalling unto the primary's surface leading to an eventual thermonuclear explosion. The event may last less -and less into the star's life- than usual when the involved primary star is a high-mass one. Core collapse-type events may lead to the formation of neutron stars, that is a star which is formed from the concentrated neutrons yielded in the last phase of the collapse. Such neutrons stars then tend to cool as neutrons further collide and keeps producing neutrinos which, as weakly interacting, are taking the star's heath away. Generally, only six progenitor stars have been found out of 3,200 supernovae observed since 1885

Magnetars, those superdense neutron stars, with an incredibly strong magnetic field and emitting in the X-rays, may sometimes be the other type of dense stars like left by a supernova explosion!

to a table of supernovae types

Miscellaneous

Type II, Ib and Ic supernovae constitute about 80 percent of the supernovae in our Milky Way Galaxy. Pulsar in M1 (the Crab Nebula) or in 3C 58 are of type II, type Ib or type Ic. Supernovae are the purveyors of heavy elements in the Universe. At the exception of type IIs which stops at iron, all supernovae are making heavier elements like gold, or uranium. Without the supernovae such elements would not exist in the Universe, hence no planets, and no humans would. The Spitzer Space Telescope, in December 2007, definitively brought the evidence that it's really the supernovae in the early Universe which provided for the dust in the Universe. A nova is another stellar event. Novae are lesser events where the star involved is not destroyed

Type Ia Supernovae in Details

Type Ia supernovae, these supernovae resulting from the interaction inside a binary system, are thought to be couples either white dwarf/red giant-headed normal star or two white dwarfs. In the first case one of the stars enters its red giant phase and spills gas onto the companion, eventually engulfing it as the system becomes included into a common envelope. This brakes the orbits and when the common envelope is ejected the two stars are closer than at the beginning. The remaining core of the giant becomes a mere white dwarf. And it's when the other star in turn begins to become a red giant spilling material onto the white dwarf that the latter eventually explodes into a supernova. After the event a supernova remnant is seen that is a bubble of gas and material running away from the explosion. The companion star surviving the explosion is ejected away retaining its orbital speed hence moving swiftly
When the system is two white dwarfs, the stars are braking each other through their gravitational waves. They eventually merge. It's this merger which triggers the supernova event

The Binary Events in More Details
Recent studies have found that a binary system may erupt as a nova each 10,000 years in full-blown explosions, as, in-between, they are feebly hiccuping every few weeks. The full-blown explosions are leaving shells of expanding matter behind. Such events are 'nova', strictly speaking, due to their repetitive nature
Novae, on the other hand, of the type Ia are thought to be originating, as far as the white dwarf involved is concerned, into white dwarfs of a relatively high mass, not having, hence, to gain much mass before reaching their limit and exploding. Such events seem not that rare, as they do occur on small scales of time
In-between the major explosions, the fact that one of the binaries orbit into the solar wind of the other one, brings to that cold places are generated into which the lower temperature allows atoms to turn into dust molecules. Those dust molecules arrange themselves into a pinwheel pattern around the binary system. That pattern of dust is blown up each time the binary system goes into an explosion

As it's still ill-known, it looks like there are fewer supernovae exploding in our Milky Way Galaxy than in the other galaxies

The case of a neutron star/companion star is specific in that they don't explode into a supernova, as the atmosphere of the neutron star only is exploding once filled with the material coming from its companion star, without destroying the star nor the system. A regular X-ray signal ticking every 120 seconds from the gas atoms mixing unto the neutron star surface, is allowing to know, when it slows to 125 seconds, that a binary (neutron/other star) system is to explode. Such a system endures about 7 to 10 bursts per day