What are Supernova and Supernovae?

Supernova are very luminous explosions caused by the death of a star. The burst of radiation often briefly outshines an entire galaxy.

Supernovas are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months.

During this short interval a supernova can radiate as much energy as the Sun is expected to emit over its entire lifespan. The explosion expels much (or all) of a star's material at a velocity of up to 30,000 km/s (10% of the speed of light), driving a shock wave into the surrounding interstellar medium. This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant.

What are the Types of Supernova?

Several types of supernovae exist. Types I and II can be triggered in one of two ways, either turning off or suddenly turning on the production of energy through nuclear fusion. After the core of an ageing massive star ceases generating energy from nuclear fusion, it may undergo sudden gravitational collapse into a neutron star or black hole, releasing gravitational potential energy that heats and expels the stars outer layers. Alternatively, a white dwarf star may accumulate sufficient material from a stellar companion (either through accretion or via a merger) to raise its core temperature enough to ignite carbon fusion, at which point it undergoes runaway nuclear fusion, completely disrupting it. Stellar cores whose furnaces have permanently gone out collapse when their masses exceed the Chandrasekhar limit while accreting white dwarfs ignite as they approach this limit (roughly 1.38 times the mass of the sun). White dwarfs are also subject to a different, much smaller type of thermonuclear explosion fuelled by hydrogen on their surfaces called a nova. Solitary stars with a mass below approximately nine solar masses, such as the Sun, evolve into white dwarfs without ever becoming supernovae.

Although no supernova has been observed unquestionably in the Milky Way since 1604, on average supernovae occur about once every 50 years in a galaxy the size of the Milky Way. They play a significant role in enriching the interstellar medium with higher-mass elements. Furthermore, the expanding shock waves from supernova explosions can trigger the formation of new stars.

Nova (plural novae) means "new" in Latin, referring to what appears to be a very bright new star shining in the celestial sphere; the prefix "super-" distinguishes supernovae from ordinary novae, which also involve a star increasing in brightness, though to a lesser extent and through a different mechanism. The word supernova was coined by Swiss astrophysicist and astronomer Fritz Zwicky and was first used in print in 1926.

History of Supernova Observations

The earliest recorded supernova, SN 185, was viewed by Chinese astronomers in 185 AD. The brightest recorded supernova was SN 1006, which was described in detail by Chinese and Islamic astronomers. The widely observed supernova SN 1054 produced the Crab Nebula.

Supernovae SN 1572 and SN 1604, the last to be observed with the naked eye in the Milky Way galaxy, had notable effects on the development of astronomy in Europe because they were used to argue against the Aristotelian idea that the universe beyond the Moon and planets were immutable.

Supernova Naming Convention

Supernova discoveries are reported to the International Astronomical Union's Central Bureau for Astronomical Telegrams, which sends out a circular with the name it assigns to it. The name is the year of discovery, immediately followed by a one or two-letter designation. The first 26 supernovae of the year are designated with a capital letter from A to Z. Afterward pairs of lower-case letters are used: aa, ab, and so on.

Historical supernovae are known simply by the year they occurred: SN 185, SN 1006, SN 1054, SN 1572 (Tycho's Nova) and SN 1604 (Kepler's Star). Since 1885 the letter notation has been used, even if there was only one supernova discovered that year (e.g. SN 1885A, 1907A, etc.) - this last happened with SN 1947A. "SN", for SuperNova, is a standard prefix. Until 1987, two-letter designations were rarely needed; since 1988, however, they have been needed every year.

Classification of Supernova

As part of the attempt to understand supernovae, astronomers have classified them according to the absorption lines of different chemical elements that appear in their spectra. The first element for a division is the presence or absence of a line caused by hydrogen. If a supernova's spectrum contains a line of hydrogen (known as the Balmer series in the visual portion of the spectrum) it is classified as Type II; otherwise it is Type I. Among those types, there are subdivisions according to the presence of lines from other elements and the shape of the light curve (a graph of the supernova's apparent magnitude as a function of time).

Role of Supernova in Stellar Evolution

The remnant of a supernova explosion consists of a compact object and a rapidly expanding shock wave of material. This cloud of material sweeps up the surrounding interstellar medium during a free expansion phase, which can last for up to two centuries. The wave then gradually undergoes a period of adiabatic expansion, and will slowly cool and mix with the surrounding interstellar medium over about 10,000 years.

The Big Bang produced hydrogen, helium, and traces of lithium, while all heavier elements are synthesized in stars and supernovae. Supernovae tend to enrich the surrounding interstellar medium with metals - elements other than hydrogen and helium. These injected elements ultimately enrich the molecular clouds that are the sites of star formation. The kinetic energy of an expanding supernova remnant can trigger star formation due to the compression of nearby, dense molecular clouds in space.

When Will The Next Supernova Explode?

There have been at least six definite supernovae recorded in the Milky Way in the whole of history, the last one occurring in 1608. Is there any way to predict when the next supernova will occur? One way to tell would be to look at stars much larger than the Sun. These stars are the best candidates for Type II supernovae.

Like earthquakes, it usually isn't possible to predict when a star will go supernova. We do however have thousands of supernova candidates that we know of. When we look at stars like Betelgeuse in Orion, which is in the final stages of evolution, we can only say that they'll go supernova somewhere in the next 10,000 to 100,000 years. In astronomy that's a very short space of time, but it's far longer than human timescales.

A much better method used by astronomers is to look at the frequency of supernovas in other galaxies and then scale them to the Milky Way. By looking at other galaxies, we can get a rate for the Milky Way of one supernova every fifty to a hundred years, which is a much more comfortable observation window. When scientists looked at these galaxies, they found that for nearby ones, Type II supernovae are the most commonly occurring. What we also know is that there hasn't been a Type II supernova in our Galaxy since the 1980s when modern-day detectors came of age. This is because Type IIs release enormous amounts of neutrinos when they explode, and we haven't detected any yet.

So when will the next supernova explode? It could be tomorrow, or it could be in a hundred years. We simply just don't know.