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Wolfram|Alpha’s coverage of the universe continues to grow. We have now added a large collection of observed supernovae in the universe to our ever-expanding compendium of astronomical knowledge.
What exactly is a supernova? It’s a catastrophic event in the life of a star.
The full details are very complex, but basically supernovae are the visible signs of the deaths of stars more massive than the Sun. As with all other stars, massive stars spend most of their lives fusing hydrogen gas into helium in their cores. This results in a buildup of “ash” (end product of fusion reactions) in the core that eventually chokes off the hydrogen fuel from the hottest area of the core. With no new fuel, there is less energy being produced to counter the gravity trying to squeeze the star’s huge mass more tightly together. The result is that the star’s core begins to collapse as gravity overtakes the outward pressure. This results in heating the core—eventually enough that the ash can begin fusing into heavier molecules, initially carbon and oxygen. The cycle repeats, each time beginning and ending with different products and creating the next fuel source. Eventually, the core contains iron. Iron cannot liberate energy from fusion, so at this point, energy generation in the core suddenly stops, and the full mass of the star comes crashing down and a shock wave rips the star apart. This explosion is called a Type II supernova and results in the formation of a neutron star (or more rarely a black hole).
In 1987, a Type II supernova known as supernova 1987A occurred in the Large Magellanic Cloud, a small satellite galaxy of the Milky Way. This was the last supernova that could be seen with the naked eye, but only from the southern hemisphere. In the year 1054, another naked-eye supernova was observed in the constellation of Taurus. This supernova, also Type II, not only created a neutron star but also left behind a visible remnant of expanding debris known as the Crab Nebula.
There are several kinds of Type I supernovae. Type Ia supernovae are a different, but related, phenomenon. In a Type Ia supernova, two stars are orbiting each other. One star is a little more massive than the Sun and has halted its fusion reactions and left only its core behind, so is a “dead” star in that sense. This is known as a white dwarf. It orbits a normal star closely enough that mass falls from the living star and is collected on the surface of the white dwarf. This particular kind of white dwarf is massive for its type and near a critical mass. The typical model proposes that when enough matter is collected from the living companion, a runaway set of nuclear reactions occurs that blows the white dwarf apart. Once again, this collapse gives off lots of energy and drastically brightens the system. Johannes Kepler observed a Type Ia supernova in our galaxy in 1604.
Type Ib and Ic supernovae are actually more similar to Type II supernovae in that they are believed to be “core collapse” supernovae. The difference is that they originate from extremely massive stars that have lost most of their outer hydrogen envelope and so have differing spectra.
In any case, the result is that a star that was typically invisible (or barely visible) from here becomes obviously visible. The word “supernova” draws its meaning from “nova”, which means “new star”. Supernovae are very rare in a given galaxy, but over the scope of all known galaxies, they can be observed several times a year. Usually, these require moderate to large telescopes to see because their parent galaxies are often very distant and therefore faint. The supernovae are often more luminous than the galaxies themselves. However, the brightness of a supernova is temporary and fades rather quickly into obscurity.
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