Supernovae: Types and Other Facts

 

A supernova is the largest explosion in space that humans have ever seen. Each blast seen in the sky is a tremendously bright and super-powerful explosion of a dying star. Supernovae (plural of supernova) can be so bright that they outshine their entire galaxies for a few days or months and can be seen from across the universe! Astronomers believe that about 2 or 3 supernovae happen every century, with an average of a supernova occurring once every 50 years in a galaxy the size of the Milky Way. Since the universe has many galaxies, a star explodes every 10 seconds. Astronomers observe a few hundred supernovae per year outside of our galaxy. The oldest recorded supernova is RCW 86, which Chinese astronomers saw in the sky in A.D. 185 and had records showing that this “guest star” stayed in the sky for eight months. 

The way a star dies depends partly on its mass. Our sun doesn’t have enough mass to explode as a supernova and will instead, once it runs out of nuclear fuel, expand into a red giant and will then gradually cool into a white dwarf. 

One type of supernovae are core-collapse supernovae, which occurs when a star several times more massive than the sun (estimates run from 8 to 15 solar masses) dies. Since massive stars burn large amounts of nuclear fuel at their cores, they produce tons of energy which causes the center to get incredibly hot. This heat creates pressure and also prevents the star from collapsing. A star is in balance with two forces, the star’s gravity, and the star’s outward pressure. The star’s gravity tries to compress the star into the tightest ball possible, but the nuclear fuel burning in the star’s core resists by creating an outward pressure. When a massive star runs out of nuclear fuel, it cools down, making the outward pressure drop. Gravity can then overcome the rest of the pressure, causing the star to collapse. This collapse happens so fast that it generates enormous shock waves that cause the outer part of the star to explode. Typically, a very dense core is left behind along with a nebula, an expanding cloud of hot gas. 

Stars that are around 20 to 30 solar masses may not explode as supernovae and instead collapse to form the densest objects in the universe, black holes. 

The second type of supernovae is thermal runaway supernovae, which occur in systems where two stars orbit each other, and at least one of them is an Earth-sized white dwarf. A white dwarf is what’s left after a star the size of the sun has run out of fuel. When one white dwarf collides with another or pulls too much matter from a nearby star, it can explode and cause a supernova. 

Stars create chemical elements that are needed to make everything in our universe. At their centers, stars convert simple elements like hydrogen into heavier elements like carbon and nitrogen. These heavier elements are essential for life. Only massive stars can make heavy elements like gold, silver, and uranium. When explosive supernovae occur, stars disperse the stored-up and newly created elements throughout space. 

Sources: spaceplace.nasa.gov, energy.gov, space.com