What Is a Supernova?
This Chandra X-ray photograph shows Cassiopeia A (Cas A, for short), the youngest supernova remnant in the Milky Way.
Credit: NASA/CXC/MIT/UMass Amherst/M.D.Stage et al.

A blindingly bright star bursts into view in a corner of the night sky — it wasn't there just a few hours ago, but now it burns like a beacon.

That bright star isn't actually a star, at least not anymore. The brilliant point of light is the explosion of a star that has reached the end of its life, otherwise known as a supernova.

Supernovae can briefly outshine entire galaxies and radiate more energy than our sun will in its entire lifetime. They're also the primary source of heavy elements in the universe. According to NASA, supernovae are "the largest explosion that takes place in space."

Various civilizations recorded supernovae long before the telescope was invented. The oldest recorded supernova is RCW 86, which Chinese astronomers saw in A.D. 185. Their records show that this "guest star" stayed in the sky for eight months, according to NASA. 

Before the early 17th century (when telescopes became available), there are only seven recorded supernovae, according to Encyclopedia Britannica. 

What we know today as the Crab Nebula is the most famous of these supernovae. Chinese and Korean astronomers recorded this star explosion in their records in 1054, and southwestern Native Americans may have seen it as well (according to rock paintings seen in Arizona and New Mexico). The supernova that formed the Crab Nebula was so bright that astronomers could see it during the day.

Other supernovae that were observed before the telescope was invented occurred in 393, 1006, 1181, 1572 (studied by famed astronomer Tycho Brahe) and 1604. Brahe wrote about his observations of the "new star" in his book, "De nova stella," which gave rise to the name "nova." A nova differs from a supernova, however. Both are sudden outbursts of brightness as hot gases are blown outward, but for a supernova, the explosion is cataclysmic and signifies the end of the star's life, according to Encyclopedia Britannica.

The term "supernova" was not used until the 1930s. Its first use was by Walter Baade and Fritz Zwicky at Mount Wilson Observatory, who used it in relation to an explosive event they observed, called S Andromedae (also known as SN 1885A). It was located in the Andromeda Galaxy. They also suggested that supernovas happen when ordinary stars collapse into neutron stars.

In the modern era, one of the more famous supernovas was SN 1987A from 1987, which is still being studied by astronomers because they can see how a supernova evolves in the first few decades after the explosion.

On average, a supernova will occur about once every 50 years in a galaxy the size of the Milky Way. Put another way, a star explodes every second or so somewhere in the universe, and some of those aren't too far from Earth. About 10 million years ago, a cluster of supernovae created the “Local Bubble,” a 300-light-year long, peanut-shaped bubble of gas in the interstellar medium that surrounds the solar system.

Exactly how a star dies depends in part on its mass. Our sun, for example, doesn't have enough mass to explode as a supernova (though the news for Earth still isn't good, because once the sun runs out of its nuclear fuel, perhaps in a couple billion years, it will swell into a red giant that will likely vaporize our world, before gradually cooling into a white dwarf). But with the right amount of mass, a star can burn out in a fiery explosion.

A star can go supernova in one of two ways:

  • Type I supernova: star accumulates matter from a nearby neighbor until a runaway nuclear reaction ignites.
  • Type II supernova: star runs out of nuclear fuel and collapses under its own gravity.

Let's look at the more exciting Type II first. For a star to explode as a Type II supernova, it must be at several times more massive than the sun (estimates run from eight to 15 solar masses). Like the sun, it will eventually run out of hydrogen and then helium fuel at its core. However, it will have enough mass and pressure to fuse carbon. Here's what happens next:

  • Gradually heavier elements build up at the center, and it becomes layered like an onion, with elements becoming lighter toward the outside of the star.
  • Once the star's core surpasses a certain mass (the Chandrasekhar limit), the star begins to implode (for this reason, these supernovae are also known as core-collapse supernovas).
  • The core heats up and becomes denser.
  • Eventually the implosion bounces back off the core, expelling the stellar material into space, forming the supernova.

What's left is an ultra-dense object called a neutron star, a city-sized object that can pack the mass of the sun in a small space.

There are sub-categories of Type II supernovas, classified based on their light curves. The light of Type II-L supernovas declines steadily after the explosion, while Type II-P's light stays steady for a time before diminishing. Both types have the signature of hydrogen in their spectra.

Stars much more massive than the sun (around 20 to 30 solar masses) might not explode as a supernova, astronomers think. Instead they collapse to form black holes.

Type I supernovae lack a hydrogen signature in their light spectra.

Type Ia supernovae are generally thought to originate from white dwarf stars in a close binary system. As the gas of the companion star accumulates onto the white dwarf, the white dwarf is progressively compressed, and eventually sets off a runaway nuclear reaction inside that eventually leads to a cataclysmic supernova outburst.

Astronomers use Type Ia supernovas as "standard candles" to measure cosmic distances because all are thought to blaze with equal brightness at their peaks.

Type Ib and Ic supernovas also undergo core-collapse just as Type II supernovas do, but they have lost most of their outer hydrogen envelopes. In 2014, scientists detected the faint, hard-to-locate companion star to a Type Ib supernova. The search consumed two decades, as the companion star shone much fainter than the bright supernova.

Recent studies have found that supernovas vibrate like giant speakers and emit an audible hum before exploding.

In 2008, scientists caught a supernova in the act of exploding for the first time. While peering at her computer screen, astronomer Alicia Soderberg expected to see the small glowing smudge of a month-old supernova. But what she and her colleague saw instead was a strange, extremely bright, five-minute burst of X-rays. 

With that observation, they became the first astronomers to catch a star in the act of exploding. The new supernova was dubbed SN 2008D. Further study has shown that the supernova had some unusual properties.

"Our observations and modeling show this to be a rather unusual event, to be better understood in terms of an object lying at the boundary between normal supernovae and gamma-ray bursts," Paolo Mazzali, an Italian astrophysicist at the Padova Observatory and Max-Planck Institute for Astrophysics, told Space.com in a 2008 interview.

Additional reporting by Elizabeth Howell and Nola Taylor Redd, Space.com contributors

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