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What is a supernova?

This Spitzer false-color image is a composite of data from the Spitzer Space Telescope. This image is a view of Kepler's supernova remnant taken in X-rays, visible light, and infrared radiation.
This Spitzer false-color image is a composite of data from the Spitzer Space Telescope. This image is a view of Kepler's supernova remnant taken in X-rays, visible light, and infrared radiation. (Image credit: NASA/JPL)

A supernova is what happens when a star has reached the end of its life and explodes in a brilliant burst of light. Supernovae can briefly outshine entire galaxies (opens in new tab) and radiate more energy than our sun (opens in new tab) 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 in the 17th century. The oldest recorded supernova is RCW 86, which Chinese astronomers spotted in A.D. 185. Their records show that this "guest star" stayed in the sky for eight months, according to NASA. 

Related: When will the sun die? (opens in new tab) 

The Crab Nebula (opens in new tab), arguably the most famous supernova, was first spotted by Chinese and Korean astronomers who recorded this star explosion in their records in 1054. Native Americans may have seen it as well, according to rock paintings found in Arizona and New Mexico. The supernova that formed the Crab Nebula was so bright that those early astronomers could see it during the day.

Other supernovae that were observed before the telescope was invented occurred in the years 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 (opens in new tab)," which gave rise to the name "nova." 

The term "supernova" was first used by Walter Baade and Fritz Zwicky at Mount Wilson Observatory (opens in new tab), who used it in relation to an explosive event they observed, called S Andromedae (also known as SN 1885A), located in the Andromeda Galaxy (opens in new tab). The scientists suggested that supernovas happen when ordinary stars collapse into neutron stars.

When stars die

On average, a supernova will occur once every 50 years in a galaxy the size of the Milky Way (opens in new tab), according to research by the European Space Agency (opens in new tab). This means a star explodes every 10 seconds or so somewhere in the universe, according to the U.S. Department of Energy (opens in new tab)

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 our solar system.

Related: Giant, galactic bubble is driving star formation, new study finds (opens in new tab)

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 (opens in new tab) that will likely vaporize our world, before gradually cooling into a white dwarf (opens in new tab).) 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.

The Hubble Space Telescope has caught the most detailed view of the Crab Nebula in one of the largest images ever assembled by the space-based observatory. (Image credit: NASA/ESA and Jeff Hester (Arizona State University).)
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Type II supernovae

Let's look at the more exciting Type II first. For a star to explode as a Type II supernova, it must be several times more massive than the sun (estimates run from eight to 15 solar masses (opens in new tab)). 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. 

Next, gradually heavier elements build up at the center, and the star forms onion-like layers of material, with elements becoming lighter toward the outside of the star. Once the star's core surpasses a certain mass (called the Chandrasekhar limit), it begins to implode. For this reason, these Type-II supernovae are also known as core-collapse supernovae.

Related: Star-smash supernova? New type of stellar explosion possibly seen (opens in new tab)

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 packs the mass of the sun in a small space.

Type II supernova sub-categories are classified based on their light curves, which describes how the intensity of the light changes over time. The light of Type II-L supernovae declines steadily after the explosion, while the light of Type II-P supernovae stays steady for a longer period 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 (opens in new tab).

This Chandra X-ray photograph shows Cassiopeia A (Cas A, for short), the youngest supernova remnant in the Milky Way. (Image credit: NASA/CXC/MIT/UMass Amherst/M.D.Stage et al.)
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Type I supernovae

Type I supernovae lack a hydrogen signature in their light spectra and are generally thought to originate from white dwarf stars in a close binary star system (opens in new tab). 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 supernovae as "standard candles" to measure cosmic distances because all are thought to blaze with equal brightness at their peaks.

Type Ib and Ic supernovae also undergo core-collapse just as Type II supernovae do, but they have lost most of their outer hydrogen layer. 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.

Related: 20-year-old supernova mystery finally solved (opens in new tab)

Watching a supernova

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

In 2008, scientists caught a supernova in the act of exploding (opens in new tab) 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 named 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 resources

For more on supernovas and their discovery, check out "The Supernova Story (opens in new tab)" (Princeton University Press, 1994) by Laurence Marschall. You may also want to consider reading A.G.W. Cameron's "Stellar Evolution, Nuclear Astrophysics, and Nucleogenesis (opens in new tab)" (Dover Publications, 2013). And for an accessible book about the entire life cycle of a star, check out "The Life and Death of Stars (opens in new tab)" (Cambridge University Press, 2013) by Kenneth Lang. 

Bibliography

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Andrea Thompson
Andrea Thompson

Andrea Thompson is an associate editor at Scientific American, where she covers sustainability, energy and the environment. Prior to that, she was a senior writer covering climate science at Climate Central and a reporter and editor at Live Science, where she primarily covered Earth science and the environment. She holds a graduate degree in science health and environmental reporting from New York University, as well as a bachelor of science and and masters of science in atmospheric chemistry from the Georgia Institute of Technology.

With contributions from