A Star That Bursts
A great source of wonder in science is the relevance of events larger or smaller than we can imagine in our lives. There are no better examples than supernovae – explosions of unimaginable fury and distance from Earth that long ago forged most of the atoms in our bodies other than hydrogen, including the ingredients of life such as carbon, nitrogen, and calcium.
Over five billion years ago – more than 1/3 the age of the universe – one or more ancient stars created most of the elements that exist on Earth, exploded, and scattered those elements into clouds of debris that later coalesced to become our own sun and its planets. In fact, the shockwaves created by exploding stars may also initiate the clumping of matter that becomes new stars and planets.
A star’s life hangs always in a balance between the energy expanding from its core where it “burns” by fusing hydrogen into helium and the gravitational energy of its own mass pulling it inward. For millions to billions of years, a star shines in this balance until it has used up so much of its fuel that the rate of fusion slows down. Then the core of the star begins collapsing, grows denser, and therefore heats up again, causing the outer layers of the star to expand hugely as it becomes a “red giant.” When our sun becomes a red giant, it will engulf Earth entirely in its outer layers. Then the core of our sun will keep fusing what it can until it runs out of fuel and collapses to become a dense white dwarf – the corpse of a star. Our sun is not expected to go supernova; it isn’t heavy enough.
The life cycle of a star. Credit: Chandra X-Ray Observatory, NASA/CXC/M.Weiss.
But if the star is more than eight times heavier than our sun, it becomes a red giant dense enough to keep fusing the products of previous fusion reactions into heavier and heavier elements in layers, like an onion, with the heaviest elements deepest inside the star. And once it starts making iron, its days are numbered. Iron can endure tremendous pressure without fusing, and when it does fuse, it can absorb a tremendous amount of energy. So, in either or both ways, an iron core ends the outpouring of energy that prevents a star from collapsing. And, suddenly, in a matter of milliseconds, the star will collapse violently under its own gravity, heating up in the center to billions of degrees Celsius. The superheated gasses then explode outward at about 10 to 25,000 miles per second (about 10% the speed of light), putting out more radiation in a fraction of a second than our sun will emit during its whole lifetime.
А mosaic image of the Crab Nebula, a six-light-year-wide expanding remnant of a star's supernova explosion, noted by Earth-bound chroniclers in 1054. Credit: NASA, ESA, J. Hester, A. Loll.
Those are known as Type II supernovae. Type I supernovae form from some dead stars that were not massive enough to explode originally. Say a star like our own sun dies peacefully and becomes a white dwarf but has a companion star; over billions of years, the white dwarf may pull in enough of the gasses of its companion star to become dense enough for fusion to restart, and then become a supernova.
Supernovae are so bright that any in our galaxy not obscured by dust clouds can be seen from Earth with the naked eye, but only a few happen in 1,000 years per galaxy. The last known in our galaxy was observed by astronomer Johannes Kepler (among others) in 1604. Another was described by Chinese and Islamic astronomers in 1006, and one by the Chinese in 185 AD.
A Hubble Space Telescope image of the supernova remnant N 63A in the Large Magellanic Cloud. Credit: NASA/ESA, The Hubble Key Project Team.
Using telescopes, supernovae become the most visible objects in distant galaxies. Since their maximum brightness is predictable, astronomers use them as “candles” to gauge the distances and speeds of faraway galaxies.
But their remains are here, now, in our bodies! The very possibility of life depends on elements created by supernovae. Just a thought to keep you in awe until next time!
“Without these supernova explosions, there are no mist-covered swamps, computer chips, trilobites, Mozart or the tears of a little girl. Without exploding stars, perhaps there could be a heaven, but there is certainly no Earth.” – Clifford A. Pickover
“Our sun, by the way … may become a white dwarf some day but apparently will never become a supernova.” – Isaac Asimov
“... the blast signatures of a detonated supernova and that of a nuclear bomb are identical.” – Eric Chaisson
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