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| Space on Jan 23, 2014

What’s a safe distance between us and an exploding star?

Scientific literature cites 50 to 100 light-years as the closest safe distance between Earth and a supernova.

A supernova is a stellar explosion – destructive on a scale almost beyond human imagining. If our sun exploded as a supernova, the resulting shock wave probably wouldn’t destroy the Earth, but it would remove all life from Earth’s surface. Also, the sudden decrease in the sun’s mass would probably free the Earth to wander off into space. What is the closest safe distance? Scientific literature cites 50 to 100 years as the closest safe distance between Earth and a supernova.

An explosion of a nearby star would leave Earth and its surface life relatively intact. But the explosion would shower us with gamma rays and other high energy radiation. This radiation could cause mutations in earthly life. Also, the radiation from a nearby supernova could change our climate.

No supernova has been known to erupt at this close distance in the known history of humankind.

Our sun isn’t the sort of star destined to explode in this way. But other stars, beyond our solar system, will suffer this fate.

Image of remnant of Supernova 1987A as seen at optical wavelengths with the Hubble Space Telescope in 2011. Via NASA, ESA, and P. Challis (Harvard-Smithsonian Center for Astrophysics).

Image of remnant of Supernova 1987A as seen at optical wavelengths with the Hubble Space Telescope in 2011. This supernova was the closest in centuries, and it was visible to the eye alone. It was located on the outskirts of the Tarantula Nebula in the Large Magellanic Cloud, a satellite galaxy to our Milky Way. It was located approximately 168,000 light-years from Earth. Via NASA, ESA, and P. Challis (Harvard-Smithsonian Center for Astrophysics).

How many potential supernovae are located closer to us than 50 to 100 light-years? The answer depends on the kind of supernova. A Type II supernova is an aging massive star that collapses. There are no stars massive enough to do this located within 50 light-years of Earth. But there are also Type I supernovae – caused by the collapse of a small faint white dwarf star. These stars are dim and hard to find, so we can’t be sure just how many are around. There are probably a few hundred of these stars within 50 light-years.

What is the closest supernova candidate? The star IK Pegasi B is the nearest known supernova progenitor candidate. It’s part of a binary star system, located about 150 light years from our sun and solar system. IK Pegasi B is a massive white dwarf — a star that has evolved past the main sequence stage of stellar evolution and is no longer generating energy via thermonuclear fusion reactions at its core. The other star (considered the primary star in the system and therefore called IK Pegasi A) is an ordinary main sequence star, not unlike our sun. When the A star begins to evolve into a red giant, it is expected to grow to a radius where the white dwarf can accrete, or take on, matter from A’s expanded gaseous envelope. When the B star gets massive enough, it might collapse on itself, in the process exploding as a supernova. Read more about the IK Pegasi system from Phil Plait at Bad Astronomy.

No one knows how often supernovae erupt in our galaxy. Scientists have speculated that the high-energy radiation from supernovae has already caused mutations in earthly species, maybe even human beings. There may be one dangerous supernova event in Earth’s vicinity every 15 million years.

Contrast that to a few million years for the time humans are thought to have existed on the planet – and four-and-a-half billion years for the age of Earth itself. So a supernova is certain to occur near Earth – but probably not in the foreseeable future of humanity.

Bottom line: Scientific literature cites 50 to 100 years as the closest safe distance between Earth and a supernova.