Saturday, February 21, 2015

What is a supernova and what is the possibility of one destroying Earth?

Supernovae are some of the most luminous objects in our night sky. A supernova occurs when a massive star collapses on itself after it runs out of fuel for its fusion process. Because iron cannot be fused with any element to create heavier atoms, the fusion process stops after iron is formed in the star’s core. Once fusion stops, there is no longer radiation pressure pushing outwards, so the star collapses upon itself with such force that a shockwave pushes through the surface and rips the star apart, creating one of the brightest explosions in space. Supernovae are very luminous but also very rare. Eight times the mass of the Sun is the minimum threshold for a supernova to occur; stars this massive are comparatively rare. Supernovae occur in the Milky Way Galaxy about once or twice per century.

Astronomers predict that the star Eta Carinae will soon undergo a supernova explosion. Although it’s over 7,500 light-years away from Earth, this supermassive star outshined Sirius, the brightest star in the night sky, from 1838 until 1858. However, because it is so far away from Earth, most of the harmful radiation that will be produced when the supernova occurs will disperse in the vacuum of space, with very little of it reaching our solar system. The closest supernova to have occurred near Earth since 1604 was 1987A, but it was still approximately 160,000 light-years away. Scientists expect Earth to receive a substantial burst of radiation from a supernova every 20 million years, enough to affect the atmosphere and the ozone layer. If the supernova occurs close enough to Earth, then life as we know it could be impacted dramatically.

How close does the supernova have to be to Earth to sufficiently impact life? At one hundred light-years and farther, a supernova poses no threat to Earth. Other than observing a bright light in our sky, we would experience no change on Earth. However, at fifty light-years away, a supernova will rip the ozone layer from our planet and destroy our magnetic field. Without a magnetic field, Earth would be bombarded by solar and cosmic radiation, causing a mass extinction of all complex life on the surface of Earth. What if a star one light-year away underwent a supernova? The closest star to us (other than the sun) is the red dwarf Proxima Centauri, which is just over four light years away. Although that star would never undergo a supernova, if in theory a star did so just one light year away from Earth, then not only our planet, but our entire solar system would be obliterated by the supernova’s shockwave.

Given current scientific estimates and predictions, the death of Earth by supernova is extremely unlikely if not impossible in the near future. So you can probably safely cross nearby supernovae off the list of possible threats to our existence on Earth, while being thankful that the ones that will occur in your lifetime will almost all be in galaxies far far away...

- Siqi Yang