Exploring Supernovae
Looking at the Different Kinds and Notable Historic Explosions
What causes some stars to explode into giant supernovae and what were some of the prominent explosions from the past?
Supernovae can be divided into two main groups: Type Ia and Type II. Types Ib and Ic are usually included with Type II because they are caused by the same general mechanism.
Type Ia supernovae occur when a medium mass star reaches the end stage of its life and becomes a white dwarf. If the white dwarf is part of a binary system, the white dwarf may begin pulling in material from the companion star, accreting material until it is pushed over the Chandrasekhar limit of about 1.44 times the mass of our sun. As it breaches this limit, it explodes in a supernova.
Type II supernovae occur when a more massive star runs out of fuel and collapses, causing a supernova explosion that may leave behind a pulsar and nebula.
A third and still theoretical type of supernova is the hypernova. This could occur when a star is so massive that it collapses directly into a black hole. The "collapsar" would then emit two jets of plasma at nearly the speed of light. A hypernova could explain the ultra-powerful and mysterious gamma-ray bursts that crop up approximately once per day, however, they are viewed as occuring at huge distances from Earth.
The most notable recent supernova occured in 1987. SN1987a is a Type II supernova that exploded not in the Milky Way but in a galaxy nearby, the Large Magellanic Cloud. Because it lay 190,000 light-years from us, it only appeared as a 4th magnitude star in the Southern Hemisphere.
The last great supernova in our own galaxy appeared in October of 1604. SN1604, also known as Kepler's Star, was a Type Ia supernova that emerged in Ophiuchus and shone brighter than any star at magnitude -2.5. It came only 32 years after another amazing Type Ia supernova, Tycho's Nova (SN1572), that lit the skies in November of 1572 in the constellation Cassiopeia. Its brightness maxxed out at -4, outshining Venus.
There have only been eight recorded naked-eye supernovae in all of history. The grandaddy of them all was in 1054, when a supernova (SN1054) reached a -6 magnitude and could be seen in daylight for 23 days and nearly two years in nighttime skies. Its intrusion into the sky was noted by Chinese, Arabs, and Native Amercians. You can still see the remains of this Type II supernova in the constellation Taurus -- it is now known as the Crab Nebula, M1.
Could we be fortunate enough to witness a naked-eye supernova in our lifetimes?
The most likely candidates are RS Ophiuchi and Eta Carinae. Both objects have sputtered before, releasing material as if threatening to make the big explosion, but then gone back into a quieter stage. Eta Carinae is a Southern Hemisphere star that lies 8,000 light-years from the Sun. The better choice for northern observers is RS Ophiuchi, a white dwarf that lies in Ophiuchus and is accumulating matter from its partner. It is nearing its "critical mass" and could explode today or 1000 years from today. It lies a mere 5,000 light-years from the Sun.
If one of these stars did explode, damage on Earth would be limited to our upper atmosphere and any objects or people in that region. Most earthlings would be protected from the gamma rays by the atmosphere. It would be a tremendous opportunity for scientists to learn more about the workings of the universe.
