Can a Star Explode without Leaving any Remnant?

About 24 hours ago now I posed the question as to whether a star could prematurely explode leaving no real trace of its existence. Many people replied "No" because stars don't just blow themselves apart willy nilly, it's almost silly to think that they do to be honest. Many also relied "Yes" because in the universe anything is possible even if only very highly improbable. At the end of the post I said that the correct answer is "Yes", now I am going to explain why.

To see the original post, see: http://www.fromquarkstoquasars.com/?p=898

When a star explodes it has to supernova, as you can imagine enormous amounts of energy would be needed to overcome the gravity of a large star and a supernova is the only way that this energy requirement can be met. It is no ordinary supernova however and this would explain why the first one wasn't officially detected until 2007, very recent times considering that supernovas in general were detected some 60 years ago.

As a general rule of thumb Type I supernova involve white dwarfs, Type II supernova are the birth of neutron stars and black holes as larger stars end their lives.

The detected supernova was designated SN 2007bi and was detected by SNFactory who have an automated robotic telescope specifically for doing supernova research. When first detected it had the spectrum somewhat similar to that of a Type Ic supernova because it lacked the hydrogen and helium lines, it wasn't like a normal Type Ic however, it was so much brighter.

This was the first detection of a pair instability supernova! The name is quite descriptive of the trigger of this large explosion and we shall shortly see. Every one of us will have heard the famous equation E=MC^2, in short this tells us that matter and energy are interchangeable, that energy has the potential to spontaneously become matter and visa versa. In the previous article we had a quick look at hydrostatic equilibrium, this effectively means that a star is in a balance between gravitational contraction and radiation pressure trying to tear the star apart. When this equilibrium is out of balance the star will either expand or contract in order to try to find a new equilibrium.

SN2007bi before it exploded was a star about 200 times the mass of our sun, suffice to say it was huge! Now our sun is small on the cosmic scale and because of this it will fuse hydrogen into helium, helium into oxygen and then it'll stop there due to lack of heat to kick start the next fusion stage. SN2007bi struggled to even get to this stage, at the time that it exploded scientists believe that it has a helium core at about 100 solar masses, so a very big core which is also a very hot core. Due to the temperature of the core at the time it was emitting very energetic gamma ray photons, so energetic in fact that for a moment these photos are able to turn into electron-positron pairs. Although these pairs only last for a fraction of a second before they fuse back together again to be come a gamma ray, gravity doesn't take a break even for a moment. This brief matter anti-matter reaction doesn't produce any needed radiation pressure for that brief moment, with the drop in outward pressure gravity wins a little. With the star contracting even only slightly it increases the pressure around the core causing it to heat up, with this extra added heat there are more highly energies photons that are generated with the potential of becoming electron-positron pairs. This is a very vicious circle, a lack in outward pressure causes the star to heat up which decreases pressure even more, a runaway contraction.

From here it doesn't take long for things to escalate out of control until finally the whole helium core undergoes a thermonuclear runaway, a 100 solar mass core effectively undergoing a helium flash! Big explosion, huge!

So there we go, a pair instability supernova can obliterate a star leaving no real trace, sort of. Although there is no remnant like a neutron star or a black hole, all of the material from the star expands out from the explosion like a usual supernova which is why we can detect and study it in the first place. In time however the ejected material will cool down and eventually stop emitting light or just become too thin (low density) that it won't be detectable by our instruments, it will become one with the universe.

§ Colin