Artist’s visualization of a collapsar: The collapsing star scenario that is one of the leading contenders as the cause of long gamma-ray bursts. This artist's concept of the collapsar model shows the center of a dying star collapsing minutes before the star implodes and emits a gamma-ray burst that is seen across the Universe. Longer bursts (more than two seconds in duration) are believed to be caused by massive star explosions.
Credit: NASA/Dana Berry, Skyworks Digital
I’m interested in long GRBs that fit the "collapsar" model of a supernova. In this scenario a huge dying star collapses into a central black hole that blasts a highly collimated beam of particles at over 99 percent the speed of light. This type of GRB is very bright. They can originate near the farthest edges of the observable Universe, and still be detected with ease. The stars linked to them are much more massive than the Sun, and typically on the order of billions of light years away, so we are talking about young stars that exploded a long time ago. And when we can look back in time like this we can start asking cosmological questions.
For me, that means cosmic chemical evolution – studying the origins of metals in space. When astronomers say metals we mean everything on the Periodic Table heavier than hydrogen and helium. We know that the metals are produced in stars and they get into the Universe by means of explosions. This process enriches the interstellar medium and gives rise to stellar generations with increasing metal content during the history of the Universe. In the early Universe there was only hydrogen and helium – the heavier and lighter elements came later. So if we see an object that is very low in metal content, we know that it was likely formed a long time ago.