It turns out there are many sources of cosmic gamma rays. Supernovae definitely make gamma rays. Black holes and neutron stars make a lot of gamma rays. Another source is active galaxies that have a black hole at the center. And pulsars, spinning neutron stars, make pulses of gamma rays. But the most common gamma ray source is the gamma-ray burst (GRB). These happen several times a day in galaxies scattered across the Universe. They produce brief, intense, flashes of gamma radiation for a few seconds that completely overwhelm every other gamma-ray source in the sky, including the Sun. A gamma-ray burst is so powerful that in a matter of seconds it releases energy equal to all of the Sun's energy generated over its 10 billion year lifetime.

Gamma-ray bursts come in two sizes – long and short. The physics are different depending on the mass of the star. Right now we only have theories when it comes to what drives short duration bursts. The leading one is that short GRBs are extremely violent collisions between two neutron stars that have been circling each other for millions of years. These stars aren't gassy, wispy giants like other stars. A neutron star is more like an atomic nucleus that's 12 km across. In long-duration GRBs, the energy is released during a supernova, the life-ending explosion of a single star.

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 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. The stars linked to them are typically on the order of billions of light years away, so we are talking about very old stars. 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 new stars throughout the Universe. But in the early Universe, there was only hydrogen and helium – the heavier and lighter elements came later. So if we see something that’s low in metal content, we know it’s very old.