When certain stars collapse, they release overwhelming blasts of energy called gamma-ray bursts – the most powerful explosion in the universe. But the cosmic leftovers of these violent outbursts have been a mystery — until now.

Two new studies suggest that when gamma-ray bursts explode, some can leave behind black holes like cosmic gravestones, while others may end up as spinning neutron stars.

Gamma-ray bursts occur when some massive stars reach the ends of their lives and exhaust their supplies of fuel for nuclear fusion in their cores. Without the pressure from fusion pushing outward, gravity wins.

In the ensuing dramatic collapse, a flood of high-energy, short-wavelength gamma-ray light is released. They are sometimes associated with supernovas – another explosive way stars die – but are separate events.

Bursts for black holes

One of the new studies found that for a certain class of the brightest, most powerful gamma-ray bursts, only black holes will do.

"We have focused only on the brightest and most extreme GRBs, arguing that the energy release from these events is too large to be powered by collapse to a neutron star (magnetar)," said study member Brad Cenko, a post-doctoral fellow from the University of California, Berkeley.

Magnetars are a type of neutron star – an object so dense its protons and electrons have merged to form neutrons. They are fast-spinning stars with extremely powerful magnetic fields.

Neutron stars are restricted in how massive they can be – if they weigh over a certain limit, then gravity would have collapsed the object further into a black hole. Black holes, on the other hand, have no upper mass limit, so can encompass any mass needed to power a gamma-ray burst.

This team used data from NASA's Fermi spacecraft, which also observes in gamma-ray light.

The two studies, both presented Wednesday at the Gamma Ray Bursts 2010 conference in Annapolis, Md., help clarify the story behind some of the universe's most violent events. The scientists said their findings don't conflict with each other.