Some Stars Bang, Others Just Switch Off

The general concept is that stars explode in a tremendous burst of light and energy when they finish their existence.

This was valid till now.

'Cause astronomers have detected a new class of stars that seem to fade away quietly, and in the dark. When stars up to eight times more massive than our Sun run out of fuel, they lose their outer layers, remaining a white dwarf. Larger stars die more violently.

When their fuel is out, their cores collapse, provoking titanic explosions called supernovas (photo). That can throw several solar masses worth of material into space and what's left in place is either a neutron star or a black hole.

Supernova emits luminous, energetic events called gamma-ray bursts (GRBs), particularly those lasting longer than two seconds. "Long" GRBs are a signal that a star is going to explode.

In June, NASA's Swift telescope detected a long GRB in a dwarf galaxy 1.6 billion light-years away in the constellation Indus. GRB 060614 lasted for 102 seconds but after that.... there was no supernova!

In another case, astronomers using the European Southern Observatory's Very Large Telescope monitored it for 50 days. "Despite our deep monitoring, no rebrightening due to a supernova was seen," said Gianpiero Tagliaferri, an astronomer at the Observatory of Brera in Italy. "It is a bit like not hearing the thunder from a nearby storm when one could see a very long-lasting flash," said Johan Fynbo of the DARK Cosmology Center at the Niels Bohr Institute of Copenhagen University in Denmark.

Fynbo's team observed the same phenomenon with another long GRB one month before. GRB 060505 occurred in a small spiral galaxy and lasted just 4 seconds. These two GRBs point to the fact that star death may not always involve a supernova explosion. "It's easy to explain away one of these anomalous events as a fluke, but two strange events give our claim some oomph," said Joshua Bloom, an astronomer at the University of California, Berkeley. "These events are observational threats to the one-to-one association between long bursts and supernovae."

Thus, some massive stars do not pass the supernova phase, releasing just a long GRB before turning into a black hole. In this case, "all the material that is usually ejected in a supernova explosion would then fall back and be swallowed," explained Guido Chincarini, an astronomer at the University of Milano-Bicocca in Italy.

Or maybe the new type of long GRB is the result of a fusion. Collisions between two neutron stars or a neutron star and a black hole also emit GRBs, but these GRBs are shorter, of less than 2 seconds, and less energetic. "Some unknown process must be at play, which we have presently no clue," said study team member Massimo Della Valle of the Osservatorio Astrofisico di Arcetri in Firenze, Italy.

"Either it is a new kind of merger which is able to produce long bursts, or a new kind of stellar explosion in which matter can't escape the black hole."