Astrophysicists believe that stars more than eight times heavier than our Sun end their lives in spectacular supernovae explosions before collapsing to form black holes.

Several solar masses worth of material are ejected into the interstellar medium during a supernova and this produces a fantastic display of light and colour.

A powerful pulse of gamma rays lasting two seconds or more is also emitted during this process and when astronomers detect such a gamma-ray burst (GRB), a number of space and Earth-based telescopes are immediately trained at the source to observe the light from the supernova.

But for two GRBs detected earlier this year, the light never came. One of the astronomers involved in the studies, Guido Chincarini of Italy’s University of Milano-Bicocca, believes that the most likely explanation for this apparent darkness is that the material usually ejected during a supernova had been sucked into a massive black hole formed by the dying star.

The two bursts were observed less than one month apart, which has led some astronomers to conclude that these events are not out of the ordinary. “This may be a more common type of explosion than we expected, possibly a new mechanism for star death,” said Pall Jakobsson of the University of Hertfordshire in the UK, who was also involved in studying the GRBs.

While GRBs have been observed without a corresponding supernova, they have always been much shorter and more energetic than supernovae bursts. This had led astronomers to classify GRBs as “long” supernovae bursts, which last more than two seconds, and “short” bursts – the latter are believed to be the result of the merger of two black holes or neutron stars. This clear distinction appears to have been shattered by the two new GRBs, which were 102 and four seconds long respectively.