When a star explodes as a supernova, the remnants of the explosion can fall inwards to form a neutron star or a black hole. Neutron stars are easy to detect because they emit powerful radio waves. Black holes, on the other hand, are difficult to detect because they do not emit any radiation. They can only be observed by their effects on material outside the black hole. However, material falling into a black hole emits radiation before it is swallowed up by the black hole.

Zampieri and co-workers have calculated the effect that the expanding cloud of hot hydrogen gas formed in the explosion has on this radiation. Initially the gas is dense and optically 'thick' at many wavelengths, which prevents light escaping. As the gas expands and cools, however, it becomes optically 'thin' and transparent at many wavelengths. This will lead to a characteristic bump in the light curve.

Many astronomers think that supernova 1987A contains a black hole, especially since there is no evidence that it formed a neutron star. However, there is no evidence for the tell-tale bump in the light curve either. It is now thought that SN1987A is simply to bright for this effect to be observed ( http://xxx.lanl.gov/abs/astro-ph/9804122). That is why attention has turned to SN 1997D, which is the dimmest Type II supernovae observed to date. According to calculations, the bump should become visible to space and ground-based telescopes in the year 2000.