A stellar mass black hole is a compact object with a mass greater than three solar masses. It is so dense and has such a powerful force of attraction that not even light can escape from it. They cannot be observed directly, but only via secondary effects—for instance, in the case of a black hole feeding on a companion star. In general, when matter falls onto a black hole it does so “quietly” by way of an accretion disc. However, there are periods when this infall is violent and bursts, producing a strong outburst of X-ray brightness.