These include a wide variety of exotica such as Seyfert galaxies, radio galaxies, and BL Lacertae objects. Since the discovery of quasars in 1963, much effort has gone into understanding their energy source. Over time, BHs have gained the widest acceptance. The key observations that led to this consensus are as follows.
Quasars have prodigious luminosities.
Yet they are tiny, because they vary on timescales of hours. From the beginning, the need for an extremely compact and efficient engine could hardly have been more apparent. Gravity was implicated, because collapse to a black hole is the most efficient energy source known. He showed that any attempt to power quasars by nuclear reactions alone is implausible. This boosts the power of a possibly small part of the quasar engine and weakens the argument that the object cannot vary on timescales less than the light travel time across it.