A massive black hole may form, on a dynamical timescale, from the runaway collapse of a dense star cluster or the instability of a massive object. Alternatively, Hills  suggests that a black hole of 10 M would be able to grow to 107 M within ~ 109 yr. It is premature to incorporate quasars into the general context of galactic evolution. However, we observe that the phenomenon was more prevalent at the epoch z 2; and it seems astrophysically unlikely that a single object could maintain a quasar-level luminosity for ~ 1010 yr (though it could "flare up" more than once if rejuvenated by an increased infall rate from its surroundings). This raises the question of what "switches off" the phenomenon, and various ideas suggest themselves: typical galaxies may be "swept clean" when the gas supply falls below a critical rate  (the explosive activity in the nucleus may itself aid the same result); a central black hole fueled by stars would swallow all those within the dense core, the capture rate thereafter being limited by the rate ( tR-1) at which dynamical relaxation can replenish the orbits that pass close to the hole [23, 29].
Solar-type stars are swallowed whole (because rT > r) when Mh exceeds a value ~ 3 x 108 M for a Schwarzschild hole and ~ 109 M for a Kerr hole. This might reduce the resulting luminosity, though of course it in no way inhibits the hole's further growth. The expected angular momentum of the hole is itself determined by the mode of accretion onto it: if there is a stably orientated disk, the hole, whatever its initial angular momentum, would spin up to a/m 0.99 if it accretes enough material to at least double its mass; but if the infalling gas or stellar debris has no preferred angular momentum, then obviously a/m -> 0. There has been some debate [43, 44] on whether black holes in the range 106-108 M could exist in the nuclei of normal galaxies without displaying energetic activity exceeding what is observed. The issue hinges on whether a tidally disrupted star need necessarily all be swallowed with high ; and on whether this would yield a brief intense flare-up (with a correspondingly short "duty cycle") or a steady luminosity from the nucleus.
An unavoidable consequence of the quasar model outlined in this paper is that massive and relatively quiescent black holes should lurk in the nuclei of many large nearby galaxies. The very compact source in the nucleus of Centaurus A could be an important and relevant clue. At a distance ~ 5 Mpc, Cen A is the closest radio galaxy. Its total radio power output is now only ~ 1042 erg s-1. An energy of ~ 1060 erg is, however, contained in the very extended radio lobes, and Cen A may have once been as powerful a radio source as (say) Cygnus A or 3C 273. If this energy were generated by a black hole (or its progenitor), the mass would be 107 M. There is a very compact radio source in the nucleus; and an X-ray source which varies on time-scales that may be as short as 2 hr. Fabian and his collaborators  have attributed the X-ray and radio properties to relatively slow quasispherical infall onto such a black hole. If this interpretation is correct, Centaurus A has a nucleus whose violent activity is almost defunct and is perhaps the closest massive black hole manifesting the effects of accretion. The compact radio sources in the nuclei of some ellipticals  can be interpreted in this way too. The very small (~ 10 a.u.) source in the Galactic Centre - a unique source in a unique location could result from a very low rate of accretion onto a black hole. The value of Mh must however be 5 x 106 M (as discussed in Professor Oort's contribution to these proceedings), implying that our own Galaxy could never have flared up into a quasar or radio source on a really spectacular scale.