http://imagine.gsfc.nasa.gov/docs/science/know_l2/black_holes.html
Black holes are the evolutionary endpoints of stars at least 10 to 15 times as massive as the Sun. If a star that massive or larger undergoes a supernova explosion, it may leave behind a fairly massive burned out stellar remnant. With no outward forces to oppose gravitational forces, the remnant will collapse in on itself. The star eventually collapses to the point of zero volume and infinite density, creating what is known as a " singularity ". As the density increases, the path of light rays emitted from the star are bent and eventually wrapped irrevocably around the star. Any emitted photons are trapped into an orbit by the intense gravitational field; they will never leave it. Because no light escapes after the star reaches this infinite density, it is called a black hole.
But contrary to popular myth, a black hole is not a cosmic vacuum cleaner. If our Sun was suddenly replaced with a black hole of the same mass, the earth's orbit around the Sun would be unchanged. (Of course the Earth's temperature would change, and there would be no solar wind or solar magnetic storms affecting us.) To be "sucked" into a black hole, one has to cross inside the Schwarzschild radius. At this radius, the escape speed is equal to the speed of light, and once light passes through, even it cannot escape.
http://imagine.gsfc.nasa.gov/docs/science/know_l2/active_galaxies.html
Most large galaxies have ~1011 Mo of stars, ~109-10 Mo of interstellar gas, and ~1012 Mo of dark matter. But at least 5% of galaxies, though it may be all of them, also have something else lurking inside...a monster in the middle!
These monsters aren't any of the typical typical horror film terrors, though they might appear in one of your favorite science fiction movies. In reality they are supermassive black holes that spew forth tremendous amounts of energy from jets on their tops and bottoms. How can these incredible objects be explained?
Long ago when galaxies were young, the stars in their cores were very closely packed. Star collisions and mergers occurred, giving rise to a single massive black hole (MBH) with perhaps 106 to 109 Mo. Gas from the galaxy's interstellar medium, from a cannibalized galaxy, or from a star that strays too close, falls onto the MBH. As in X-ray binary star systems, an accretion disk forms, emitting huge amounts of light across the electromagnetic spectrum (infrared to gamma-rays). The MBH plus accretion disk produces the phenomena seen in active galactic nuclei (AGN). Below you see optical and radio images of the active galaxy NGC 4261. The central object, accretion disk, and lobes are all visible.
The different types of AGN are variations on this theme. Many galaxies today (including our Galactic center??) may have a quiet MBH which happens not to have recently accreted gas. Seyfert galaxies have accretion onto a moderate-mass MBH, while the more luminous quasi-stellar objects (i.e. quasars) have accretion onto a high-mass MBH.
In ~10% of the AGN, the MBH + accretion disk somehow produce narrow beams of energetic particles and magnetic fields, and eject them outward in opposite directions away from the disk. These are the radio jets, which emerge at nearly the speed of light. Radio galaxies, quasars, and blazars are AGN with strong jets, which can travel outward into large regions of intergalactic space. Many of the apparent differences between types of AGN are due to our having different orientations with respect to the disk. With Blazars and Quasars, we are looking down the jet. For Seyferts, we are viewing the jet broadside.
Considerable uncertainties remain. Exactly how are jets produced and accelerated? Where do the clouds producing the broad emission lines come from? Can we empirically confirm that a MBH is actually present?
