{i also know that an accretion disc composed of gases is formed around a black hole .....
what is the relation betweent he formation of this disc and the rotation black hole ????is there any or not????}

Accretion disc forms around black hole when there is some matter left around black hole after star collapsed (some outer mass of star can be scattered around) or - most often - if something else comes dangerousely close to the black hole (for example, a former companion star in a binary system in which one star collapsed into black hole).

Then this stuff falls on black hole, and due to conservation of angular momentum it does not fall straight, but rather spirals around (because it is highly unlikely that it had zero angular momentum to begin with). Also if the black hole rotates fast, then rotating space drags falling matter in the direction of rotation of black hole. Due to collisions infalling matter gets very hot (and becomes plasma - mainly protons and electrons as most of stuff in Universe is made of hydrogen). This plasma cloud due to collisions (and partly due to its own gravity) flattens into disk shape (with disc axis coinsiding with the direction of total angular momentum of falling matter). Again due to collisions initiaslly irregular disc quickly gets more and more circular as it spirals into black hole (eventually entering it via event horizon equator). In this accretion spiral plasma rotates with almost orbital velocity - therefore the closer to event horizon the faster it moves, and plasma speed may approach essential fraction of light speed before it crosses event horizon. Due to fast collisions at that high speed the temperature of inner plasma approaching black hole is very high and thus such plasma radiates energy predomonantly in x-ray and gamma-ray range (instead of predominant visible radiation for cold peripheral plasma).

Nevertheless, there is plenty of visible radiation left on the tail of black-body distribution to see blue/red shifted lines and to measure the orbital velocity of spiraling plasma at different distances from the hole and thus calculate the mass of black hole. Also, circling electrons emit strong synchrotron radiation in the plane of accretion disk tangentially to their orbits (also in gamma-ray range). Fast rotation of conducting plasma generates very strong magnetic field which shows up in Zeeman-broadened lines and which also allowes small amount of some superfast plasma particles to escape in the direction along lines of magnetic field (jetting in perpendicular to accretion plane direction).

If a black hole happen to be close to densely populated by stars center of galaxy and is groving big by swallowing more and more stars and planets, it may eventually shift the center of galaxy onto itself (more and more stars will orbit groving heavy gravitating object).

When a compact object (like a neutron star) approaches event horizon, the last few seconds as it stretches by tidal forces and crosses event horozon it shines brightly (but shortly) in x-ray and we may see it as a short x-ray burst. Energy released can be few % of mass of such object, so it may outshine entire galaxy for a few seconds.

• better yet how about spin,spin? - wildratsci - March 31, 2002 - 16:57 UTC