## Suppose that the blackhole formed from a collapsing star.

Because the horizon of this blackhole would be very small -- only about 3 kilometers -- and as weobserved above, as long as you stay well outside the horizon, a blackhole's gravity is no stronger than that of any other object of thesame mass.

## What if the Sun *did* become a black hole for somereason?

The short answer is that the Big Bang gets away with it because it is expanding rapidlynear the beginning and the rate of expansion is slowing down. Space can be flat evenwhen spacetime is not. Spacetime's curvature can come from the temporal parts of thespacetime metric which measures the deceleration of the expansion of the universe. So the total curvature of spacetime is related to the density of matter, but there is acontribution to curvature from the expansion as well as from any curvature of space. The Schwarzschild solution of the gravitational equations is static and demonstrates thelimits placed on a static spherical body before it must collapse to a black hole. The Schwarzschild limit does not apply to rapidly expanding matter.

## How can you tell whetherthe unseen compact object is a black hole?

If the black hole lasts forever, then the light may takearbitrarily long to get out, and that's why she doesn't see you crossthe horizon for a very long (even an infinite) time.

Back in the 1970's, Stephen Hawking came up withtheoretical arguments showing that black holes are not really entirelyblack: due to quantum-mechanical effects, they emit radiation.

## This combination of black and white holes is called a wormhole.

A stellar remnant such as aneutron star would be unable to support itself against gravity, andwould collapse to a black hole.) The combination of such massestimates and detailed studies of the radiation from the accretiondisk can supply powerful circumstantial evidence that the object inquestion is indeed a black hole.

## Black Hole Theory and Hawking Radiation - Black Holes …

Ina review article in the 1992 issue of Annual Reviews of Astronomy andAstrophysics, Anne Cowley summarized the situation by saying thatthere were three such systems known (two in our galaxy and one in thenearby Large Magellanic Cloud) for which very strong evidence existsthat the mass of the invisible object is too large to be anything buta black hole.

## Blackholes, Wormholes and the Tenth Dimension : …

(The technique isquite similar to the one we described above for supermassive blackholes in galactic centers: the faster the star is moving, the strongerthe gravitational force required to keep it in place, and so the moremassive the invisible companion.) If the mass of the compact objectis found to be very large very large, then there is no kind of objectwe know about that it could be other than a black hole.

## Every Black Hole Contains Another Universe?

The matter in theaccretion disk gets very hot as it falls closer and closer to theblack hole, and it emits copious amounts of radiation, mostly in theX-ray part of the spectrum.

## What Is a Black Hole? What Is the Event Horizon?

In particular,in some binary systems containing a compact object such as a blackhole, matter is sucked off of the other object and forms an "accretiondisk" of stuff swirling into the black hole.