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Black Holes - Astronomy


Black Holes: What Are They?

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 only thing that would change would be the Earth's temperature. 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.

The Schwarzschild radius can be calculated using the equation for escape speed.

vesc = (2GM/R)1/2
For photons, or objects with no mass, we can substitute c (the speed of light) for Vesc and find the Schwarzschild radius, R, to be
R = 2GM/c2

 

If the sun was replaced with a black hole that had the same mass as the sun, the Schwarzschild radius would be 3 km (compared to the sun's radius of nearly 700,000 km). Hence the Earth would have to get very close to get sucked into a black hole at the center of our solar system.

(NASA Material)


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GOODS Chandra Deep Field South GOODS Chandra Deep Field South (June 01, 2004)
RXJ1242 MGG 11 (April 10, 2004)
RXJ1242 RX J1242-11 (February 18, 2004)
SS 433 SS 433 (January 5, 2004)
Cygnus X-1, XTE J1650-500 & GX 339-4 Cygnus X-1, XTE J1650-500 & GX 339-4 (September 17, 2003)
Chandra Deep Fields Chandra Deep Fields (June 19, 2003)
Sagittarius A* Sagittarius A* (January 6, 2003)
SS 433 SS 433 (December 11, 2002)
NGC 6240 NGC 6240 (November 19, 2002)
XTE J1550-564 XTE J1550-564 (October 3, 2002)
Sagittarius A* Sagittarius A* (September 5, 2001)
XTE J1118+480 XTE J1118+480 (May 07, 2001)
Black Hole Event Horizon (Illustration) Black Hole Event Horizon (Illustration) (January 11, 2001)
M82 Black Hole M82 Black Hole (September 12, 2000)
Pictor A Pictor A (June 6, 2000)
NGC 4151 NGC 4151 (June 5, 2000)
NGC 3783 NGC 3783 (May 25, 2000)
CXO 0312 Fiore P3 CXO 0312 Fiore P3 (March 20, 2000)
Type 2 Quasar Type 2 Quasar (March 20, 2000)
NGC 5548 NGC 5548 (February 17, 2000)
Galactic Center Galactic Center (January 14, 2000)

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