问天者
发表于 2003-4-12 21:32
|
显示全部楼层
|阅读模式
来自： 中国–四川–成都 教育网/成都理工大学教育网
A nearby black hole, hurtling through the plane of our galaxy like a cannonball,
has given what some astronomers say is their best evidence yet that stellar-mass
black holes are made in supernova explosions. The black hole, called GRO J1655-40
, is streaking across space at a rate of 250,000 miles per hour. That speed is fo
ur times faster than the average velocity of the stars in that galactic neighborh
ood. The most likely "cannon blast" is the explosive kick of a supernova, one of
the universe's most titanic events.
Even though, by definition, black holes swallow light, the runaway black hole ha
s a companion star, allowing astronomers to track it. NASA Hubble Space Telescope
's sharp view allowed astronomers to measure the black hole's motion across the s
ky in images taken in 1995 and 2001. Combining the Hubble data with separate meas
urements of its radial motion toward Earth taken from ground-based telescopes yie
lds the true "space velocity" of the black hole, and shows that it is streaking a
cross the plane of our Milky Way in a highly elliptical orbit.
"This is the first black hole found to be moving fast through the plane of our g
alaxy," says Felix Mirabel of the French Atomic Energy Commission and the Institu
te for Astronomy and Space Physics of Argentina. "This discovery is exciting beca
use it shows the link of a black hole to a supernova," aside from observing gamma
-ray busts from hypernovae (even more powerful stellar explosions), which are bel
ieved to make black holes. Mirabel's results appear in the November 19 issue of A
stronomy and Astrophysics.
Though the black hole is roughly heading in our direction, it is at a "safe" dis
tance, 6,000 to 9,000 light-years away, in the direction of the constellation Sco
rpius. Mirabel believes the black hole may have been born in the inner disk of ou
r galaxy, where the highest rate of star formation is taking place.
An aging, evolved star whirls around the black hole, completing one orbit just e
very 2.6 days. The hole is slowly devouring the companion, which apparently survi
ved the supernova that originally created the black hole. This process makes blow
torch-like jets that stream away from the black hole at a significant fraction of
the speed of light. It is the second "microquasar" discovered in our galaxy (mea
ning that it is a scaled-down model of monster black holes at the cores of extrem
ely active galaxies, called quasars.)
Astronomers have known about stellar-mass black holes (ranging anywhere from 3.5
to approximately 15 solar masses) since the early 1970s. The only conceivable me
chanism for making such black holes would be the implosion of the core of a star
when it dies. The implosion sends out a shockwave that rips the rest of the star
to shreds as a supernova. If the surviving core is greater than 3.5 times our Sun
's mass, no forces can stop the collapse, and it will shrink to an infinitely sma
ll and dense singularity.
Astronomers have catalogued even faster-moving neutron stars catapulted by a sup
ernova explosion. The black hole is moving relatively slower because it has much
more mass and so has more resistance to being accelerated.
Release Date: 10:00AM (EST) November 18, 2002
Release Number: STScI-2002-30 |
|
置顶卡
变色卡
千斤顶
显身卡
