Black
Hole’s Dinner Bell is Ringing
February
8, 2012
The supermassive black hole at the center of our galaxy is about to
consume a tasty morsel. Astronomers are salivating as they prepare to
witness a cosmic feast that may last several years.
Supermassive black holes have been found at the centers of most major
galaxies; some are 10 billion times more massive than our Sun. Our own
galaxy, the Milky Way, has a much more modest central black hole,
Sagittarius A* (pronounced “A star”). Sag A*, as its friends call it,
is a mere pipsqueak by comparison, “only” 4 million times more massive
than our Sun.
Black holes increase their mass by sucking in surrounding matter —
stars, gas clouds, and the odd planet. It seems that Sag A* has been a
very picky eater, consuming thousands of times less matter than the
central black holes of other galaxies, and therefore emitting thousands
of times less radiation. Sag A*’s modest appetite has made our galaxy a
much safer place for life.
But now, Sag A*’s dinner bell is ringing.
In December 2011, Astronomers from Germany, the U.S. and Chile reported
observations of a gas cloud coming too close to Sag A* for comfort.
Their observations were made with the European Southern Observatory’s
VLT radio telescope array. They’ve identified a gas cloud moving toward
Sag A* at 5 million miles per hour. The cloud contains three times
Earth’s mass, or only 2 trillionth’s of Sag A*’s mass. So this will be
a modest meal — equivalent to a person eating a bonbon only 1/1000th of
an inch across.
The gas cloud’s projected orbit will take it to within 24 billion miles
of Sag A* by mid- 2013. Only stars S2 and S14, (S2 at 11 billion miles
and S14 at 7 billion miles), have orbits that venture closer to Sag A*.
All of these orbits lie far beyond the 8 million mile radius of Sag
A*’s event horizon (its Schwarzchild radius and the point of no
return). Created by Marc Schartmann, below is an illustration of Sag
A*’s vicinity at the Milky Way’s center — the blue ellipses show the
orbits of nearby stars and the red arc represents the orbit of this gas
cloud. The actual size of the gas cloud is much smaller than
illustrated here.
While stars can survive passing within a few billions of miles from Sag
A*, this gas cloud won’t. The reason is that stars are far denser and
have much stronger self-gravity than gas clouds. This gas cloud’s
average radius is 12 billion miles compared with Earth’s 4000 miles and
our Sun’s 400,000 miles. With its mass spread over this great volume,
the gas cloud’s self-gravity is only one-billionth of what’s needed to
resist Sag A*’s gravitational tidal forces — the black hole will shred
the gas cloud to pieces. The color-coded image below shows the extent
of the gas cloud as actually observed from 2000 through 2011, as well
as computer projections of its demise from 2012 through 2016.
Once shredded, the gas cloud’s debris will be dragged into the
accretion disk surrounding Sag A* and ultimately pulled within its
event horizon to oblivion. Black hole feasting has been observed in
remote galaxies — this is what drives quasars and active galactic
nuclei (AGN’s). But now it will be happening up close and personal, in
our own backyard “only” 27,000 light-years (160,000 trillion miles)
away. Astronomers are eager to carefully observe and document Sag A*’s
feast. We’ll be close enough to get a good look at the action, but far
enough away not to become part of the mayhem — our solar system is in
the Goldilocks Zone of our galaxy.
Best Regards,
Robert
* * * * * * * * *
* * * * * * * * * * * * * * * * * * * * * * * * * * *
* * * * * * * * * * * * * * * *
Dr Robert
Piccioni,
Author of "Everyone's
Guide to Atoms, Einstein, and the Universe",
Can Life Be
Merely An Accident?"
and "A
World Without Einstein"
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