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Ashes from massacred
planets hide black holes
PLANETS and asteroids may be smashing into each other by the thousand
around monster black holes. Dust from the collisions could explain why
many of these colossal objects, which would otherwise shine brightly as
they swallow nearby matter, are hidden from view.
Most galaxies like the Milky Way harbour a central black hole with a mass
upwards of a million suns. While the Milky Way's black hole is currently
inactive
, many in other galaxies are swallowing up gas from dense discs
that surround them, producing copious X-rays and other radiation in the
process.
In addition to providing fodder for the black holes, these discs may give
birth to millions of stars, explaining the dense star clusters seen in some
galaxies' hearts.
These stars may in turn host a lot of planets. But they might have trouble
holding onto them, says a team led by Sergei Nayakshin at the University
of Leicester, UK.
That is because the stars are crowded together so tightly that frequent
gravitational encounters between them would strip planets away, sending
them careening through space. En route, they would often crash into each
other at speeds much higher than anything that happens in our solar
system. Here, collisions typically occur at less than a few tens of
kilometres per second. But around a supermassive black hole, objects zip
around so fast that crashes would happen at up to 1000 kilometres per
second, pulverising the colliding objects.
That would be bad news for any life on the rocky bodies, but Nayakshin
thinks the intense X-rays and ultraviolet radiation so close to a black hole
would prevent life from forming on these planets in the first place. "It
would be unimaginable that you could have life in these sorts of
environments," he says.
However, dust from the crashes could explain why many actively feeding
black holes are hidden behind a fat doughnut-shaped dust shroud, the
team says in an upcoming issue of Monthly Notices of the Royal
Astronomical Society.
It is not clear how much dust each doughnut might contain, but assuming
it is about 300 suns' worth, that could be produced by grinding up planets
from around 100,000 to a million stars, the team estimates. The idea is
reasonable, says Richard Mushotzky at the University of Maryland in
College Park, adding that the doughnuts have "so far been very difficult to
explain".
The dust may also help new stars to form, since it absorbs radiation from
the feeding black hole that might otherwise push away star-forming gas.
Ashes from massacred planets hide black holes - space - 19 September 2011 - New Scientist
planets hide black holes
PLANETS and asteroids may be smashing into each other by the thousand
around monster black holes. Dust from the collisions could explain why
many of these colossal objects, which would otherwise shine brightly as
they swallow nearby matter, are hidden from view.
Most galaxies like the Milky Way harbour a central black hole with a mass
upwards of a million suns. While the Milky Way's black hole is currently
inactive
, many in other galaxies are swallowing up gas from dense discsthat surround them, producing copious X-rays and other radiation in the
process.
In addition to providing fodder for the black holes, these discs may give
birth to millions of stars, explaining the dense star clusters
seen in somegalaxies' hearts.
These stars may in turn host a lot of planets. But they might have trouble
holding onto them, says a team led by Sergei Nayakshin at the University
of Leicester, UK.
That is because the stars are crowded together so tightly that frequent
gravitational encounters between them would strip planets away, sending
them careening through space. En route, they would often crash into each
other at speeds much higher than anything that happens in our solar
system. Here, collisions typically occur at less than a few tens of
kilometres per second. But around a supermassive black hole, objects zip
around so fast that crashes would happen at up to 1000 kilometres per
second, pulverising the colliding objects.
That would be bad news for any life on the rocky bodies, but Nayakshin
thinks the intense X-rays and ultraviolet radiation so close to a black hole
would prevent life from forming on these planets in the first place. "It
would be unimaginable that you could have life in these sorts of
environments," he says.
However, dust from the crashes could explain why many actively feeding
black holes are hidden behind a fat doughnut-shaped dust shroud, the
team says in an upcoming issue of Monthly Notices of the Royal
Astronomical Society.
It is not clear how much dust each doughnut might contain, but assuming
it is about 300 suns' worth, that could be produced by grinding up planets
from around 100,000 to a million stars, the team estimates. The idea is
reasonable, says Richard Mushotzky at the University of Maryland in
College Park, adding that the doughnuts have "so far been very difficult to
explain".
The dust may also help new stars to form, since it absorbs radiation from
the feeding black hole that might otherwise push away star-forming gas.
Ashes from massacred planets hide black holes - space - 19 September 2011 - New Scientist