According to this article, the mysterious super-high speed stars are
more numerous and fastest than previously though. What if one of these
"hypervelocity" stars came through Sun's neighborhood? How dangerous
it could be for life on Earth?

What would be safe distance a star moving at about 700 - 1000 km/s
could get near Sun without disrupting the planetary system?



http://space.newscientist.com/channe...ays-stars.html


Puny black holes can eject Milky Way's stars

NewScientist.com news service

David Shiga




Tiny black holes near the galaxy's centre can fling stars out of the
Milky Way at break-neck speeds, a new study suggests. Previously, only
the supermassive black hole there was thought to be able to produce
these "hypervelocity" stars.

The researchers say the small black holes could actually be ejecting
more stars than the central black hole does on its own, and that
observing the stars could shed light on the elusive population of
mini-black holes themselves.

Physicist Jack Hills at Los Alamos National Laboratory in New Mexico,
US, predicted in 1988 that the monster black hole at our galaxy's
centre should disrupt pairs of stars, capturing one and launching the
other out of the galaxy altogether. Since 2004, astronomers have found
nine stars travelling at high speed out of our galaxy - all
presumably flung there by the giant black hole, which weighs about 3.6
million times the mass of the Sun.

But a new study suggests that many of the ejected stars are getting
kicked out by a swarm of much smaller black holes at the galaxy's
centre. The study was carried out by Ryan O'Leary and Avi Loeb, both at
the Harvard Smithsonian Center for Astrophysics in Cambridge,
Massachusetts, US.

Close encounters

The smaller black holes are about 10 times more massive than the Sun.
Some studies suggest as many as 25,000 small black holes may orbit the
supermassive black hole at the galaxy's centre, having migrated there
from other parts of the galaxy after being nudged out of their original
orbits by passing stars.

When one of these little black holes near the galactic centre passes
very close to a star, it can sometimes fling the star out of the
galaxy, while itself moving closer to the supermassive black hole, the
study says.

This process could eject stars at an even greater rate than the
disruption of binary stars by the supermassive black hole itself, Loeb
says. "The rate could account for a substantial fraction or maybe most
of the events we have seen so far," he told New Scientist.

If so, studying the sheer number of hypervelocity stars, as well as
their trajectories and speeds, could help astronomers determine how
many black holes are there and how they are distributed in space, Loeb
says.

These have proven to be elusive subjects of study because existing
telescopes do not have the resolution to see within a fraction of a
light year of the central supermassive black hole, where these little
black holes are thought to be concentrated most heavily.

Speed limit

Margaret Geller, an astronomer at the Smithsonian Astrophysical
Observatory who was on the team that found the first hypervelocity star
in 2004, says encounters with small black holes could account for some
of the ejected stars.

She says it is not likely to account for all such stars, however. "It's
tough for them to get the highest velocity objects," she told New
Scientist.

The fastest known hypervelocity star is moving at 709 kilometres per
second with respect to the galaxy. The star would have been slowed by
the galaxy's gravity on its way out, so it must have been moving at
about 1200 kilometres per second initially. But Loeb argues that small
black holes can eject stars at up to 2000 kilometres per second, fast
enough to explain all known hypervelocity stars.

He and Geller agree that searching for even faster stars would be a
good way to test the small black hole mechanism. The supermassive black
hole could fling stars out with starting velocities of 4000 kilometres
per second, faster than anything the small black holes could account
for, Loeb says.

"If the velocity distribution does extend to high speeds, then it
argues for the other mechanism," he says.