Mystery Swirls Around 'Dark Stars'
David
Mystery Swirls Around 'Dark Stars'
By Charles Q. Choi
SPECIAL to SPACE.com
posted: 21 December 2009
08:21 am ET
When the very first stars lit up, they may have been fueled by the
dark matter that has long eluded scientists.
These "dark stars," first born nearly 13 billion years ago, might
still exist today. Although they would not shed any visible light,
astronomers might detect these invisible giants — some 400 to 200,000
times wider than our sun and 500 to 1,000 times more massive — because
they should spew gamma rays, neutrinos and antimatter and be linked
with clouds of cold, molecular hydrogen gas that normally would not
harbor such energetic particles.
If scientists find these stars, they could aid the search to discover
and identify dark matter. They could also help solve the mystery of
why black holes formed much faster than expected.
Scientists think unseen, as-yet unidentified dark matter makes up
about 95 percent of all matter in the universe. They know it exists
because galaxies rotate faster than can be explained by the visible
matter within them.
Among the main candidates for what dark matter is are WIMPs, or weakly
interacting massive particles. One type of WIMP that scientists
theorize exists is called a neutralino. These particle can annihilate
each other, generating heat. They would also produce quarks and their
antimatter counterparts, antiquarks, which would collide to emit gamma
rays, neutrinos and antimatter such as positrons and antiprotons.
The researchers calculated that in the newborn universe, some 80 to
100 million years after the Big Bang, as proto-stellar clouds of
hydrogen and helium tried to cool and shrink, annihilating neutralinos
would have kept them hot and large. The result might be dark stars,
fueled by dark matter instead of nuclear energy as in normal stars.
These would have been made up largely of normal matter, mostly in the
form of hydrogen and helium, but would be vastly larger and fluffier
than the sun and current stars.
"It's a completely new type of star with a new power source," said
researcher Katherine Freese, a theoretical physicist at the University
of Michigan.
Originally researcher Paolo Gondolo, a particle astrophysicist at the
University of Utah, wanted to dub these new, theoretical kinds of
invisible stars "brown giants," similar to dim but smaller, Jupiter-
sized stars known as "brown dwarfs." But he said his collaborators
insisted on calling them "dark stars," after the song "Dark Star"
first played in 1967 by the revered rock band The Grateful Dead.
"There is a dark star song by Crosby, Stills, Nash and Young, too,
that we had in mind," Freese said.
"It was a good name," Gondolo noted.
Although dark stars are made up of less than 1 percent dark matter,
"it's very important," he explained. "It converts all of its mass to
energy with 100 percent efficiency, under Einstein's equation, E=mc2.
Normal stars that rely on nuclear energy convert just a small fraction
of its mass to energy, 1 or 2 percent."
There was initially skepticism as to whether dark matter densities
were high enough in the early universe to support the creation of dark
stars. "However, we've checked it and so have two other groups, and
they agree with us," Freese said.
Dark stars could be detected with the next-generation James Webb Space
Telescope currently scheduled for launch in 2014. "It may be that
these stars eventually cluster together, and clusters of them might be
visible with the James Webb Space Telescope," Gondolo said.
If scientists do discover dark stars, "they would tell us a very
important thing — that dark matter is made of elementary particles,"
Gondolo said. "At this moment, we know absolutely nothing about what
dark matter is made of. We know where dark matter is, how much is
there, but we don't know its nature."
Dark stars might also explain why black holes formed much faster than
expected. Astronomers have found black holes that existed only a few
hundred million years after the Big Bang, yet current theories suggest
they should have taken longer to form. Dark stars might have collapsed
into black holes very early, Gondolo said, because they might be very
short-lived and could have formed when the universe was young.
In addition, dark stars could solve a puzzle seen with stars in the
galactic halo, the murky, roughly spherical part of the galaxy
extending past the main, visible component. "There's an abundance of
elements in the very old halo stars that's hard to explain, and dark
stars can explain that — they would end up creating the chemical
abundances needed," Gondolo said.
It is unlikely any dark stars are being formed today, Freese said.
"The early universe was more compact than it is now, and everything
was denser, including the amount of dark matter one had at any one
place," she explained. "Now the universe has expanded and things are
less dense, making it harder to make dark stars today."
It remains uncertain just how long dark stars might live, Freese said.
"The ones that formed in the early universe could have continued as
long as they had dark matter to power them," Freese said. "They start
at the center of dark matter 'halos' — giant spherical globes of dark
matter — and these are always merging with other ones, so some might
have burned out their dark matter fuel very early and become either
normal stars or collapsed, but it remains an open question if any have
survived until now."
tarkin
site.
http://chandra.harvard.edu/photo/2008/g19/more.html
They say it is the youngest supernova yet discovered. (Nice pictures
by the way -- it looks like a winged disc!) The supernova is near the
center of our galaxy. Naturally it would be expanding. In my
opinion, this object is not our Planet X. (Though conspiracy
theorists might say that this is a misdirection ploy, saying, nothing
to see here.)
Tarkin