News

Our Galaxy Could Have 100
Billion Failed Stars Lurking in
The Darkness
But not many near our Solar System.
Astronomers have estimated that the Milky Way
could contain roughly one brown dwarf for every
two other stars, potentially tripling a previous
estimate that left us wondering where they
were all hiding.
The new research was based on a star cluster
outside of our Solar System's immediate
neighbourhood, suggesting our corner of the
galaxy isn't exactly typical as far as star
production goes.
Back in 2012, observations made by NASA's
Wide-field Infrared Survey Explorer (WISE) on
nearby star clusters indicated only one in seven
stars was a brown dwarf, fewer than the 1:1
ratio astronomers had expected.
The odd results were considered preliminary,
and now by turning their gaze a little further
from home an international team of astronomers
have found much to their relief our galaxy has
far more of these dark suns than first appeared.
Brown dwarfs are somewhere between a planet
and a main sequence star, in that they're
massive objects that just fall short of squeezing
its matter enough to spark a fusion reaction.
That makes them of particular interest to
astrophysicists who want to know how stars are
born.
Such "failed stars" don't shine brightly like their
bigger, more successful siblings, but they can
reach masses in the order of 90 times the mass
of Jupiter, or about 10 percent the mass of our
Sun.
Being kind of dull makes them hard to spot,
limiting brown dwarf hunting to star clusters
less than 1500 light years away – just over the
Solar System's back fence in a galaxy 100,000
light years across .
After stumbling across a cluster containing an
unexpected number of brown dwarfs, the
researchers used the European Southern
Observatory's Very Large Telescope in northern
Chile to analyse the stars in a far off cluster
called RCW38, a young bunch of stars 1.7
kiloparsecs (just over 5500 light years) away.
Its density and richness in massive stars made
it an ideal place to look at the formation of
brown dwarfs in such an environment and test
whether the previous finds were anomalies.
It turns out astronomers were pretty much on
the money about how common failed stars are.
"We've found a lot of brown dwarfs in these
clusters. And whatever the cluster type, the
brown dwarfs are really common. Brown dwarfs
form alongside stars in clusters, so our work
suggests there are a huge number of brown
dwarfs out there," says Aleks Scholz from the
University of St. Andrews in Scotland.
Fortunately the stars are hot enough to emit a
faint glow in infra-red, so while they don't stand
out in the brilliance of other nearby stars, they
can be seen.
To actually detect the faint bodies of brown
dwarfs in the cluster, the astronomers used
technology that accounted for distortions in the
Earth's atmosphere in conjunction with a high
resolution infra-red camera and spectrometer.
Based on their counts, there could be anywhere
between 25 and 100 billion brown dwarf stars in
the Milky Way, with one failed star for every 2
to 5 main sequence star.
That puts the clusters near our Solar System at
the lower end of the spectrum.
Since brown dwarfs are so hard to spot, the
researchers are intentionally being conservative
with their estimates, so it's likely that for every
star sparkling in the galaxy, there's another one
that just wasn't big enough to shine.
"They are ubiquitous denizens of our Milky Way
galaxy," says Ray Jayawardhana from York
University in Canada.
With the recent discovery of nearly all stars
being born with at least a twin, it's possible that
there is a lot of celestial sibling rivalry going on
over our heads.
The research is currently available on the pre-
review website arXiv.org , and will be presented
at this year's National Astronomy Meeting in the
UK .