A new study done has shown that stars are all the while single,
not double as we thought.
Astronomers Had it Wrong: Most Stars are
Single
For more than 200 years, astronomers thought that most of the stars
in our galaxy had stellar companions. But a new study suggests the
bulk of them are born alone and never have stellar company.
Since planets are believed to be easier to form around single
stars, the discovery could mean planets are more common as
well.
Conventional wisdom on double star systems, called binaries, goes
as far back as the late 1700s. More sophisticated observations made
in the 20th century seemed to confirm the numerical dominance of
pairs.
Stellar surveys found that more than half of all Sun-like stars
were part of multiple systems. For more massive stars, like O- and
B-type stars, the number was estimated to be as high as 80
percent.
A few stars, like the North Star, Polaris, have two or more
companions.
"If you go out and look at the all visible stars in the night sky
and ask, 'How many of those are binary?' the answer is, 'Most of
them,'" said study author Charles Lada of the Harvard-Smithsonian
Center for Astrophysics (CfA). "The assumption was that because
most bright stars were binaries, all stars would tend to be
binaries."
The catch, however, is that most stars in the Milky Way are not
bright stars like our Sun, but dim, low-mass stars called red
dwarfs.
Red dwarfs
Scientists estimate that red dwarfs make up to 85 percent of the
stars in our Galaxy. These stars are about one-fifth as massive as
the Sun and up to 50 times fainter.
Red dwarfs are so dim that it's only been in the past decade or so
that technology has improved to the point where astronomers can
study them in detail. And they've found that only about 25 percent
of red dwarfs have stellar companions.
Lada concludes that upwards of two-thirds of all star systems in
the galaxy are single, red dwarf stars. His finding, announced
today, was detailed in a recent online edition of Astrophysical
Journal Letters.
"It's an important realization," said Christopher McKee, an
astronomer from the University of California, Berkeley who was not
involved in the study. "The facts that [Lada] points out were
available for a long time and people just hadn't put them
together."
The skew towards singleness only applies to red dwarfs, however.
It's still true that more than half of the brighter, more massive
stars in the galaxy have companions.
Star birth
Recent observations show that half a dozen new stars are produced
each year in the Milky Way, but many of the details surrounding
their births remain unclear. According to the classical theory,
stars form when large clouds of dust and gas in space collapse
under the force of gravity. If these dense cores of matter reach a
critical limit, internal nuclear fires ignite and new stars are
born.
However, this theory doesn't explain why most large stars tend to
have companions.
"There are theories that can form binary stars and theories that
can form single stars, but there none that explain why you form
more singles than binaries," Lada told SPACE.com.
Astronomers have some ideas though. Observations have shown that
the large clouds that serve as stellar nurseries for massive stars
are more turbulent than small clouds where red dwarfs are thought
to typically come from. It could be that this turbulence causes
massive stars to form in groups of two or more.
"If the [large clouds] have some initial turbulent motions, then as
they collapse, they will tend to split up," Lada explained.
Forming planets
Lada's finding could be good news for planet-hunters. Although some
studies suggest that planet formation around binary star systems is
more common than previously thought, most astronomers believe that
making planets is still easier around single stars. It's thought
that when there are two or more stars, the gravitational forces
between them hinder matter from clumping into cores dense enough to
form planets.
If single stars are the rule and not the exception, as the new
finding suggests, then extrasolar planets may be more common as
well.
"Red dwarfs may be a fertile new hunting ground for finding
planets, including ones similar in mass to the Earth," Lada
said.
One such planet may have already been found. Astronomers recently
discovered a cold, rocky planet that is about 5.5 times the mass of
Earth around a red dwarf 28,000 light-years away.
