More
than half of all stars are in multiple systems: binary stars, or even
triplets or quadruplets, that orbit one another. No one is quite sure
how or why they form, but the effects can be significant, for example
influencing the character of their planets. Our Sun is uncommon in
having no companion star, perhaps suggesting that its configuration of
planets is equally uncommon.
There are two principal ideas about how multiple stars form:
fragmentation in the early stages of birth, or the gravitational capture
of a nearby star later on. Computer simulations of star formation find
that both are reasonable possibilities, and so astronomers have been
trying to make observations to refine the models and the conclusions.
Writing in this week’s journal Nature, Alyssa Goodman and her
collaborators report finding a nearby stellar nursery where quadruplets
are being born. The region is in the star forming molecular cloud in
the direction of the constellation of Perseus, about 825 light-years
away. Scientists have known for decades about a protostar in this area,
a dense core of material that is developing into a small star about
one-tenth of a solar-mass in size.
Using radio wavelength observations of dense molecular gas, ammonia
in particular, the team discovered that around this protostar are
several filamentary gas structures in which they detected three other
condensations. The other three embryos are two to three times more
massive than the main protostar, and models suggest they will become
stars soon - in roughly forty thousand years. The longest dimension of
the complex is only about ten thousand astronomical units (one AU is the
average distance of the Earth from the Sun), and so these objects are
close enough together for gravity to be the major influence in their
development; velocity measurements confirm that the objects are
physically associated.
It is possible – even likely - that during the stars’ development
their orbital motions will prompt the ejection of one or two members
from the system, but for now it appears that at least one binary pair
will survive for longer times. Other stellar systems need to be
examined in order to see how widespread these young multiplets really
are, but the new results support models in which multiple stars form
very early in the stellar womb.