Scientists
have long known that stars are formed from swirling clouds of gas and dust that
coalesce. But why some of these stellar nurseries give rise to ordinary stars
like our sun and others can pop out stars 15 to 30 times as massive is
something of a conundrum.
Astronomers
working with the Submillimeter
Array in Mauna Kea, Hawaii think they may have found the key difference:
turbulence.
The
Submillimeter Array (SMA) is so named because it probes the universe in
wavelengths of light from 0.3 to 1.7 millimeters (0.01 to 0.07 inches). Most
radiation in this range comes from the cold interstellar gas and dust from
which stars and planets form.
The ability
to monitor this radiation comes in handy when astronomers are trying to peer
into stellar
nurseries, whose cocoons — all that dust and gas —block visible light.
A team of
astronomers at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., used the SMA to probe two such cocoons located 15,000 light years away in
the constellation Serpens Cauda.
"The
SMA enables us to see the dust and gas in the cocoon with amazing details, and
to probe the initial stages of massive
star formation," said team member Qizhou Zhang.
The team's
findings will be detailed in an upcoming issue of The Astrophysical Journal.
A baby
star is born
The basics
of star formation are pretty clear: a cloud of cosmic gas tucked away in
some part of a galaxy spins and coalesces under the pull of gravity. As it does
so, the gas grows denser and hotter until nuclear fusion ignites and a baby
star is born.
The
gravitational pull that condenses the gas also tends to fragment it, fracturing
the condensing cloud into smaller and smaller pieces. Astronomers think this
fragmentation may inhibit the formation of massive stars because the resulting
pieces are too small — they eventually become mundane-sized stars like our sun.
But young,
massive stars are clearly seen in some clouds; for example, the Orion Nebula
(located in the Hunter's sword of the Orion constellation) is host to a cluster
of newborn stars called the Trapezium that are many times more massive than our
sun and 100,000 times as bright.
So some
mechanism must allow the behemoth babies to form in these birthing clouds.
Counteracting
gravity
Some
astronomers posit that such young, massive stars are the result
of collisions between smaller nascent stars. But this method requires an
"extreme environment," Zhang told SPACE.com, and the stellar
nurseries they have examined don't have a high enough density of protostars for
collisions to occur.
Another
proposal that some force must be counteracting gravity and suppressing
fragmentation in the clouds, allowing the massive stars to form outright, gathering
gas as smaller stars do.
Two such
forces are known to be at work in gas clouds: thermal pressure and turbulence.
The thermal pressure is the result of the intense heat radiating from the
protostars. The turbulence is likely the result of spiral waves in galaxies,
supernovas interacting with the clouds, or outflows of material from newborn
stars, said Zhang's coauthor, Thushara Pillai.
Previous
work had suggested that thermal pressure was the strongest influence opposing
fragmentation, but the new SMA study finds that turbulence is more important,
at least at certain spatial scales.
"What's
unique about these SMA observations is that we can check some of the hypotheses
for massive star formation against the observations for the first time,"
Zhang said. "Unlike what has been assumed in theoretical models, we found
that fragmentation is suppressed in these clouds, not by stellar heating but
rather by turbulence."
This was
contrary to previous theories because it was thought that "when you
trigger those turbulence [events], they die down very quickly," Zhang
said. But that doesn't seem to be the case, and some feedback that perpetuates
the turbulence must be in play.
While this
study has shed some light on massive star formation, it's a first step that
will be followed-up by a more comprehensive survey of the regions where massive
stars form that will look for signatures of stellar outflow and potential
feedbacks of turbulence
"We
have just started to understand the initial conditions in distant, massive
star-forming regions," Thushara Pillai said. "A large survey that we
have launched with the SMA will, in the near future, reveal the nature of more
of such objects."
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