Sky and Telescope - June 2015 - 13

IN BRIEF
STELLAR I Before They Were (Binary) Stars

Citizen Scientists Find "Yellowballs."
Thanks to volunteers working with Zooniverse's Milky Way Project, astronomers
have discovered a new signature marking
a hidden phase of star formation. The
project's aim is to ﬁnd cavities carved
out by the winds of newborn, massive
stars. Before they emerge from their dusty
cocoons, though, the not-yet-stars must
grow from cool clumps of dense gas into
ready-to-burn protostars, heating up their
surroundings in the process. When a
citizen scientist known by the username
kirbyfood came across a mysterious fuzzy
yellow thing and posted it in the Zooniverse
forum, a professional astronomer tagged
the object with the name #yellowball.
Soon that identiﬁer marked another 928
objects. Researchers think the yellowballs
mark the in-between phase of transition
from cool clumps of gas and dust to newly
formed stars, Charles Kerton (Iowa State)
and colleagues report in the February 1st
Astrophysical Journal. Their yellow color
comes from the combined glow of warm
dust (red) and organic molecules (green) in
false-color infrared images from the Spitzer
Space Telescope.

Astronomers have taken a behind-thescenes look at a set of dense gas clumps,
catching a quadruple star system in the
ﬂeeting act of formation. Jaime Pineda
(ETH Zurich, Switzerland) and colleagues report the observations in the
February 12th Nature, and they add some
much-needed evidence to the theoretical
playing ﬁeld.
It's easy to make multiples in theory.
One idea is that a single star-forming
clump might split into several, like fraternal twins in the womb. Another proposes
that the disk of material that feeds a
forming star might be gravitationally
unstable, fragmenting and collapsing into
another star that orbits the ﬁrst. Some
theorists have also suggested complex
three-body encounters that can lead to
stellar capture or a modiﬁcation of existing partnerships.
But Pineda says these theories have
a hard time explaining the formation of
wide binaries, stellar siblings separated
by thousands of astronomical units (a.u.).
Instead, his observations point to a fourth
option: fragmenting ﬁ laments.
Pineda's team used the Very Large
Array in New Mexico to image radio
waves emitted from ammonia molecules
in the Perseus star-forming region. This
radiation traces the presence of dense
gas and reveals long ﬁ laments that have
crumbled into four distinct clumps in a

star-forming core called Barnard 5, which
lies 815 light-years away. One of these
clumps contains a well-known protostar, a star that hasn't yet ignited its core
fusion. The other three clumps surround
the protostar at distances ranging from
3,300 to 11,400 a.u.
The protostar and the three surrounding clumps each contain between a tenth
and a third of the Sun's mass, based oﬀ
the submillimeter-wavelength brightness
as seen with the James Clerk Maxwell
Telescope on Mauna Kea. The authors
estimate that the clumps' gravitational
collapse will take roughly another 40,000
years, so the stars' ﬁnal masses will
depend on how much gas they can collect
in that time. Gas ﬂowing along the ﬁlaments might continue to feed the growing
clumps, or the individual masses could
fragment further even as they continue to
collect gas from their surroundings.
The clumps are smaller than what you
might predict if you simply pit gravity's
inward pull against the thermal motion
of gas molecules. Instead, it looks like
random ﬂows of turbulence have broken
up the condensations within this ﬁ lament. Turbulent fragmentation isn't a
new idea, but observational conﬁrmation
has only recently entered the realm of
possibility, in part due to the VLA's massive upgrade in 2011.

the matrices of tetrataenite within Imilac
and Esquel record changes of strength
and direction of the magnetic ﬁeld produced by their parent bodies over time -
and the eventual shutoﬀ of the ﬁeld once
each asteroid's core solidiﬁed.
These are some of the ﬁrst observations of how an asteroid's magnetic ﬁeld
changes in time, notes planetary scientist
Ben Weiss (MIT), who was not involved
in the study. The measurements show
that asteroid magnetic ﬁelds probably
were generated a lot like that of Earth:

by the motion of iron-rich ﬂuid in a core
that is turning solid. The motion would
have been driven by the expulsion of
iron-depleted material from the core as it
"froze." Previous research assumed that
convection in these bodies was thermally
driven, like boiling water, which transfers
heat with physical motion from a pot's
bottom to top. However, the magnetic
activity the two meteorites record lasted
well beyond what thermally driven convection could have sustained.

Eta Carinae's X-ray Pulse. Ranking as
the most massive, most luminous star
within 10,000 light-years of us, Eta Carinae has baﬄed astronomers ever since it
unexpectedly ejected a vast shell of matter
in the 1840s. They now think it's actually
a binary star whose components have
roughly 90 and 30 times the Sun's mass.
Recently, space observatories found that
Eta Carinae creates strong X-ray outbursts
every 5½ years, whenever the paired stars
are closest in their highly elongated orbit,
separated by only about 225 million km
(140 million miles) - roughly Mars's distance from the Sun. Computer simulations
by Thomas Madura (NASA Goddard Space
Flight Center) and others suggest that the
secondary's thin, high-speed stellar wind
collides violently with the primary's slower,
denser wind, creating a superheated shock
boundary that generates a torrent of X-rays.

■ EMILY POORE

■ J. KELLY BEATTY

■ MONICA YOUNG

■ MONICA YOUNG

SkyandTelescope.com June 2015

13
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Sky and Telescope - June 2015

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