Sky and Telescope - June 2016 - 14

News Notes

STELLAR I Stars Form in Turbulent Times
New images of four disks around protostars conﬁrm that stars grow in violent
ﬁts and bursts.
Right ascension oﬀset (arcseconds)
2

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HAUYU BAOBAB LIU ET AL. / SCIENCE ADVANCES 2016

Declination oﬀset (arcseconds)

V1735 Cyg

Arm
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Arc

900 AU

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log (polarized intensity/millijansky per arcsecond 2)

This Subaru Telescope image of polarized
intensity shows the clumpy disk around V1735
Cygni. The region nearest the star is masked.

Stars form when pockets of gas in a
cloud collapse under their own gravity
into protostars, fed by spinning gaseous
disks. Observations have revealed that
protostars are far too faint if they're feeding slowly and steadily from these disks.
At the same time, a handful of protostars
brighten by a hundredfold or more in
just a matter of decades before they settle
back down. Looking at these clues side
by side, astronomers drew the obvious
conclusion: accretion is episodic. But they
haven't found direct evidence until now.
Hauyu Baobab Liu (Academia Sinica
Institute of Astronomy and Astrophysics, Taiwan) and colleagues have now
detected pile-ups in the disks around four
extremely bright protostars, manifested
as clumps, arcs, and streamers. The
team reports the near-infrared observations February 5th in Science Advances.
The researchers are now looking to see
whether these structures also appear at
other wavelengths.

June 2016 sky & telescope

Hide Behind the Milky Way

sample - 1.6 to 2 solar masses - about
half are "depressed," the team reported
January 6th at the American Astronomical Society meeting in Kissimmee,
Florida, and in the January 21st Nature.
The mass boundary between "normal"
and depressed red giants is also the transition point from non-convective to convective cores in the giants' progenitors. So
astronomers can track the strength of a
star's internal magnetic ﬁeld using these
brightness patterns.
However, red giants don't have convective cores. But they used to, before their
core-hydrogen fusion shut oﬀ. Thus the
ﬁelds muﬄing the variations are fossils.
Magnetic ﬁelds are like strings, Fuller
says, and they get twisted and tangled by
the churning, convective plasma. Once
this boiling motion disappears, the ﬁeld
essentially freezes in place. So long as
there are no big, bulk motions to disrupt
the ﬁeld, it should just stay put.

Astronomers have found hundreds of
galaxies hidden behind the Milky Way.
Researchers have been mapping out the
local cosmic structure for decades, with
much success. But there are still some
unexplored corners. These generally lie
along the zone of avoidance, the strip of sky
covered by our spiral galaxy's disk.
To peer through the galactic plane,
astronomers often observe the 21-cm emission line of neutral hydrogen (HI). This
radio wavelength passes through the dusty
interstellar muck lying between us and its
source. It also reveals the source's distance:
astronomers estimate a galaxy's distance
based on how fast it's moving away from
us, calculated from how much the galaxy's
spectral lines have shifted. So any redshift
in the 21-cm line's wavelength tells us how
far away the HI-ﬁlled galaxy is.
An ongoing mapping project, called
HIZOA, has surveyed most of the zone of
avoidance that's visible from the Northern Hemisphere. Reporting in the March
Astronomical Journal, Lister StaveleySmith (University of Western Australia)
and colleagues have now released their
analysis of the entire Southern Hemisphere ZOA, or HIZOA-S.
The survey covers more than 1,800
square degrees and probes out to about
250 million light-years, on the scale of our
home supercluster, Laniakea (S&T: Dec.
2014, p. 16). HIZOA-S found 883 galaxies, one-third of which had never been
cataloged, and less than 10% of which had
ever had their distances measured. In all,
HIZOA has detected 957 galaxies.
There are no big surprises, only new
details. For example, the team found
extensions to the Norma Supercluster, also
known as the Norma Wall or the Great
Attractor. The Great Attractor is the major
valley in our cosmic watershed, toward
which the Milky Way and many of its
neighbors are being pulled by gravity, like
tributaries feeding a river. The wall centers
on the massive Norma Cluster, ACO 3627.
The new data show that the wall stretches
roughly 250 million light-years long. ✦

■ CAMILLE M. CARLISLE

■ SHANNON HALL

STELLAR I Old Stars' Fossil Fields
Astronomers have conﬁrmed that
strong magnetic ﬁelds are frozen in place
deep inside aging stars. Stars create
magnetic ﬁelds through the convection
of hot, ionized gas. Where that convection
happens depends on how massive the
star is: low-mass stars, including the Sun,
have convective envelopes around a nonconvective core, but stars a little bulkier
than the Sun have convective cores.
Recently, Jim Fuller (Caltech) and
colleagues found that strong core ﬁelds
could explain the oddly weak, on-and-oﬀ
brightening behavior of a sample of red
giant stars (S&T: Feb. 2016, p. 10). Now,
Dennis Stello (University of Sydney),
Fuller, and their team have expanded this
work to 3,600 red giants observed with
the Kepler spacecraft. The astronomers
found that here, too, some red giants had
"muﬄed" variations, and how much they
were suppressed depended on how massive the star was, with a stronger eﬀect
at higher masses. For the heftiest of the

COSMOLOGY I Galaxies

Sky and Telescope - June 2016

Table of Contents for the Digital Edition of Sky and Telescope - June 2016