Sky & Telescope - October 2021 - 11

STARS
Moon-size White Dwarf
Is Smallest Ever Found
MOST WHITE DWARFS are collapsed
stellar cores. But in the July 1st Nature,
Ilaria Caiazzo, Kevin Burdge (both at
Caltech), and colleagues report evidence
of a white dwarf born from the union
of two smaller versions - and this one
might be about to collapse.
Squeezed-together electrons support
white dwarfs against gravity, so the
more massive it is, the more compact
it becomes. But when a pair of white
dwarfs merges, the total mass may tip
the scale over the Chandrasekhar limit,
beyond which runaway nuclear reactions
detonate the stellar cinder. But if
both white dwarfs are small enough,
their union might not trigger ignition.
Burdge found such a survivor while
sifting through Zwicky Transient Facility
data for minutes-long brightness
variations. Follow-up observations confirmed
this white dwarf
rotates every 7 minutes
(rather than hours).
A fast spin is expected
for a merger product and
should generate a strong
magnetic field. Using the
W. M. Keck I Telescope
on Mauna Kea, Hawai'i,
Caiazzo and colleagues
found that the white
dwarf's magnetism is on
the extreme end - more
than a billion times
stronger than Earth's.
Caiazzo's team then used the white
This artist's concept
compares the white
dwarf's size to the Moon.
The exact mass limit
depends on its composition,
which is likely
changing as heavy nuclei
in the core capture electrons
and remove their
outward pressure. Electron
capture itself accelerates
as heavy atoms
settle into the core over
time. Eventually, fusion
could restart, exploding
or further collapsing the
white dwarf.
However, the star's core is also cooldwarf's
brightness and distance to
estimate its size: At 4,300 kilometers
(2,700 miles) across, it's just bigger
than the Moon. That diameter corresponds
to a mass between 1.327 and
1.365 Suns' worth, depending on its
composition.
It remains unclear if, when, and how
the white dwarf might collapse further.
ing. It will crystallize anywhere between
10 to 100 million years from now,
preventing heavy elements from settling
and perhaps staving off collapse.
Whatever its ultimate fate, the
detection of this young object just 134
light-years away suggests there may be
many more merger-born white dwarfs
out there awaiting discovery.
¢MONICA YOUNG
IN BRIEF
A Menagerie of Stellar Nurseries
Astronomers with the Physics at High Angular Resolution in Nearby Galaxies
(PHANGS) group have created the first high-resolution maps of the big,
dense gas clouds that form stars inside galaxies. The collaboration used the
Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to study some
40,000 molecular clouds within 90 nearby, star-forming galaxies.
ALMA mapped carbon monoxide (orange), a tracer of hard-to-see hydrogen
gas, enabling astronomers to observe gaseous structures on the
scale of about 300 light-years, which happens to be around the size of an
individual cloud. Within these regions of dense gas and dust, up to tens of
thousands of stars form over a few million years.
While astronomers previously thought that all stellar nurseries were essentially
the same, the new maps, shown at the June virtual meeting of the
American Astronomical Society, illustrate that the clouds' characteristics vary
with their location within their host, with those near a galaxy's
center being more massive, dense, and turbulent. The results
will also appear in the Astrophysical Journal Supplement Series.
¢ LAUREN SGRO
The Giant That " Blinked "
A red giant star nearly disappeared
from the sky for about 200 days in
2012. In April that year, only 3% of its
light was arriving at Earth, but by July
its brightness was back to normal. The
huge dip, cataloged as VVV-WIT-08,
only happened once over the 17 years
that two surveys, VISTA Variables in the
Via Lactea and Optical Gravitational
Lensing Experiment, monitored that
portion of the sky. The star's smooth
dimming and recovery contrasts with
the flat-bottom dips that occur when
exoplanets cross in front of their stars.
This object was big, with a radius
of at least 0.25 a.u., and elliptical in
projection. Leigh Smith (University
of Cambridge, UK) and colleagues
suggest in the Monthly Notices of the
Royal Astronomical Society that the
star might have a companion, either a
planet or another star, surrounded by a
disk. Such a disk could be both large
enough and of the right shape to block
the giant star's light - and it may do
so again one day.
¢ MONICA YOUNG
sk yand tele scope .o r g * OCTOBER 2021 11
WHITE DWARF AND MOON: GIUSEPPE PARISI; FACE-ON SPIRALS: ALMA (ESO
/ NAOJ / NRAO ) / PHANGS / S. DAGNELLO (NRAO)
http://skyandtelescope.org
Sky & Telescope - October 2021

Table of Contents for the Digital Edition of Sky & Telescope - October 2021
