Sky & Telescope - December 2023 - 10

NEWS NOTES
STARS
A Jekyll-Hyde White Dwarf
ASTRONOMERS HAVE FOUND a
bizarre, two-faced white dwarf: one side
hydrogen, the other helium.
White dwarfs are the dense embers
left behind when Sun-like stars die.
The atoms in these erstwhile stellar
cores separate by mass, the heavier stuff
sinking and the lighter stuff rising to
the surface. Since hydrogen is the lightest
element, it winds up on top, with
helium underneath.
But sometimes when a white dwarf
cools down from its initial blaze (more
than 100,000 kelvin) to a slightly
less startling 35,000K, convection in
the helium layer begins to dilute the
overlying hydrogen. Eventually, the
white dwarf continues its slow chill-out
wrapped in a helium blanket.
But helium doesn't conquer at once.
The newfound ZTF J203349.8+322901.1
may be an extreme example of this
transition in action.
Ilaria Caiazzo
(Caltech) and others
discovered the object
using the Zwicky
Transient Facility.
Follow-up observations
revealed something
odd: The dwarf
appeared to vary in
both brightness and
composition as it spun
around every 15 minutes.
When the brighter side pointed at
Earth, the team detected only hydrogen
in the dwarf's spectrum; the other side
showed only helium.
Reporting the find in the August 3rd
Nature, Caiazzo and colleagues emphasize
that they don't know for sure why
the white dwarf - which they've nicknamed
Janus for the two-faced Roman
god of transition - looks this way. But
they think the reason has to do with the
object's magnetic field.
If localized to part of the surface,
a magnetic field could easily choke
The Drama of Starbirth
 Artist's illustration of
Janus, a two-faced cinder
of a star that appears to
have one side of hydrogen
and the other of helium
convection in the area,
preventing mixing.
Conversely, where the
field is weaker, helium
could still ride convection
to the surface.
Alternatively, a magnetic
field could suppress gas pressure
and density over part of the surface,
forming an " ocean " of hydrogen.
Either way, invoking magnetism is
" perfectly reasonable, " says Alexandros
Gianninas (Connecticut College),
who has studied similar, though less
extreme, white dwarfs in transition.
The changeover won't last forever,
though. Eventually, as J20233 cools,
convection will strengthen and the
object will become a nondescript,
helium-sheathed white dwarf.
¢CAMILLE M. CARLISLE
Some 1,400 light-years away in the
southern constellation of Vela, the
Sails, is a dark nebula dense with
dust that blocks out background
stars. With the James Webb Space
Telescope (JWST), astronomers can
see through and into this cloud (seen
here in ethereal blue), revealing a pair
of forming stars known as Herbig-Haro
46/47. Discovered in 1977, HerbigHaro
46/47 has served as one of the
prototypes of this unique celestial
phenomenon ever since. The orange
diffraction spikes point to the protostars'
central location. The dense
disk of dust and gas that feeds them
isn't directly visible but is apparent by
its shadow, cast as two dark cones
above and below the stars, seen between
the spikes. While huge masses
of dusty gas feed the still-growing
stars, it's the paltry plasma speeding
away from the center that dominates
the image. These jets provide an outlet
for the angular momentum of the
inward-spiraling gas in the disk, so
they are key both to maintaining the
stars' growth and, for astronomers, to
understanding it.
¢ MONICA YOUNG
10 DECEMBER 2023 * SKY & TELESCOPE
TWO-FACED WHITE DWARF: K. MILLER / CALTECH / IPAC; JWST IMAGE OF STELLAR JETS: NASA /
ESA / CSA; IMAGE PROCESSING: JOSEPH DEPASQUALE (STSCI), ANTON M. KOEKEMOER (STSCI)

Sky & Telescope - December 2023

Table of Contents for the Digital Edition of Sky & Telescope - December 2023

Contents
Sky & Telescope - December 2023 - Cover1
Sky & Telescope - December 2023 - Cover2
Sky & Telescope - December 2023 - 1
Sky & Telescope - December 2023 - Contents
Sky & Telescope - December 2023 - 3
Sky & Telescope - December 2023 - 4
Sky & Telescope - December 2023 - 5
Sky & Telescope - December 2023 - 6
Sky & Telescope - December 2023 - 7
Sky & Telescope - December 2023 - 8
Sky & Telescope - December 2023 - 9
Sky & Telescope - December 2023 - 10
Sky & Telescope - December 2023 - 11
Sky & Telescope - December 2023 - 12
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Sky & Telescope - December 2023 - Cover3
Sky & Telescope - December 2023 - Cover4
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