Sky & Telescope - August 2021 - 10

NEUTRON STAR: NASA'S GODDARD SPACE FLIGHT
CENTER; MILKY WAY HALO STARS: NASA / ESA /
JPL-CALTECH / CONROY ET. AL. 2021
NEWS NOTES
NEUTRON STARS
What's Inside Neutron
Stars?
SIZE MEASUREMENTS OF two neutron
stars have narrowed down what
kinds of exotic matter might exist in
their extremely dense cores. There,
where densities reach roughly twice
that of an atomic nucleus, matter might
transform, breaking neutrons into their
constituent quarks or forming even
more exotic particles.
To probe this ultra-dense matter,
astronomers measure neutron stars'
mass and radius, use these values to
calculate how pressure changes with
density (a relation known as the equation
of state), and then compare this to
nuclear physicists' predictions.
But measuring their size is tricky.
Neutron stars' gravity is so extreme, it
bends light leaving the surface like a
funhouse mirror, making the objects
appear bigger than they are. Anna Watts
(University of Amsterdam) and Cole
MILKY WAY
A Dark Matter " Wake " in Our Galaxy's Outer Reaches
THE LARGE MAGELLANIC CLOUD
(LMC), the Milky Way's biggest satellite,
is plowing through - and pulling
on - the fog of dark matter that surrounds
our galaxy. Sparse stars fl oat like
the tips of dark matter icebergs in this
galactic ocean. Now, astronomers have
defi nitively detected a pile-up in those
Images of the Milky Way and the Large
Magellanic Cloud are overlaid on a map of the
surrounding galactic halo. Shades of blue represent
stellar density, with lighter blue regions
indicating stellar pileups.
stars (and presumably the dark matter,
too) that the LMC has left in its wake as
it makes its fi rst pass. The results appear
in the April 22nd Nature.
Charlie Conroy (Center for AstroMilky
Way
LMC " wake "
j
e
c
10 AUGUST 2021 * SKY & TELESCOPE
physics, Harvard & Smithsonian)
and colleagues plotted distances and
positions for 1,301 giant stars between
200,000 and 330,000 light-years from
the galactic center using data from the
European Space Agency's Gaia satellite
and NASA's Wide-fi eld Infrared
Survey Explorer. Two regions on the sky
stand out for their thickness of
halo stars: one below the
galactic plane and one
above. To the south,
a stellar pile-up
extends toward
the Large MagelLarge
Magellanic Cloud
(LMC)
lanic Cloud. The
slightly less dense
region of stars in
the north is even larger,
Miller (University of Maryland, College
Park) led two independent teams using
the Neutron star Interior Composition
Explorer (NICER) telescope aboard
the International Space Station to see
through this light-bending effect.
By analyzing changes in brightness
as short as 100 nanoseconds, Watts's
and Miller's teams mapped hotspots
rotating on the surfaces of two neutron
stars, J0030+0451 and J0740+6620, and
derived their true size. They published
results for J0030 in 2019 and in April,
the teams presented new results for
J0740 at a virtual meeting of the American
Physical Society.
At 1.4 times the mass of the Sun,
J0030 barely had enough
heft to collapse into a
neutron star; meanwhile,
J0740's 2.1 solar masses
This animation frame shows
an example of the hotspots
that whirl around on the
surfaces of the pulsars J0030
and J0740.
make it the most massive neutron star
known. " Our new measurements of
J0740 show that even though it's almost
50% more massive than J0030, it's
essentially the same size, " Watts says.
" That challenges some of the more
squeezable models of neutron star cores,
including versions where the interior is
just a sea of quarks. "
Yet, even though the observations
rule out quark soup cores, they also suggest
that the internal pressure is more
intense than previously realized, which
may rule out simpler neutron cores, too.
Some hybrid scenarios incorporating
both neutrons and quarks might still
work. More exotic particles are still in
play, too.
" Our fervent hope is
that at least we're able
to make a lot of nuclear
physicists sweat, " Miller
says, " because this [result]
is not easy to get into
their models. "
MONICA YOUNG
spanning a quarter of the sky.
The team compared the observed
density of stars to computer simulations
tracing the gravitational effects of
the Large Magellanic Cloud as it passes
around the Milky Way for the fi rst time.
Nicolás Garavito-Camargo (University
of Arizona) led the work on the simulations,
which show that a wake of stars
(and dark matter) should trail the satellite
as it falls through our galaxy's halo.
The expected wake matches the location
of the dense region of stars below the
plane, though the observed contrast is
even stronger than predicted.
Meanwhile, the Milky Way itself
should move in response to the massive
dwarf, which has a tenth of our
galaxy's heft. As the pair's center of
mass shifts, a weaker but wider-ranging
effect gathers stars (and dark matter)
closer together in a large region above
the galactic plane.
This result helps astronomers refi ne
their understanding of the complex
dark matter halo around our galaxy.
MONICA YOUNG
P
m
r
d
o
t
e
o
t
i
o
n
p
a
th
Sky & Telescope - August 2021

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