Sky and Telescope - November 2018 - 11

IN BRIEF

GALAXIES

Andromeda's Encounter with a Galactic Sibling
tion 2 billion years ago,
when the merger ended.
ran into a large companIt could also have ripped
ion billions of years ago,
M32
away M32p's disk, creating
astronomers claim July
the giant stream of metal23rd in Nature Astronomy.
rich stars in Andromeda's
Richard D'Souza and
halo and explaining M32's
Eric Bell (both at Unicompact structure.
versity of Michigan)
But Puragra Guhaexamined independent,
p The oddly compact satelThakurta
(University of
large-scale simulations to
lite galaxy M32 might be the
California,
Santa Cruz),
understand how Andromremnant of a past merger.
who wasn't involved in
eda-size galaxies form. To
the study, doubts M32's involvement.
reproduce our neighbor's massive and
If M32 were the remnant of a major
metal-rich stellar halo, the simulations
require a major collision within the past merger, then it ought to have a stream
of tidal debris both ahead of and behind
5 billion years, with disrupting effects
it, he says - but while a giant stream of
continuing until 2 billion years ago.
stars and gas trail the satellite, nothing
The researchers conclude that M32,
lies in front of it.
Andromeda's satellite, is the most likely
Nevertheless, D'Souza and Bell
remnant of the encounter. The parent
maintain that their work makes a
galaxy, dubbed M32p, was likely similar
strong case that something massive
in mass to the Milky Way.
interacted with Andromeda. More
The collision explains many of the
observations of stellar motions and
galaxies' characteristics, D'Souza says. A
composition might help bolster the case.
gravitational encounter could have triggered Andromeda's bout of star forma■ ELIZABETH HOWELL
OUR NEIGHBOR GALAXY

STARS

Gap in Gaia Data Reveals
Stellar Interiors

A NDRO MEDA A ND M32: TORBEN H A NSEN / CC BY 2.0

ASTRONOMERS WORKING with data

from the European Space Agency's Gaia
mission (S&T: Aug. 2018, p. 9) have discovered a new feature due to a change
in the structures of low-mass stars.
Astronomers often use the centuryold Hertzsprung-Russell diagram to
determine stars' properties based on
their color and brightness. Now, a
team of astronomers led by Wei-Chun
Jao (Georgia State University) has
announced the discovery of a narrow
but distinctive gap in this diagram in
the July 1st Astrophysical Journal Letters. While most stars follow a smooth
continuum, a tiny hiccup appears in the
luminosities of low-mass M stars.
The gap in luminosities corresponds
to the mass where M stars transition
from a fully convective interior to a

structure more like the Sun's, with
a convective envelope surrounding a
denser, radiative core.
Most theoretical models of stellar
structure don't predict this hiccup. Yet
James MacDonald and John Gizis (both
at University of Delaware) suggest in
the October 21st Monthly Notices of the
Royal Astronomical Society that the gap is
a feature of existing theory.
The gap, they explain, comes from
changes in the efﬁciency of the stars'
power source. The boiling motions in
the cores of lower-mass stars help mix
intermediate products of fusion reactions, allowing them to fuse hydrogen more efﬁciently. When the core
becomes too dense for convection, that
steady increase in efﬁciency stops. This
sudden change creates a dearth of stars
at a particular brightness; however,
some questions remain. Clearly, the HR
diagram still has secrets to reveal.

Small Satellite to Answer
Big Question
HaloSat, a CubeSat deployed from the
International Space Station on July 13th,
is on the hunt for the universe's missing
mass. While the identity of dark matter
remains a mystery, even "normal" baryonic
matter isn't all accounted for: Astronomers
tallying up stars, planets, galaxies, gas,
and dust only come up with about half of
the expected mass. Some of this missing
matter may reside in million-degree gas
that surrounds galaxies, but the temperatures are so extreme that observational
signatures are faint. HaloSat detects X-rays
with energies between 400 and 2,000 electron volts, which enables it to find emission
from ionized oxygen associated with superhot halo gas. With a 100-square-degree
field of view, HaloSat will efficiently survey
the entire sky to determine the extent of
the Milky Way's gaseous halo. The satellite
will avoid a major source of noise, created
as the solar wind interacts with Earth's
atmosphere, by only taking observations
during its 45-minute passes over Earth's
nightside. HaloSat is expected to operate
for a year. Read more about the mission at
https://is.gd/halosat.
■ DAVID DICKINSON

Sea Meteorite Recovered
On the evening of March 7th, a fireball lit
up skies along western Washington and
Oregon. The object responsible, later
estimated to be 2 metric tons, broke apart
some 25 km (16 miles) off the coast of
Washington state. Now, a team of scientists has announced the possible recovery
of fragments from the fall. The first sonarbased search effort with Exploration Vessel
Nautilus didn't turn up anything, but the
team had better luck with a seven-hour
visual search using a pair of remotely operated vehicles named Argus and Hercules.
Using a suction hose sampler, magnetic
plate, and sediment scoop, the researchers
attracted and sorted out meteoritic debris.
A preliminary analysis by Marc Fries,
Cosmic Dust Curator at NASA's Johnson
Space Center, revealed two small fragments characteristic of the smooth fusion
crust that forms during a bolide's plunge
through Earth's atmosphere. If verified, the
find will mark the first meteorite recovery
from the seafloor after an observed fall.
■ DAVID DICKINSON

■ MONICA YOUNG
sk yandtelescope.com * N OV E M B E R 2 018

http://is.gd/halosat http://www.skyandtelescope.com

Sky and Telescope - November 2018

Table of Contents for the Digital Edition of Sky and Telescope - November 2018