Sky & Telescope - May 2020 - 8

NEWS NOTES

Leading arm of
the Magellanic
Stream
Plane of the Milky Way

Magellanic Stream
Small
Magellanic
Cloud

Large
Magellanic
Cloud

MILKY WAY

Baby Stars Found in Ancient Part of Our Galaxy
ASTRONOMERS HAVE DISCOVERED a
collection of young stars in a surprising
location: the Milky Way's halo.
Adrian Price-Whelan (Flatiron Institute) discovered the stars while digging
through data collected by the European
Gaia mission (S&T: Mar. 2020, p. 34).
The newfound stars are 117 million
years old and move together as a set.
They're similar to open clusters like the
Pleiades in both age and mass. But at
94,000 light-years away, the new stars
are 200 times farther away than the
Seven Sisters. They're also more spread
out, spanning some 2,500 light-years,
and are not gravitationally bound.

p A newfound group of young stars, the PriceWhelan cluster sits on the periphery of the Milky
Way. These stars probably formed from material
originating from the Magellanic Clouds.

By rights, the stars shouldn't be out
there at all. There's not much starmaking gas in the Milky Way's halo, as
most of the gas is too hot and diffuse
to collapse into new suns. What little
cool gas there is resides in the Magellanic Stream, a long gaseous ribbon
that travels with the Large and Small
Magellanic Clouds through our galaxy's
outer regions.
And the new stars appear to be sailing right in front of that ribbon of gas.

COSMOLOGY

Lensed Quasars Shed Light on Dark Matter, Dark Energy
GRAVITATIONALLY LENSED QUASARS

imaged by the Hubble Space Telescope
are helping astronomers measure fundamental properties of the universe.
Two research teams reported results
from these cosmic mirages at the winter
meeting of the American Astronomical
Society in Honolulu.
In one case, Anna Nierenberg (JPL /
Caltech), Daniel Gilman (University of
8

M AY 2 0 2 0 * S K Y & T E L E S C O P E

California, Los Angeles), and colleagues
studied eight such gravitationally lensed
quasars, where the gravity of a massive
foreground galaxy bends light from a
background quasar. Most of the lensing
mass is actually in the galaxy's dark
matter halo. So astronomers can use the
relative brightness within each set of
quasar images to determine the distribution of mass in the foreground halo.

■ CAMILLE M. CARLISLE

The researchers found that dark matter clumps up in blobs just 1/100,000 times
the mass of the Milky Way. If dark matter consisted of "warm," or fast-moving,
particles, it wouldn't be able to form
such low-mass chunks. Dark matter
must instead consist of "cold," or slowmoving, particles. The results appear in
two companion studies published in the
Monthly Notices of the Royal Astronomical Society.
Meanwhile, another group of
astronomers, led by Sherry Suyu (Max

D. NIDE V ER E T A L. / NR AO / AUI / NSF / MELLING ER / LEIDEN
/ A RG EN TINE / BONN SURV E Y / PA RK ES OBSERVATORY /
WESTERBORK OBSERVATORY / ARECIBO OBSERVATORY

Price-Whelan 1

To investigate the stars' properties,
David Nidever (Montana State University), Price-Whelan, and colleagues took
spectra of 28 of the brightest stars in
the group. The measurements showed
that the stars have a fairly pristine composition. Both their composition and
velocities match that of the leading arm
of the Magellanic Stream. In two papers
in the Astrophysical Journal, the team
concludes that these stars likely formed
from the gas at the head of the Magellanic Stream.
Murmurs of excitement from fellow astronomers met Price-Whelan
and Nidever when they reported these
results at the winter meeting of the
American Astronomical Society in
Honolulu. Observers have been looking
for stars associated with the Magellanic Stream for decades, explains
Jeremy Bailin (University of Alabama),
in part to help pin down its distance.
It's difficult to determine the distance
to hydrogen gas, but based on the stars'
distances, the astronomers estimate
that the Magellanic Stream is roughly
half as far away as previously thought.
The stars lie in front of the stream,
not in it. The team thinks this offset
occurs because the halo's hot gas drags
on the gaseous stream but not on the
stars. Over time, the gas slows down
and falls behind the stars. In fact, the
stars' ages match when the stream last
passed through our galaxy's outer disk.
That passage could have compressed the
stream's gas, spurring starbirth.



Sky & Telescope - May 2020

Table of Contents for the Digital Edition of Sky & Telescope - May 2020

Contents
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