Sky & Telescope - June 2022 - 10

NEWS NOTES
STARS
A Star Where It Shouldn't Be
THE CREPE-LIKE DISK of our galaxy,
just 1,000 light-years thick, is where
almost all the Milky Way's stars form.
So astronomers were baffled when,
years ago, they found a massive star
3,000 light-years above the galactic
plane. This star was far too young
to have traveled out of the disk after
its birth, but a birthplace in the halo
seemed equally unlikely.
Now, Douglas Gies (Georgia State
University) and colleagues have proposed
a possible backstory for this stellar
wanderer, HD 93521. It's currently
a giant, rapidly rotating star with 17
times the Sun's mass, but perhaps it
didn't start this way. They suggest this
star was once a close pair of lower-mass
stars that merged. The
team presented this scenario
in the February 1st
Astronomical Journal.
Based on new distance
and motion measurements
from the European
Space Agency's Gaia satellite,
the team confirmed
that the 5-million-yearold
star would have taken 39 million
years to fly up from the disk. Yet observations
show no signs of birth in the
halo, so the star must have come from
the disk - but how?
Gies's team suggests that a merger
took place within an ejected star system.
The collision stirred their interiors,
bringing fresh hydrogen to the merged
star's core and resetting the evolutionary
clock. While HD 93521 appears to
The O-type star HD 93521 is
in the Milky Way's halo, far from
the star-forming galactic plane.
be 5 million years old,
its " age " might mark the
time of its merger, not of
its birth.
This scenario could
also explain the star's
fast rotation. Although
the star is 7.4 times the Sun's girth at
its equator, it completes a full rotation
in just 21 hours, compared to the Sun's
24.5-day equatorial period.
Ian Howarth (University College
London), who first recognized the star's
unique properties in 1993, says the
team has resolved long-standing ambiguities
surrounding this star: " They
make a persuasive case. "
¢MONICA YOUNG
-1.8 -1.0
Spectral index
1.0
The Heart of the Milky Way
The South African Radio Astronomy
Observatory has released a striking
series of images showing the complex
and chaotic center of our galaxy, part
of a study to appear in the Astrophysical
Journal.
Unaffected by the large quantities
of dust that obscure this region in visible
light, radio waves reveal a scene
teeming with star birth and star death
as well as nearly 1,000 mysterious
10 JUNE 2022 * SKY & TELESCOPE
radio filaments. Discovered in the early
1980s, the filaments can extend up
to 150 light-years and appear highly
organized. Some even come in pairs
or clusters. Cosmic rays streaming
from our galaxy's center, perhaps from
our resident supermassive black hole,
might compress and illuminate the
galaxy's ambient magnetic field.
Besides the filaments, our galaxy's
inner 650 light-years are also home to
young, massive stars, puffy remnants
of supernova explosions, and shells
of ionized gas surrounding massive
stars. Astronomers knew of many of
these objects, but the team, led by
Ian Heywood (University of Oxford,
UK), has already reported several
new features.
¢ JURE JAPELJ
View additional images and details at
https://is.gd/GCradio.
ISOLATED STAR: ALADIN SKY ATLAS; GALACTIC CENTER:
SARAO / I. HEYWOOD / J. C. MUÑOZ-MATEOS
https://www.is.gd/GCradio
Sky & Telescope - June 2022

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