Sky & Telescope - July 2022 - 10

TWO STARS: ESO / L. CALÇADA; MILKY WAY: © STEFAN PAYNE-WARDENAAR / MPIA
NEWS NOTES
BLACK HOLES
" Closest Black Hole "
Doesn't Exist After All
A CONTESTED CANDIDATE for the
closest black hole to the solar system
has proven to be a mirage, a team of
astronomers has concluded.
The putative black hole lay in the
binary HR 6819, some 1,000 light-years
away in Telescopium. HR 6819 appears
to be a single, bright star, but its spectrum
reveals two: a rapidly spinning
Be star that's bright blue and skirted by
a disk of hot gas, and a second, also bluish
but fainter B-type star.
In 2020, Thomas Rivinius (European
Southern Observatory, Chile) and
colleagues teased apart the binary's
spectrum to discover that the B-type
MILKY WAY
Multitudes of Stars Reveal Our Galaxy's Early Years
HUNDREDS OF THOUSANDS of subgiant
stars in our galaxy have provided a
window into the Milky Way's history.
Maosheng Xiang and Hans-Walter
Rix (both at Max Planck Institute for
Astronomy, Germany) constructed a
sort of population pyramid of some
250,000 subgiants in the Milky Way.
Subgiant stars still fuse hydrogen in a
thick shell around their helium cores,
and previous research has established a
relationship between their luminosity
and age. The European Gaia mission
provided the stars' positions, distances,
star was moving through
a 40-day orbit. The Be star
showed no clear signs of
movement, so the team
proposed a third, invisible
companion - perhaps a
black hole.
But as we reported at
the time (S&T: Aug. 2020,
p. 8), other astronomers were skeptical.
To settle the debate, Rivinius's team and
some of the skeptics joined forces. Using
imaging and spectroscopic instruments
at the Very Large Telescope in Chile,
the team was able to split the stars
apart and confirm that they lie only
0.3 astronomical unit from each other,
not at the much wider separation that
would require a close-in, third object to
explain the 40-day orbit. Abigail Frost
New data show that
HR 6819, previously suspected
to be a triple system with a
black hole, is in fact a system
of two stars and no black hole,
as shown here in an artist's
illustration.
(KU Leuven, Belgium)
heads up the team's report
in the March Astronomy & Astrophysics.
Instead of containing a black hole,
the astronomers explain, HR 6819 is
actually a unique type of stellar system,
in which the Be star has sucked the
atmosphere off its companion. It has
spun itself up and vested itself in gas in
the process. The system provides a rare
chance to study this kind of " stellar
vampirism " in action.
¢CAMILLE M. CARLISLE
and motions, while the Large Sky Area
Multi-Object Fiber Spectroscopic Telescope
in China gave the stars' temperatures
and chemical composition.
In the March 24th issue of Nature,
Thick
disk
Thin
disk
Xiang and Rix used this population
census to put together a timeline of
the major events in our galaxy's early
history. The analysis sheds light on the
origin of the Milky Way's thick disk,
the fluffier pancake of older stars that's
some 6,000 light-years thick. (Newer
stars populate the thin disk, which is
only about 1,000 light-years thick).
From their data, Xiang and Rix conclude
that the thick disk of stars started
forming around 13 billion years ago,
800 million years after the Big Bang.
Star formation in the thick disk
Bulge and bar
Galactic plane
Dust lanes
r
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3
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,
This illustration shows the basic structures of the Milky Way Galaxy.
10 JULY 2022 * SKY & TELESCOPE
peaked 11 billion years ago, likely a
direct result of the merger between our
budding galaxy and a smaller intruder
nicknamed Gaia Enceladus. In addition
to contributing most of the Milky Way's
halo stars, the incoming galaxy also
caused new stars to form. The stellar
baby boom halted 3 billion years later,
though starbirth continued at a more
moderate pace in the galaxy's thin disk.
Future Gaia data releases may enable
an even more detailed reconstruction of
the Milky Way's history.
¢GOVERT SCHILLING
(
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Sky & Telescope - July 2022

Table of Contents for the Digital Edition of Sky & Telescope - July 2022
