Sky & Telescope - July 2023 - 10

CLOUD ELONGATION: A. CIURLO ET AL. / UCLA GCOI / W. M. KECK OBSERVATORY;
SATELLITE STREAK: NASA / ESA / KRUK ET AL. / NATURE 2023 / CC BY 4.0
NEWS NOTES
BLACK HOLES
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The Milky Way's Black Hole Spaghettified a Cloud
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TWO DECADES of observations show
a dusty gas cloud elongating as it
approaches our galaxy's supermassive
black hole.
That black hole, called Sgr A*, exerts
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lar cloud, dubbed X7, in images of the
galactic center taken since 2002 using
the adaptive optics system on the Keck
Observatory atop Mauna Kea, Hawai'i.
Starting in 2006, the team also colX7
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X7
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tidal forces on any objects nearby, pulling
harder on the nearer side than on
the farther side, stretching them out -
or spaghettifying them - in the process.
Astronomers spotted one particuS0-14
X7
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lected
spectroscopic data, which give
additional information about the
cloud's movements.
By combining these measurements,
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Anna Ciurlo (University of California,
Los Angeles) and colleagues show in the
February 20th Astrophysical Journal that
X7 is on its way toward the black hole.
It will pass within some 3,200 astronomical
units (a.u.; 18 light-days) of
Sgr A* in 2036. Already, the cloud is
nine times as long as it is wide.
X7 won't survive its upcoming pass,
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so it must be younger than its 170-year
orbit. Ciurlo's team therefore suggests
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that the gas was ejected recently when a
pair of stars collided.
That scenario has support in the
form of another object: a star cocooned
in dust known as G3, whose orbit is
surprisingly similar to X7's. G objects
are thought to be products of stellar
mergers. If so, X7 might represent the
dust and gas expelled during the birth
of the merged star at G3's core.
Stefan Gillessen (Max Planck
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Institute for Extraterrestrial Physics,
Germany), who wasn't involved in the
study, agrees with Ciurlo and colleagues,
calling the study " very nice
work. " He adds that this kind of gaseous
lump might represent a typical meal for
Sgr A*. However, whether we'll see the
supermassive black hole feed on the 50
Earths' worth of mass that the cloud
contains depends on how long it takes
to flow into the dark maw.
" For sure we will see how X7 is
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pImages from the Keck Observatory show the elongation of a gas cloud over two decades. Each
frame is 1 arcsecond across. The star S0-14 is labeled for reference.
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LIGHT POLLUTION
Satellite Trails Mar
Hubble Images
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WHEN SPACEX LAUNCHED its
first Starlink satellites in 2019, and
astronomers realized just how bright
they would be, CEO Elon Musk glibly
tweeted that telescopes would simply
have to go to space. Only it turns out
that, depending on their orbits, space
observatories aren't safe from light pollution
either.
A new accounting of satellite trails
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in Hubble Space Telescope images,
published March 2nd in Nature, shows
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that the chance of satellites affecting an
image has doubled to 6% over the past
two decades, largely before the Starlink
era. The study serves as a baseline for
comparison against future studies, in
which the impacts of Starlink and other
satellites will become more noticeable.
The team, led by Sandor Kruk (Max
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Planck Institute for Extraterrestrial
Physics, Germany), started with comments
on an online forum run by the
Hubble Asteroid Hunter citizen-science
project. Volunteers perusing images
for short, curved asteroid trails noted
other image anomalies, such as the
long, straight streaks of satellites. Kruk
and colleagues used this assortment of
satellite trails to train two machinelearning
algorithms, which then picked
out satellite streaks from observations
taken between 2002 and 2021.
Kruk's team found that the increase
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in satellite trails (50%) roughly corWhen
satellites pass through a telescope's
field of view, they leave behind trails, such as in
this Hubble image.
10 JULY 2023 * SKY & TELESCOPE
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torn apart by the black hole, " Ciurlo
says. " After that, who knows? We'll be
watching! "
¢MONICA YOUNG
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responds to the increase in number of
satellites (40%). Kruk's team estimates
that satellites will interfere with at least
a fifth and perhaps up to half of all
Hubble images within the decade.
Other telescopes, such as ESA's Char2021
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acterising Exoplanets Satellite (CHEOPS)
and NASA's Near-Earth Object
Wide-field Infrared Survey Explorer
(NEOWISE), also conduct observations
from low-Earth orbit. Studies of satellites'
impacts on these space observatories
are in the works.
Kruk and others have acknowledged
that while some satellite trails can be
removed, that isn't always possible.
That's why, on March 20th in Nature
Astronomy, John Barentine (Dark Sky
Consulting) and colleagues instead
proposed that governments regulate
the number of satellites launched into
near-Earth space. Restrictions of the
objects allowed at certain altitudes, they
write, may be the only way forward for
sustainable space.
¢MONICA YOUNG & JAN HATTENBACH
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Sky & Telescope - July 2023

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