Sky & Telescope - January 2025 - 11

STARS
See the Spotted
Surface of Polaris
ASTRONOMERS HAVE USED
an array of telescopes to reconstruct
images of the surface
of Polaris for the first time.
The new images have been
published in the August 20th
Astrophysical Journal.
Polaris is the nearest
Cepheid variable star to Earth,
and it has a much fainter
stellar companion. A team
of astronomers led by Nancy
Evans (Smithsonian Astrophysical
Observatory) initially
set out to map their mutual
30-year orbit, but that takes
time and patience.
" The small separation and
large contrast in brightness
between the two stars makes
it extremely challenging to
resolve the binary system during their
closest approach, " Evans says.
To pull it off, Evans turned to the
Center for High Angular Resolution
Astronomy (CHARA) array, a set of six
1-meter telescopes on the summit of
Mount Wilson in California. By adding
the new data to older measurements,
including some from the Hubble Space
Telescope, the team has now covered
three-quarters of the stars' long orbit.
In the process, they've upped the estiCHARA
observations of the surface
of Polaris, taken over four nights
between 2018 and 2021
mate of Polaris's mass from 3.5
to 5.1 solar masses.
In undertaking this study,
the team also used a camera
attached to CHARA to capture
fuzzy views of Polaris's surface.
The images show that Polaris
is 46 times the diameter of
the Sun. Remarkably, they also
show surface details: large
bright and dark spots on its
surface that change over time.
The starspots may help
explain Polaris's peculiarities,
such as its unusually
low pulsation amplitude; the
difference between its dimmest
and brightest points is smaller
than that of other Cepheids.
The spots may add " noise " that
muddles the pulsation's signal.
The starspots also open
the door to measuring Polaris's spin.
Astronomers have already noted a 120day
variation in Polaris's pulsations,
and Evans speculates that this time
interval could mark the star's rotation.
" We plan to continue imaging
Polaris in the future, " says team member
John Monnier (University of Michigan).
" We hope to better understand
the mechanism that generates the spots
on the surface of Polaris. "
¢COLIN STUART
New Volcano on Io
NASA's Juno mission has spotted a new volcano on Jupiter's moon Io: The image
at left shows the new feature to the east of an existing feature. The volcano is fresh
since the Galileo mission, which saw only featureless terrain in the same region in
1997 (inset). Now, multiple lava flows and deposits cover a region about 180 kilometers
(110 miles) on a side. Michael Ravine (Malin Space Science Systems), speaking
at the 2024 Europlanet Science Congress on behalf of the JunoCam team, presented
images of this " large, complicated volcanic feature that appears to have formed from
nothing. " The new volcano spewed sulfur into space, which then fell back onto Io's
surface, staining the area to the east of it red. On the western side, two dark streams
of lava run for about 100 km, pooling at the farthest point. There, the lava's heat
vaporized surface material, creating two overlapping dark gray deposits. The new
images come from several close passes by Io conducted during Juno's extended
mission. All of JunoCam's images are publicly available at missionjuno.swri.edu.
¢ MONICA YOUNG
sk yand tele scope .o r g * JANUARY 2025 11
IN BRIEF
Omega Centauri Has
No Mid-size Black Hole
Astronomers recently discovered
evidence for a mid-size black hole in
Omega Centauri (S&T: Nov. 2024, p. 9).
But the globular cluster might instead
contain tens of thousands of stellarmass
black holes, according to a study
posted on the arXiv astronomy preprint
server. The study, led by Andrés
Bañares-Hernández (University of La
Laguna, Spain), included observations
of the motions of more than 1,000 stars
as well as timing data for five pulsars.
Combined, the measurements suggest
thousands of stellar-mass black holes,
containing 200,000 to 300,000 Suns'
worth of mass, are spread throughout
the cluster center. Or, if there were
a single, larger black hole, it would
contain less than 6,000 solar masses
- significantly less than the 8,200 to
50,000 solar masses estimated by the
previous study, which was based on the
motions of seven unexpectedly speedy
stars. One reason for the mass discrepancy
is that Bañares-Hernández's team
estimates a higher mass for the cluster,
which leads to a higher escape velocity.
That means individual stars can speed
around the center without requiring a
giant black hole to keep them bound.
Simon Portegies Zwart (Leiden University,
The Netherlands), who was not
involved in either study, says that, while
he'd found the potential discovery of a
single, larger black hole " super exciting, "
the evidence just isn't there yet.
¢ARIELLE FROMMER
POLARIS'S BRIGHT AND DARK SPOTS: EVANS ET AL. / ASTROPHYSICAL JOURNAL 2024; IO: NASA / JPLCALTECH
/ SWRI / MSSS / EUROPLANET

http://missionjuno.swri.edu/ http://skyandtelescope.org

Sky & Telescope - January 2025

Table of Contents for the Digital Edition of Sky & Telescope - January 2025