Sky & Telescope - March 2019 - 11

STAR CLUSTERS

Stars in M11 Hide Their True Colors
THE WILD DUCK CLUSTER - or Messier 11 (M11) - is one of the most enticing
telescopic sights in the summer. There are some 2,900 stars here, a rich assemblage
of suns that for some resembles a ﬂock of ducks in ﬂight. Astronomers have long
believed that all the stars in an open cluster such as M11 were born in a single generation and therefore should be close in age. But the Wild Duck presents a mystery.
Stars of similar brightness (and presumably similar mass and, thus, evolutionary
stage) display different colors, which are generally a good indicator of age.
To get to the bottom of this mystery, Beomdu Lim (Kyung Hee University, South
Korea) and colleagues examined the spectra of stars in M11. To their surprise,
it isn't the stars' ages that cause the spectral variety but their rotation. Spectra
revealed that the stars are spinning at different rates. This range of spins leads to
differences in star color and ultimately stellar lifetimes.
"The effects of rotation on stellar evolution were often neglected in the past,"
says study co-author Yaël Nazé (University of Liège, Belgium). The team published
their ﬁndings November 5th in Nature Astronomy.
Most of a star's life is spent on the main sequence burning hydrogen in its core
(see page 14). When its core hydrogen is exhausted, the star switches to other fuels
and burning strategies, causing it to expand, redden, and leave the main sequence.
But the faster a star spins, the better it mixes hydrogen into its core and the
longer it can remain on the main sequence - 15% to 62% longer - compared to
slower-rotating cousins of similar mass. Fast rotation also deforms a star's shape.
As its diameter expands, the equatorial regions cool and redden. The fast rotators
appear redder than slow rotators while still basking on the main sequence. So the
Wild Duck Cluster mimics two stellar populations when only one exists.
■ BOB KING

p This picture of the open cluster M11, or
the Wild Duck Cluster, was taken using the
European Southern Observatory's 2.2-meter
telescope at La Silla Observatory in Chile.

ESO

IN BRIEF
Landing Site Selected
for Mars Rover

Quasar Cannibalizes
its Neighbors

Ghostly Galaxy Orbits
Milky Way

After a five-year selection process, NASA has
chosen Jezero Crater in the Martian northern
hemisphere as the landing site for its next
Mars rover - dubbed Mars 2020 - scheduled to launch in July 2020. The 30-milediameter crater has landforms as old as 3.6
billion years and appears to have once been
flooded with water early in the history of Mars.
It also contains many promising geological
targets, including at least five varieties of
rock as well as an assortment of carbonates
and clays. These are crucial, as the rover
will specifically look for signs of life past and
present on Mars. "Getting samples from this
unique area will revolutionize how we think
about Mars and its ability to harbor life," said
Thomas Zurbuchen (NASA) in a November
19th press release. However, the site will be
a challenging place for the rover to land and
operate. Small craters and large boulders litter
the terrain. There are also depressions filled
with wind-driven sand, spots that could act as
sand traps for the rover.
■ DAVID DICKINSON

The most luminous known galaxy in the
universe - a beast designated W2246 that
blazes with the light of 350 trillion suns - appears to be feeding the gargantuan black hole
at its center by cannibalizing three neighboring galaxies, researchers report in the November 30th Science. The galaxy hosts a quasar,
a brilliant fountain of radiation likely powered by material falling onto the black hole.
Astronomers have suspected that the most
likely thing capable of feeding such voracious
appetites is other galaxies, but evidence has
been largely circumstantial. Now, Tanio DíazSantos (Diego Portales University, Chile) and
colleagues have found rivers of dust streaming onto W2246 from three adjacent galaxies,
indicating that the foursome is merging. The
researchers argue that this quadruple merger
is responsible for powering the quasar. And if
W2246 is typical of similar behemoths, then
perhaps all ultraluminous quasars in the early
universe are powered by galaxies stripping
each other for parts.
■ CHRISTOPHER CROCKETT

A ghost of a galaxy has been lurking on the
farside of the Milky Way, hiding this whole
time behind the wall of stars that constitutes
the disk of our galaxy. Dubbed Antlia 2, the
ephemeral satellite sits about 420,000 lightyears away and is almost as large as the
Large Magellanic Cloud - our galaxy's biggest known companion - and yet about one
four-thousandth as bright. Gabriel Torrealba
(Academia Sinica, Taiwan) and colleagues
discovered Antlia 2 while sifting through the
most recent data from the European Gaia
spacecraft, which is charting the positions
and speeds of over a billion stars in and
around our galaxy (see page 26). The newly
discovered galactic satellite is an oddball.
It's either too faint for its size or too large for
its brightness - and it may hint at a whole
population of tenuous dwarf galaxies around
the Milky Way that have yet to reveal themselves. The researchers report their findings
November 9th on the astronomy preprint site
arXiv.org.
■ CHRISTOPHER CROCKETT
sk yandtelescope.com * M A RCH 2 019

http://www.arxiv.org http://www.skyandtelescope.com

Sky & Telescope - March 2019

Table of Contents for the Digital Edition of Sky & Telescope - March 2019