My first Magazine Sky & Telescope - 03.2019

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 ﬂ ock of ducks in ﬂ ight. Astronomers have long believed that all the stars in an open cluster such as M11 were born in a single gen- eration 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 ﬁ ndings 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. IN BRIEF Landing Site Selected for Mars Rover Quasar Cannibalizes its Neighbors Ghostly Galaxy Orbits Milky Way After a fi ve-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 — sched- uled to launch in July 2020. The 30-mile- diameter crater has landforms as old as 3.6 billion years and appears to have once been fl ooded with water early in the history of Mars. It also contains many promising geological targets, including at least fi ve varieties of rock as well as an assortment of carbonates and clays. These are crucial, as the rover will specifi cally 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 fi lled 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 — ap- pears to be feeding the gargantuan black hole at its center by cannibalizing three neighbor- ing galaxies, researchers report in the Novem- ber 30th Science. The galaxy hosts a quasar, a brilliant fountain of radiation likely pow- ered 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 Díaz- Santos (Diego Portales University, Chile) and colleagues have found rivers of dust stream- ing 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 light- years away and is almost as large as the Large Magellanic Cloud — our galaxy’s big- gest 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 them- selves. The researchers report their fi ndings November 9th on the astronomy preprint site arXiv.org. ■ CHRISTOPHER CROCKETT sk yandtelescope.com • M A RCH 2 019 11

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