My first Magazine Sky & Telescope - 02.2019

NEWS NOTES SUPERNOVAE Low-key Blast Marks Possible Birth of Neutron Star Duo tional waves washed over Earth, herald- ing the collision of two neutron stars in a far-off galaxy. Such collisions appear to be the birthplace of many of the heaviest elements such as gold, plati- num, and uranium (S&T: Feb. 2018, p. 32). Now an unusual, rapidly brighten- ing supernova might help researchers understand how such neutron star duos arise in the ﬁrst place. The supernova, designated iPTF- 14gqr, went off in the outskirts of a galaxy some 900 million light-years away. Detected in October 2014, this supernova seemed odd right away. Most supernovae take a few weeks to hit their peak brightness. This one did so in less than 7 days, suggesting it had only a relative paucity of debris to clear out of the way. Kishalay De (Caltech) and colleagues estimate that SN iPTF14gqr released only one-ﬁfth of the mass of the Sun — most supernovae expel several Suns’ worth of gas. What’s more, the team deduced that the star was about 1.5 times as massive as the Sun shortly before the explosion. But for a star to go boom, its original mass needs to be at least eight times as massive as the Sun. Something must have stripped the star of most of its mass before it died. The most likely candidate, the team argues in the October 12th Science, is a com- panion neutron star. Here’s how the team thinks the story played out: Two massive stars once orbited each other. One of them exploded, leaving its core behind as a neutron star. This neutron star then stripped its buddy of most of its gas. By MAGELLANIC CLOUDS Evidence Mounts for Magellanic Collision THE PAST DECADE HAS seen astrono- mers’ understanding of the Magellanic Clouds — two dwarf galaxies near the Milky Way — completely overthrown, resulting in new revelations about the violent and ongoing formation of our own galaxy. Now, a team led by Sally Oey (University of Michigan) has made a discovery that bears out predic- tions that these two galaxies once col- lided: A big chunk of the Small Magel- lanic Cloud (SMC) is moving toward the Large Magellanic Cloud (LMC) and the Magellanic Bridge of gas and stars that joins them. Astronomical wisdom once held that the LMC and SMC had been orbiting the Milky Way for billions of years, but a landmark study in 2007 showed that the dwarf galaxies are likely falling toward the Milky Way for the ﬁ rst time (S&T: Oct. 2012, p. 28). That realiza- tion introduced a new puzzle: If the Magellanic Bridge wasn’t stripped from the LMC and SMC by the powerful gravity of the Milky Way, as previously suspected, how did it get there? t This picture of the Small Magellanic Cloud is composed of two images from the Digitized Sky Survey 2. 14 FE B RUA RY 2 019 • SK Y & TELESCOPE p The moments before (left), during (middle), and after (right) Supernova iPTF14gqr appeared in the outskirts of a spiral galaxy. the time the second star exploded — and became known as SN iPTF14gqr — there wasn’t much gas left. The explosion would also have produced a neutron star, leaving two neutron stars in a tight orbit. “This is the ﬁrst example of a super- nova that produced a compact binary system that is tight enough to eventually merge and produce gravitational waves,” De says. Unfortunately, at nearly 1 bil- lion light-years away, the high-energy light from any putative neutron stars is much too faint to detect. ■ CHRISTOPHER CROCKETT In 2012, researchers offered an answer. The Magellanic Clouds must have had a close encounter with each other in the recent past — perhaps even a direct collision — to pull the bridge’s stars and gas away so strongly. In the November 1st Astrophysical Journal Letters, Oey and her collabora- tors present support for this hypothesis based on stellar position and veloc- ity data from the Gaia satellite. They discovered that many of the stars in the eastward “wing” of the SMC are mov- ing in concert, a result that can only be explained by the LMC’s gravitational force acting on the galaxy globally. With further study, astronomers might be able to use the observed bulk motion to understand the direct colli- sion in even more detail: how long ago it occurred, at what speed, and at what angle. The better astronomers under- stand the past trajectories of these galaxies, the more precisely they can predict their future fates, which will likely see them accumulated into the Milky Way. ■ EMILY SANDFORD DSS2 LAST YEAR, A TSUNAMI of gravita-

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