My first Magazine Sky & Telescope - 03.2019

NEWS NOTES Puny Star Might Be Survi- vor from Early Universe THE FIRST STARS to light up the uni- verse — known as Population III stars — were likely giants, hundreds of times heftier than the Sun. But a recently discovered puny star in our galaxy might be an ancient specimen that shows how the ﬁrst stellar generation could have sprouted some runts that still live among us. The star in question, estimated to be about 13.5 billion years old, contains few elements heavier than helium, a p The 13.5-billion-year-old binary system 2MASS J18082002–5104378 lies in the yellow box just off the plane of our galaxy. sign that it was born before other stars spewed lots of atoms such as carbon, oxygen, and iron into space. Astrono- mers know of similarly pristine stars, but two things make this one stand out. It’s just 14% as massive as the Sun, barely weighty enough to ignite fusion, and it’s the slimmer half of a binary system, which gives a clue about how it formed. Kevin Schlaufman (Johns Hopkins University) and colleagues report in the November 10th Astrophysical Journal that the star likely formed via disk frag- mentation — gas clumping together in a disk that once swirled around the more massive of the duo. Researchers have debated whether or not this can happen around stars with a paucity of elements heavier than helium. If disk fragmentation can happen for 10 M A RCH 2 019 • SK Y & TELESCOPE stars as pristine as these two — which aren’t themselves from the ﬁ rst stellar generation — then it should also have been possible for Sun-sized stars to form in disks around gargantuan Population III stars, the team argues. Unlike ﬁ rst- generation behemoths, which would have shone brieﬂ y then exploded long ago, their low-mass bretheren might still be with us today. “That’s the reason we should keep looking for low-mass Population III stars in the galaxy,” says Schlaufman. “We shouldn’t despair. There’s good rea- son to think they’re still there.” ■ CHRISTOPHER CROCKETT COSMOLOGY Starlight Tally Reveals History of Star Formation FOUR THOUSAND octillion octillion 4x10 84 . octillion — or That’s roughly the number of photons emitted by all the stars in all galaxies throughout time. Using this new tally, astronomers have pieced together an updated timeline of how the rate of star formation has waxed and waned over cosmic history, research- ers report November 30th in Science. The new timeline jibes with earlier independent estimates: The cosmic star- forming factories were at their most proliﬁc about 3 billion years after the Big Bang and have been gradually slow- ing down ever since. At its peak, the universe churned out 10 times as many stars per year as it does now. “Our galaxy and the entire universe were lit up like a Christmas tree,” says coauthor Marco Ajello (Clemson University). Starlight that escapes galaxies con- tributes to a haze of photons known as the extragalactic background light. This light is tricky to measure directly. So a team of researchers known as the Fermi LAT collaboration looked at how this light interferes with gamma rays emit- ted from blazars, wellsprings of radia- tion powered by supermassive black holes at the cores of galaxies. To make their tally, researchers analyzed data on 739 blazars (and one gamma-ray burst) collected over 9 years by the Fermi Gamma-ray Space Telescope. The light from these blazars took anywhere from 200 million to 11.6 billion years to reach Earth. By measuring how speciﬁc wavelengths of gamma-ray light from each blazar have been suppressed, the team estimated the intensity of the extragalactic back- ground light at various epochs. This in turn enabled the researchers to recon- struct the rate of star formation across most of cosmic time. ■ CHRISTOPHER CROCKETT q This map of the entire sky from the Fermi Gamma-ray Space Telescope marks the location of 739 blazars (green crosses) used to estimate the extragalactic background light. A STARS

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