Supersonic bullets shoot through the Orion Nebula

 作者:舒寺晶     |      日期:2017-06-13 06:01:12
By David Shiga (Image: Gemini Observatory) Bullet-like clumps of gas hurtle through the Orion stellar nursery at supersonic speed in a new image from the Gemini North observatory. The unusual structures are revealed in unprecedented detail by newly commissioned laser-equipped optics. The so-called ‘bullets’ are located in the Orion Nebula, a star-forming region about 1500 light years from Earth. Each of the few dozen observed is a dense clump of gas about as wide as Pluto’s orbit around the Sun. Tearing through the surrounding medium of thin gas at 400 kilometres per second, the bullets create shock waves in front of them. As the shock waves propagate, they produce lengthy glowing trails – each about 400 times longer than our entire solar system. Based on the length of the trails and the bullets’ speed, the bullets appear to have formed simultaneously less than 1000 years previously. Exactly what triggered their birth is unknown. But they may be related to a nearby, bright source of infrared light thought to be a moderately heavy star that is partially obscured by dust. The bullets might have been thrown off this star like shrapnel, says James Stone of Princeton University in New Jersey, US. Massive stars can blow off large amounts of gas – called a stellar wind – in all directions. Gas expelled by the star in the past surrounds the star and slows down as it moves outwards. So if an especially strong outburst of wind slams into this surrounding ‘slow’ gas, it may fragment into small clumps, he says. The dense clumps would continue moving outwards faster than the speed of sound in the gas they travel through, creating shock waves behind them. “It’s very much like an airplane flying at supersonic speeds,” Stone told New Scientist. The image was made as part of a test of a laser guide star system on the Gemini North telescope atop Mauna Kea in Hawaii, US. The laser creates a glowing spot, or artificial star, in the sky at an altitude of about 90 kilometres that can be used as a reference to measure how the atmosphere is distorting light. This information is used to automatically adjust the shape of a flexible, shape-changing mirror on the telescope that cancels the distortion of light due to the Earth’s atmosphere, producing sharper images. The improved image resolution should allow the evolution of the bullet tracks to be studied, says Michael Burton of the University of New South Wales in Sydney, Australia. “Small changes in the structures are expected from year to year as the bullets continue their outward motion,