news center

Science: A tunnel driven through the Galaxy's fog

Science: A tunnel driven through the Galaxy's fog

作者:梁丘裔  时间:2019-03-03 09:07:01  人气:

By KEN CROSWELL in BOSTON An astronomer in the US has discovered a huge tunnel through our Galaxy. The tunnel, which is virtually free of neutral, or non-ionised, gas, is at least 1000 light years long and stretches from the Sun to the star Beta Canis Majoris and beyond. It may allow astronomers to observe distant objects that emit extreme ultraviolet light, a type of radiation which is usually absorbed by intervening neutral gas. Barry Welsh of the Center for Extreme Ultraviolet Astrophysics in Berkeley, California, discovered the tunnel while extending the work of two other American astronomers: Priscilla Frisch and Donald York of the University of Chicago. In 1983, they discovered that the Sun is in a void which contains very little neutral gas (Astro-physical Journal (Letters), vol 271, p L59). The void, dubbed the ‘Local Bubble’, extends 100 light years in most directions. But it reaches even farther in the direction of Beta Canis Majoris, about 650 light years away. Now Welsh has determined that this extension is in fact a huge tunnel that stretches well beyond the star itself. Welsh used the 1.5-metre telescope at Cerro Tololo Inter-American Observatory in Chile. He determined the amount of neutral gas lying in front of Beta Canis Majoris and in front of 10 other stars in a similar direction. All the stars are at different distances from the Sun. Specifically, Welsh measured the amount of neutral sodium gas in front of each star. Neutral sodium gas absorbs orange light – at wavelengths of 5890 and 5896 angstroms – from stars that lie behind it. Astronomers can estimate how much neutral sodium gas lies in front of the star from the precise amount of light absorbed at these wavelengths. This, in turn, reveals just how much neutral gas, mostly hydrogen, there is. In front of most of the stars that were less than 1000 light years away Welsh found no neutral sodium gas. But he did find sodium absorption in a star 1100 light years from the Sun, and in stars that were more distant than this. Welsh concludes that the neutral-free gas tunnel to Beta Canis Majoris is at least 1000 light years long, with the Sun located near one end. He estimates that the tunnel is at least 160 light years across. Although the tunnel lacks much neutral gas, he says, it may contain some ionised gas. The tunnel lies in the disc of the Milky Way. Imagine the Sun at the centre of a giant clock, with 6 o’clock as the direction of the Galactic centre. The tunnel extends from the Sun in the direction of 10 o’clock. Welsh speculates that supernovae, or violently exploding stars, may have created the long tunnel. The blast wave of a supernova is capable of ionising, or even sweeping clear, the neutral gas from a region as large as a few hundred light years across. However, in order to create the 1000-light-year tunnel, says Welsh, several supernovae would have been needed. The tunnel is more than a mere curiosity to astronomers. Regions devoid of neutral gas are vital for astronomers who hope to observe stars at extreme ultraviolet wavelengths. Until recently, such wavelengths have been ignored because neutral hydrogen absorbs it in the same way fog absorbs visible light. Extreme ultraviolet radiation arises from very hot objects and has a wavelength betwee 100 and 912 angstroms, shorter than that of normal ultraviolet radiation (see ‘A new wave in astronomy’, New Scientist, 30 September 1989). Later this year, NASA will launch the Extreme Ultraviolet Explorer satellite,