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  3. About 2,500 of so-called supernovae are known inside our galaxy and beyond. But exactly what they were before they exploded is n

About 2,500 of so-called supernovae are known inside our galaxy and beyond. But exactly what they were before they exploded is n

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问题     About 2,500 of so-called supernovae are known inside our galaxy and beyond. But exactly what they were before they exploded is not so clear. The hunt for supernovae, from their origins to their long-term effects, is heating up rapidly. Understanding these incendiary objects is important in part because they are responsible for creating most of the elements in the universe, including the stuff of which people, plants and planets are made. A supernova is relatively easy to detect because of the intense radiation it casts off in visible light and other wavelengths. Figuring out what the star looked like prior to the explosion, however, requires probing into the archives of astronomy.
    A separate team of supernova hunters did just that, using a decade of Hubble Space Telescope images to search for possible pre-supernova stars. Their new findings support existing theory of supernova mechanics, which holds that only very massive stars explode. When a massive star burns up all its hydrogen fuel, it casts its outer layers into space and then collapses into a dense neutron star or black hole. Theorists believe that stars must be about 10 to 20 times the mass of the sun to support such an explosive scenario. Such heavy objects have brief lives, typically less than 20 million years, compared with the sun, which is middle-aged and already 4. 6 billion years old.
Ground-based telescopes that observed the actual supernova explosions are not as accurate as Hubble, however, due to air turbulence, so scientists require follow-up Hubble observations to see if they are in the right positions. One has already been eliminated, Alexei Filippenko of the University of California at Berkeley said, but he’s confident at least a couple of the remaining five will prove to be actual precursor stars. Astronomers have known for decades that the universe is expanding. But in the late 1990s they began to realize that the expansion is occurring at an ever-faster rate. This suggests that some mysterious, unseen force is at work across great distances, breaking the will of gravity that would otherwise rein in the universe eventually.
    Supernovae are useful in this research because they can be seen from far away. Astronomers measure how much an exploded star’s light has stretched, which tells them the speed at which the object is receding. By comparing this to nearby supernovae, researchers can refine the universal expansion’s rate of acceleration. In particular, the orbiting observatory’s keen new eyesight can be used to analyze pinpricks of light from very distant objects and learn what they are and what they’re made of. This so-called spectroscopy technique is just like using a prism to break white light into its constituent colors.
What is this passage mainly about?
选项 A、Acceleration of supernovae.
B、Expansion in the universe.
C、Exploration of galaxy.
D、Explosion of supernovae.
答案D
解析 本文主要讲的是什么?[A]超新星的加速。[B]宇宙的膨胀。[C]探索银河。[D]超新星的爆炸。本文主要讲述科学家们对超新星爆炸前的状况的探索。他们发现只有巨星才能爆炸，并且通过测量爆炸星星光的延伸可以得知该星的衰减速度，而且还可以对宇宙的膨胀速度进行更为准确的测量。因此可知，全文主要围绕超新星的爆炸展开，正确答案是[D]。其他三个选项都是文章的一个侧面，不能概括全文内容。
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