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这是星际尘埃的题目原文,从GRE NO。书,COPY下来的。大家看看吧。 冥):
The dark regions in the starry night sky are not pockets in the universe that are devoid of the stars as had long been thought.Rather ,they are dark becasue of interstellar dust that hides the stars behind it.Although its visual effect is so pronounced, dust is only a minor constituent of the material,extremely low in density ,that lies between the stars.Dust accounts for about one percent of the total mass of interstellar matter.The rest is hydrogen and helium gas ,with small amounts of other elements .The interstellar material,raher like terrestrial clouds,comes in all shapes and sizes.The average density of interstellar material in the vicinity of our Sun is 1000 to 10000 times less than the best terrestrial laboratory vacuum.It is only because of the enormous interstellar distances that so little material per unit of volume becomes so significant.Optical astronomy is most directly affected,for although interstellar gas is perfectly transparent,the dust is not. |
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