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  3. The explosion of a star is an awesome event. The most violent of these cataclysms, which produce supernovae, probably destroys a

The explosion of a star is an awesome event. The most violent of these cataclysms, which produce supernovae, probably destroys a

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问题     The explosion of a star is an awesome event. The most violent of these cataclysms, which produce supernovae, probably destroys a star completely. Within our galaxy of roughly 100 billion stars the last supernova was observed in 1604.  Much smaller explosions, however, occur quite frequently, giving rise to what astronomers call novae and dwarf novae.  On the order of 25 novae occur in our galaxy every year, but only two or three are near enough to be observed. About 100 dwarf novae are known altogether. If the exploding star is in a nearby part of the galaxy, it may create a "new star" that was not previously visible to the naked eye. The last new star of this sort that could be observed clearly from the Northern Hemisphere appeared in 1946. In these smaller explosions the star loses only a minute fraction of its mass and survives to explode again.
    Astrophysicists are fairly well satisfied that they can account for the explosions of supernovae. The novae and dwarf novae have presented more of a puzzle. I shall describe recent investigations that have provided important new information about these two classes of exploding star. The picture that emerges is quite astonishing.  It appears that every dwarf nova--and perhaps every nova--is a member of a pair of stars. The two stars are so close together that they revolve around a point that lies barely outside the surface of the larger star. As a result the period of rotation is usually only a few hours and their velocities range upward to within a two-hundredth the speed of light,
    Astronomers use the term "cataclysmic variable" to embrace the three general classes of exploding star: dwarf novae, novae, and supernovae. A cataclysmic variable is defined as a star that suddenly and unpredictably increases in brightness by a factor of at least 10. Dwarf novae are stars that increase in brightness by factor of 10 to 100 within a period of several hours and decline to their former brightness in two or three days. In this period they emit some 1038 to 1039 ergs of energy. At maximum brilliance a dwarf nova shines about as intensely as our sun, previously it had been only about a hundredth as bright. The number of outbursts ranges anywhere from 3 to 30 a year, but for any one star the intervals have a fairly constant value. Moreover, the maximum bright ness from outburst to outburst is the same within a factor of two for a given star. The dwarf novae are often referred to, after their prototypes, as U Geminornm or SS Cygni stars. (The stars of each constellation are designated by letters or numbers.) A subgroup of dwarf novae, called Z Camelopardalis stars, do not always descend to minimum bright ness between outbursts but may stay at some intermediate level for several months.  
The reason why dwarf novae explode is ______.
选项 A、not known
B、a cataclysmic variable
C、that they are twin stars
D、that they lose a small part of their mass
答案A
解析 矮新星爆炸的原因尚不清楚。作者在第二段开头说，天文物理学家们很满意，因为他们能解释超新星爆炸的原因。同超新星相比，新星和矮新星提出了更大的一个谜，这就说明矮新星爆炸的原因尚不清楚。
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