【天文学】流星雨是怎么产生的？

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（嗷呜~~~想去看流星雨~~~）


原文：
A meteor（流星） stream is composed of dust particles that have been ejected from a parent comet at a variety of velocities. These particles follow the same orbit as the parent comet, but due to their differing velocities they slowly gain on or fall behind the disintegrating comet until a shroud of dust surrounds the entire cometary orbit. Astronomers have hypothesized that a meteor stream should broaden with time as the dust particles' individual orbits are perturbed by planetary gravitational fields. A recent computer-modeling experiment tested this hypothesis by tracking the influence of planetary gravitation over a projected 5,000-year period on the positions of a group of hypothetical dust particles. In the model, the particles were randomly distributed throughout a computer simulation of the orbit of an actual meteor stream, the Geminid. The researcher found, as expected, that the computer model stream broadened with time. Conventional theories, however（递进）, predicted that the distribution of particles would be increasingly dense toward the center of a meteor stream. Surprisingly, the computer model meteor stream gradually came to resemble a thick walled, hollow pipe.
Whenever the Earth passes through a meteor stream, a meteor shower occurs. Moving at a little over 1,500,000 miles per day around its orbit, the Earth would take, on average, just over a day to cross the hollow, computer-model Geminid stream if the stream were 5,000 years old. Two brief periods of peak meteor activity during the shower would be observed, one as the Earth entered the thick-walled "pipe" and one as it exited. There is no reason why the Earth should always pass through the stream's exact center, so the time interval between the two bursts of activity would vary from one year to the next.
Has the predicted twin-peaked activity been observed for the actual yearly Geminid meteor shower? The Geminid data between 1970 and 1979 show just such a bifurcation, a secondary burst of meteor activity being clearly visible at an average of 19 hours(1,200,000 miles)after the first burst. The time intervals between the bursts suggest the actual Geminid stream is about 3000 years old.

译文：
流星流是由母彗星以不同的速度喷射出的尘埃粒子组成的。这些粒子沿着与母彗星相同的轨道运行，但由于它们的速度不同，它们慢慢接近或落在正在解体的彗星后面，直到整个彗星轨道被一层尘埃包围。天文学家假设，流星流应该随着时间的推移而变宽，因为尘埃粒子的各自轨道受到行星引力场的干扰。最近的一项计算机建模实验通过跟踪行星引力对一组假设尘埃粒子位置的影响来验证这一假设。在该模型中，粒子随机分布在计算机模拟的一个实际流星流——双子座的轨道中。研究人员发现，正如预期的那样，计算机模型流随着时间的推移而变宽。还有需要补充的是，传统理论(递进)预测，颗粒的分布将越来越密集向流星流的中心。令人惊讶的是，计算机模型的流星流逐渐变得像一个厚壁的空心管。
每当地球经过流星流时，就会发生流星雨。地球以每天150多万英里的速度绕其轨道运行，如果这条计算机模拟的空心双子座河有5000年的历史，那么它平均只需一天多一点的时间就能穿过。在流星雨期间可以观察到两个短暂的流星活动高峰期，一个是在地球进入厚壁“管道”时，另一个是在它退出时。没有理由说明为什么地球总是要经过这条流的确切中心，所以两次活动爆发之间的时间间隔每年都不一样。
预测的双峰活动是否被实际观测到每年的双子座流星雨?1970年至1979年的双子座数据就显示了这样一个分叉，在第一次流星爆发后的平均19小时(120万英里)，可以清楚地看到流星活动的二次爆发。这些爆发之间的时间间隔表明，真正的双子座流大约有3000年的历史。

课代表笔记：

• Describing a new theoretical model and noting that it explains the nature of observations made of a particular natural phenomenon
  
• Meteor streams are composed of dust particles derived from comets.
  
• The research described in the first paragraph was undertaken in order to test the hypothesis that meteor streams become broader as they age.
  
• During the Earth's passage through a meteor stream if the conventional theories mentioned in highlight text were correct?：A meteor activity would gradually increase to a single, intense peak, and then gradually decline. (这个要有空间想象能力。想象彗星以及流星群分布在围绕太阳公转的轨道上，就像一条带子一样，传统理论认为带子的中心的陨石颗粒是密集的，但假说是讲该流星群带是中空的，就跟一条通心粉一样。而地球公转轨道与流星群轨道形成一定角度，所以每当地球共转到与流星群轨道的焦点的时候，将会穿过流星群带，因此如果是中空的，你在地球上就会看到两次流星雨峰值，但如果是中间密集形的，你就只会看到一次密集的流星雨。)
  
• The computer-model Geminid meteor stream provides an accurate representation of the development of the actual Geminid stream. ---- This is an assumption underlying the last sentence of the passage.