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最终，所有的恒星都会消耗掉它们的核燃料，坍缩成三种状态之一——白矮星、中子星或黑洞。

Eventually, all stars consume their nuclear fuel and collapse into one of three final states-white dwarf, neutron star, or black hole.

白矮星是非常小的恒星，密度比任何已知的陆地物质都要大。

White dwarfs are extremely small stars with densities greater than any known terrestrial material.

人们相信白矮星起源于低质量或中等质量的恒星。

It is believed that white dwarfs began life as low-mass or medium-mass stars.

随着恒星年龄的增长，它开始坍缩——变成“矮星”，因为它不再产生足够的辐射热量来抵消其引力的收缩力。

As the star ages, it begins to collapse—become a "dwarf" because it no longer produces sufficient radiant heat to counteract the contracting force of its gravity.

虽然有些白矮星不比地球大，但这种白矮星的质量可以达到太阳的1.4倍。

Although some white dwarfs are no larger than Earth, the mass of such a dwarf can equal 1.4 times that of the Sun.

A spoonful of such matter would weigh several tons.

当一颗恒星收缩成白矮星时，它的表面会变得非常热、非常亮。

As a star contracts into a white dwarf, its surface becomes very hot and very bright.

但是，如果没有持续的能量来源，它会慢慢变冷、变暗。

But, without a continuing source of energy, it will slowly become cooler and dimmer.

虽然没有观测到白矮星，但白矮星的最后阶段一定是一个小的、冷的、不发光的天体，称为黑矮星。

Although none have been observed, the terminal stage of a white dwarf must be a small, cold, nonluminous body called a black dwarf.

一项对白矮星的研究得出了一个乍一看可能令人惊讶的结论：最小的白矮星质量最大，而最大的白矮星质量最小。

A study of white dwarfs produced what might at first appear to be a surprising conclusion: the smallest white dwarfs are the most massive, and the largest white dwarfs are the least massive.

对此的解释是，质量越大的恒星具有更大的引力，而更大的引力将恒星收缩成一个比质量较小的恒星更小、密度更大的物体。

The explanation for this is that a more massive star has a greater gravitational force, and that greater gravitational force contracts the star into a smaller, more densely packed object than one gets with a less massive star.

因此，最小的白矮星来自于比大白矮星更大更重的恒星的坍缩。

Thus, the smallest white dwarfs came from the collapse of stars that were larger and more massive than those the large white dwarfs came from.

将大恒星坍缩成小恒星的观点延伸开来，我们可以预测，非常大的恒星会坍缩成比白矮星更小、质量更大的恒星。

Extending this idea of larger stars collapsing to smaller stars leads to the prediction that very large stars would collapse to stars even smaller and more massive than white dwarfs.

这些天体被命名为中子星，被认为是超新星事件（一颗巨大的、质量非常大的恒星的坍缩）的残留物。

Named neutron stars, these objects are thought to be the remnants of supernova events (the collapse of a large, very massive star).

如果地球坍缩到中子星的密度，它的直径将相当于一个足球场的长度，而豌豆大小的这种物质的样本将重达1亿吨。

If Earth were to collapse to the density of a neutron star, it would have a diameter equivalent to the length of a football field, and a pea-sized sample of this matter would weigh 100 million tons.

这大约是原子核的密度；因此，中子星可以被认为是大的原子核。

This is approximately the density of an atomic nucleus; thus, neutron stars can be thought of as large atomic nuclei.

虽然中子星的表面温度很高，但它们的小尺寸会极大地限制它们的亮度。

Although neutron stars have high surface temperatures, their small size would greatly limit their luminosity.

因此，视觉定位将是极其困难的。

Consequently, locating one visually would be extremely difficult.

然而，理论预测中子星会有很强的磁场。

However, theory predicts that a neutron star would have a very strong magnetic field.

此外，当一颗恒星坍塌时，它会旋转得更快，这与滑冰运动员在手臂收缩时旋转得更快的原因是一样的。

Further, as a star collapses, it will rotate faster, for the same reason ice-skaters rotate faster as they pull in their arms.

如果太阳坍缩到中子星的大小，它的自转速度将从每25天一次增加到每秒近1000次。

If the Sun were to collapse to the size of a neutron star, it would increase its rate of rotation from once every 25 days to nearly 1,000 times per second.

这种旋转的恒星产生的无线电波会被集中成两束狭窄的光束，就像警车或救护车上的旋转灯一样。

The radio waves generated by such rotating stars would be concentrated into two narrow beams, like the rotating light on a police car or ambulance.

如果地球碰巧在这个光束的路径上，当无线电波掠过时，这颗恒星就会出现忽明忽暗或脉动的现象。

If Earth happened to be in the path of this beacon, the star would appear to blink on and off, or pulsate, as the radio waves swept past.

在20世纪70年代早期，在蟹状星云中发现了一种发射短脉冲射电能量的源，称为脉冲星（脉冲射电源）。

In the early 1970s, a source that radiates short pulses of radio energy, called a pulsar (pulsating radio source), was discovered in the Crab Nebula.

对这种射电源的目视检查显示它是星云中心的一颗小恒星。

Visual inspection of this radio source revealed it to be a small star centered in the nebula.

第一颗中子星就这样被发现了。

The first neutron star had been discovered.

中子星是由密度最大的物质构成的吗？

Are neutron stars made of the densest matter possible?

答案是否定的。在超新星事件中，质量大于3个太阳质量的恒星的残骸显然会坍缩成比中子星更小、密度更大的物体。

No. During a supernova event, remnants of stars greater than three solar masses apparently collapse into objects even smaller and denser than neutron stars.

尽管这些物体会非常热，但它们的引力会非常大，以至于光都无法逃离表面。

Even though these objects would be very hot, their gravity would be so immense that even light could not escape the surface.

因此，它们会从人们的视线中消失。

Consequently, they would literally disappear from sight.

这些令人难以置信的天体被恰当地命名为"黑洞"。

These incredible bodies have appropriately been named "black holes."

任何离黑洞太近的物体都会被其不可抗拒的引力卷进去并湮灭。

Anything that moved too near a black hole would be swept in by its irresistible gravity and annihilated.

天文学家怎样才能找到一个其引力场阻止所有物质和能量逃逸的物体呢？

How can astronomers find an object whose gravitational field prevents the escape of all matter and energy?

理论预测，当物质被拉入黑洞时，它会变得非常热，并在被吞没之前释放出大量的x射线。

Theory predicts that as matter is pulled into a black hole, it should become very hot and emit a flood of x-rays before being engulfed.

第一个黑洞x射线源是在1971年由卫星探测器发现的。

The first black hole x-ray sources were discovered in 1971 by detectors on satellites.

The word "terminal" in paragraph 2 is closest in meaning to

It can be inferred from paragraph 2 that the reason white dwarfs have been observed and black dwarfs have not is that

According to paragraph 3, among stars that eventually became white dwarfs, why do the largest and most massive stars produce the smallest white dwarfs?

According to paragraph 5, neutron stars are very difficult to locate visually because

Why does the author mention "ice-skaters"?

According to paragraph 6, black holes cannot be directly observed because

The word "engulfed" in paragraph 7 is closest in meaning to

According to paragraph 7, what is the connection between x-rays and black holes?

Look at the four squares  that indicate where the following sentence could be added to the passage. At this rate of rotation, a magnetic field would generate electromagnetic radiation in the radio wave range.  Where would the sentence best fit? Click on a squareto add the sentence to the passage.

Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

Stellar Remnants Eventually, all stars consume their nuclear fuel and collapse into one of three final states-white dwarf, neutron star, or black hole. White dwarfs are extremely small stars with densities greater than any known terrestrial material. It is believed that white dwarfs began life as low-mass or medium-mass stars. As the star ages, it begins to collapse-become a "dwarf—because it no longer produces sufficient radiant heat to counteract the contracting force of its gravity. Although some white dwarfs are no larger than Earth, the mass of such a dwarf can equal 1.4 times that of the Sun. A spoonful of such matter would weigh several tons. As a star contracts into a white dwarf, its surface becomes very hot and very bright. But, without a continuing source of energy, it will slowly become cooler and dimmer. Although none have been observed, the terminal stage of a white dwarf must be a small, cold, nonluminous body called a black dwarf. A study of white dwarfs produced what might at first appear to be a surprising conclusion the smallest white dwarfs are the most massive, and the largest white dwarfs are the least massive. The explanation for this is that a more massive star has a greater gravitational force, and that greater gravitational force contracts the star into a smaller, more densely packed object than one gets with a less massive star. Thus, the smallest white dwarfs came from the collapse of stars that were larger and more massive than those the large white dwarfs came from. Extending this idea of larger stars collapsing to smaller stars leads to the prediction that very large stars would collapse to stars even smaller and more massive than white dwarfs. Named neutron stars, these objects are thought to be the remnants of supernova events (the collapse of a large, very massive star). If Earth were to collapse to the density of a neutron star, it would have a diameter equivalent to the length of a football field, and a pea-sized sample of this matter would weigh 100 million tons. This is approximately the density of an atomic nucleus; thus, neutron stars can be thought of as large atomic nuclei. Although neutron stars have high surface temperatures, their small size would greatly limit their luminosity. Consequently, locating one visually would be extremely difficult. However, theory predicts that a neutron star would have a very strong magnetic field. Further, as a star collapses, it will rotate faster, for the same reason ice-skaters rotate faster as they pull in their arms. If the Sun were to collapse to the size of a neutron star, it would increase its rate of rotation from once every25 days to nearly 1,000 times per second. The radio waves generated by such rotating stars would be concentrated into two narrow beams, like the rotating light on a police car or ambulance. If Earth happened to be in the path of this beacon, the star would appear to blink on and off, or pulsate, as the radio waves swept past. In the early 1970s, a source that radiates short pulses of radio energy, called a pulsar (pulsating radio source), was discovered in the Crab Nebula. Visual inspection of this radio source revealed it to be a small star centered in the nebula. The first neutron star had been discovered. Are neutron stars made of the densest matter possible? No. During a supernova event, remnants of stars greater than three solar masses apparently collapse into objects even smaller and denser than neutron stars. Even though these objects would be very hot, their gravity would be so immense that even light could not escape the surface. Consequently, they would literally disappear from sight. These incredible bodies have appropriately been named "black holes. Anything that moved too near a black hole would be swept in by its irresistible gravity and annihilated. How can astronomers find an object whose gravitational field prevents the escape of all matter and energy? Theory predicts that as matter is pulled into a black hole, it should become very hot and emit a flood of x-rays before being engulfed. The first black hole x-ray sources were discovered in 1971 by detectors on satellites.