托福阅读材料之黑洞 Black Holes

　　在遥远的外太空有许多神秘的天体，黑洞就是其中之一。至今依然有科学家对于黑洞的存在性持有质疑的观点，但是大部分的天文学家还是相信射电望远镜的观测数据，对黑洞的存在表示认同。从今天起我们将分三次连载这篇关于黑洞的科技文，希望对提高大家的阅读能力有所帮助。

　　Astronomers think the object shown in this Chandra X-ray Observatory image (in box) may be an elusive intermediate-mass black hole. Located about 32 million light-years from Earth in the Messier 74 galaxy (M74), this object emits periodic bursts of x-rays at a rate that suggests it is much larger than a stellar-mass black hole but significantly smaller than the supermassive black holes found at the centers of galaxies. Few such middling black holes have been discovered, and scientists aren't sure how they form.

　　Black holes are the cold remnants of former stars, so dense that no matter—not even light—is able to escape their powerful gravitational pull.

　　While most stars end up as white dwarfs or neutron stars, black holes are the last evolutionary stage in the lifetimes of enormous stars that had been at least 10 or 15 times as massive as our own sun.

　　When giant stars reach the final stages of their lives they often detonate in cataclysms known as supernovae. Such an explosion scatters most of a star into the void of space but leaves behind a large "cold" remnant on which fusion no longer takes place.