托福閱讀：黑洞 Black Holes

　　在遙遠(yuǎn)的外太空有許多神秘的天體，黑洞就是其中之一。至今依然有科學(xué)家對(duì)于黑洞的存在性持有質(zhì)疑的觀點(diǎn)，但是大部分的天文學(xué)家還是相信射電望遠(yuǎn)鏡的觀測(cè)數(shù)據(jù)，對(duì)黑洞的存在表示認(rèn)同。從今天起我們將分三次連載這篇關(guān)于黑洞的科技文，希望對(duì)提高大家的閱讀能力有所幫助。

　　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.