GRE Reading Comprehension: JiJing 352-GRE阅读机经352篇 - 755E2NL9OGNJX7SAK

(This passage is excerpted from material published in 2000.) During a search for massive compact halo objects (MACHOs) – objects the size of planets but that do not orbit a star – astronomer Charles Alcock did not find many. He did find, however, something that produced a gravitational effect so strong that it implied an object with a mass six time that of the Sun. If this had been an ordinary star, it would have been visible in its own light. Given that it was not, Alcock reasoned at the time that it was probably a black hole. Although it is impossible to draw strong conclusions from a single observation, the tiny chance a black hole has of being discovered this way makes it plausible that such lone black holes are common.