GRE Reading Comprehension: JiJing 352-GRE阅读机经352篇 - 7L557MN_447M85K5K

The bright star we call Sirius is really a binary star – a gigantic white star known as Sirius A and a far smaller dwarf called Sirius B. Sirius A is truly massive – about twice the size of our sun and around 25 times more luminous. Seen with the naked eye, the two stars coalesce into a single bright object in the nighttime sky. Sirius's existence was recorded by the ancient Egyptians, who used its initial date of visibility – the date at which the earth's rotation moves it far enough from the glare of our sun to be clearly seen – as the basis for their calendar. Its extreme brightness frightened the ancient Greeks, who ascribed to it evil powers. It wasn't until 1844 that astronomer Friedrich Bessel determined that Sirius must have an invisible companion star, and in 1862, American Alvan Graham Clark finally observed the tiny Sirius B.