X-Ray Sources: Sco X-1

The brightness of a celestial object as it appears from the Earth does not take into account the distance of the object from the Earth. Thus it is not a reliable indication of any physical property of the object. So even though Sco X-1 appears bright in the X-ray sky, it does so primarily because it is so close to us. An extragalactic X-ray source such as 3C273, although it may appear only one thousandth as bright as Sco X-1, in reality produces about one hundred million times as much energy as does Sco. Its faintness is because it is about 2 billion light years from the Earth!

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