For any object, there is a critical radius, called the Schwarzschildradius, at which its mass will form a black hole.
对于任何一个物体,都存在一个被称为史瓦西半径的临界半径,在这个半径以内的质量将形成一个黑洞。
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The X-rays are sent off into space before the matter crosses the Schwarzschildradius and crashes into the singularity. Thus we can see this X-ray emission.