Expert: Philip Stahl - 2/20/2004

Question
I was watching TV and saw for the first time a actual star get absorbed by a Black Hole. What got my attention was the Xray blast that seam to not be affected by the Black hole in all directions! It light up the Galaxy if I am not mistaken.
How was this possible? How could the Xrays escape so easy in all direction? Is it a time thing? the Xrays escape before the gravity absorb the star? As part of the event?
I am confused about this... In my mind's eye the Xrays be absorbed also? Maybe it was because the star was not completely absorbed?
The Xray seamed to defy gravity...Is it?
Please help me understand.
Peter.  

Answer
Hello.

I haven't seen the program of which you speak, but x-rays generally indicate the accretion of the outer layers of gas of a companion star, to the black hole. Thus, as the gaseous layers are pulled into the hole, x-rays are produced.

It is important to recognize that just because something appears in the vicinity of a black hole, doesn't necessarily mean it will be sucked into it.

In the case of x-rays, or other forms of electromagnetic radiation, it must first be within the black hole's "event horizon" to be pulled into it. This is an outer region surrounding the hole, within which nothing can escape. Even light (or x-rays).

Since we detect black hole presence mainly by x-rays produced from accretion of gas from a neighboring star, it follows that these x-rays must be beyond the limit of the event horizon. (Otherwise we wouldn't detect anything!)

So it isn't a case of "defying gravity" but simply lying beyond the limit of event horizon, which prevents any escape!