THE BLACK HOLES

 

 

In these images, courtesy of HEASARC, you can see how the sky would appear to an observer moving towards a black hole.

1. Approaching the black hole

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The first picture shows what a hypothetical observer, moving towards the black hole, would see while approaching: at the centre, towards the right, the three stars of the Orion belt; bottom left the star Sirius, and, above them the star Betelgeuse.
While approaching the black hole, to the observer the stars appear shifted outwards, because the beams of light that they emit are curved. On the other hand, for the same reason the stars placed on the other side of the black hole from the position of the observer, are visible.
At the end of the picture, the observer is only 42 Km away from the black hole.

2. Around the black hole

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While the observer approaches the black hole and begins moving around it, due to the deflection of the luminous beams, he has the impression that the sky moves in a strange way. The luminous beams are so curved that they can reach the observer from any direction in space, even from behind the black hole.
The strong curving and the strengthening of the rays of light are responsible for the fact that very weak stars, situated behind the black hole, with respect to the observer's position, become visible.
It is possible to recognize some sort of ring around the black hole, formed by the light of the stars deflected by the black hole: such formation is called   Einstein ring .

3. Approaching the event horizon

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As you get closer, the gravitational field of the black hole is so intense that it produces two distinct images of the Orion constellation. The observer is now approaching the event horizon, a spherical region around the black hole representing the boundary beyond which no signal can escape. Here, the gravitational force is so strong that the luminous beams can only be attracted towards the black hole or move in circles around it. For example, a photon could be emitted by the back of the observer, complete a circle around the black hole and reach his eyes, so he would see his own back!!!
At the photon sphere, no signal emitted outwards can leave the black hole. From here, the sky appears at the back of the observer, concentrated in half the space it normally occupies.

4. A look from the event horizon

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If you looked towards the black hole, you would only see a dark region. The outward stars, on the contrary, are visible and appear blue, because the light they emitted, which fell towards the black hole, became more energetic. You could also see stars weakened in luminosity due to the huge gravitational force that deforms light, and stars in front of the observer, when they actually are behind, because the light emitted has been curved and completed a circle around the black hole.
Once beyond the event horizon, light can only travel towards the black hole, but it cannot escape.

5. A trip around the event horizon

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The observer is now situated on the event horizon, the position of which is easy to recognize: it is the imaginary line dividing the sky from the black hole.
While moving around the black hole, the sky appears to move in a strange way. Here, an Einstein ring becomes an invisible line above the event horizon. The stars opposite the black hole, with respect to the observer's position, are apparently approaching this line with a high angular velocity and appear very enlarged.
The observer is still above the event horizon. While moving towards it, the sky would apparently become "flat" and concentrate in one point on the opposite side of the black hole, but this event is not shown in the picture. We do not know how the sky would appear from the event horizon, within the black hole.