Black holes, although emitting no light of their own, interact with their environment because of their enormous gravitational fields. They usually form an accretion disc of material orbiting faster and faster as it is pulled towards the event horizon.
Friction in theaccretion discheats up the material and produces X-rays, which reveal the presence of theblack holeto astronomers. In many cases, the accretion disc, combined with strong magnetic fields, channels the flow of matter and energy away from the black hole, in powerful ‘jets’ that are at right angles to the disc.
If there is no material available to a black hole, then the accretion disc eventually dissipates. The black hole has stopped ‘feeding’ and there is no material producing X-rays. The black hole has become ‘quiescent’ or ‘dormant’ and the only way to detect its presence is through its gravitational effect on nearby objects.
The black hole may not be dormant forever. Other material may eventually come into range of the black hole’s influence and kick-start the process again. The black hole will become ‘active’ again.
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Asked by: Eleanor Smith, via email
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