How do scientists look for black holes in space?
1 Answer
Black holes are "seen" through their interactions with other objects. The various interactions show up in X-ray emissions, the motion of surrounding stars, and most recently gravitational waves.
Explanation:
X-ray emissions
Most likely black holes are detected through X-ray emissions. If a massive star that has collapsed is part of a binary system, the dense core can pull gas from the remaining "live" star. The gas spirals inwards, gains energy from the gravitational field, and it gets so hot that its emissions are mostly X-rays. A compact object that emits primarily X-rays, next to a light emitting star, is likely a black hole if it appears to have enough mass. Cygnus X-1 is the best known such candidate.
A larger-scale version of this process is seen in the center of some galaxies, producing emissions so bright that that they outshine all the surrounding stars (https://en.wikipedia.org/wiki/Quasar).
Motion of Surrounding Stars
Massive black holes, containing millions of solar masses or more, can exert strong gravitational forces on surrounding stars. We detect this effect through the rapid, sharply curved motion of the stars. The central black hole in our galaxy is detected in this way (http://www.nasa.gov/mission_pages/chandra/multimedia/black-hole-SagittariusA.html).
Gravitational Waves
The most certain way to detect black holes is to detect the gravitational waves produced when they collide with other black holes or other objects. The gravitational waves are made of the same "stuff" as the gravity of the black holes and so offer the clearest indication of such black holes. Recently the Laser Interferometer Gravitational-wave Observatory (LIGO) detected such gravitational waves from a black-hole collision (http://news.mit.edu/2016/second-time-ligo-detects-gravitational-waves-0615).
