Back in the 1780s, when Pierre Simon Laplace and John Michell first propounded the existence of ‘dark stars’ with tremendous gravitational fields, their hypothesis had very few takers. How could the audience believe in the existence of something that wasn’t visible in the first place?
Since so much of our scientific research, experimentation, analysis and even the most basic ideas of perception are based on our ability to see things, how do astronomers that talk about the transformation of black holes and supermassive black holes detect those invisible entities in the first place?
What are black holes?
Black holes are regions of space-time that have such a strong gravitational field that nothing, not even light, can escape them. You can read about black holes in detail here: What Are Black Holes And How Common Are They?
As the name signifies, a ‘black’ hole is essentially black, and in the vast darkness of space, it becomes invisible. So, how can we say for sure that a black hole exists at a certain location in space?
Behavior of neighboring objects
Suppose you know that an invisible man is in a room. Assuming that you don’t have any Batman-esque gadget at your disposal, what would be your best approach of determining his approximate position in the room? You would look for any signs of movement in any object in the room, such as a creak of the door, the twitch of the curtain, shuffling papers or shattering glass (if the invisible man is really clumsy).
Astronomers use a more technologically advanced version of the same technique – they observe and measure the unusual or inexplicable movements of stars and clusters of dust and gas around these invisible entities to predict the presence of black holes.
Here’s a 1-minute video from NASA that depicts how a black hole devours a neighboring star and reveals its location:
If a large star or a disk of gas or dust behaves as though it’s under the influence of something immensely dense and heavy (heavier than at least thrice the mass of sun), then it’s assumed that the motion is caused by a black hole. The features of the particular black hole, like its size and mass, are then determined by measuring the effects that it has on any neighboring bodies.
Emission of radiation
With such a high mass and gravitational field, neighboring stars or other bodies sometimes fall and funnel into a disk around black holes. These bodies then heat up to millions of degrees and swirl around the disk so fast that they heat up to incredible temperatures and emit X-rays. These rays, in turn, can be detected from the X-ray telescopes that we have back on Earth.
The Chandra X-Ray Observatory is one such orbiting X-ray telescope that has helped detect a number of black holes and given us a great deal of insight into their structural features.
Gravity Lensing effect
Out of the many theories that originated from Einstein’s general theory of relativity, astronomers rely on the one that claims gravity can bend space. Therefore, if a cluster of stars or a galaxy passes near a black hole, a redistribution of matter occurs that causes light to bend as it travels to the observer, i.e. telescopes on Earth. This phenomenon is known as gravitational lensing, and plays a vital role in detecting black holes in space.
To conclude, black holes are not visible, per se, but the effect that they have on their neighbors is what helps us determine their position in the vast blackness of space.
- Chandra X-Ray Observatory / NASA
- Cool Cosmos / California Institute of Technology