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Magazine How one image captures 21 hours of a volcanic eruption. Science Why it's so hard to treat pain in infants. It is part of a system called HR that also contains two stars.
The team was looking for pairs of stars in which one was a type that rotates so quickly it flings off material from its equator, creating a kind of ring from its own plasma.
They found HR , which has one of those plus one normal star, but the normal star appeared to be orbiting an empty spot of space once every 40 days. That turned out to be a black hole at least four times as massive as the sun, invisible because it is not actively devouring any material. We encounter radio waves often in our daily lives, for example in radios and cell phones. The black holes in the Universe do not emit any detectable type of light. However, astronomers can still find them and learn a lot about them.
They do this by measuring the visible light, X-rays and radio waves emitted by material in the immediate environment of a black hole.
For example, when a normal star orbits around a black hole we can measure the speed of the star by studying the visible light that it emits. Knowledge of this speed can be combined with the laws of gravity to prove that the star is in fact orbiting a black hole, instead of something else.
It also yields the mass of the black hole. Alternatively, when gas orbits around a black hole it tends to get very hot because of friction.
It then starts emitting X-rays and radio waves. So black holes can also often be found and studied by looking for bright sources of X-rays and radio waves in the sky. There are many other types of electromagnetic radiation as well.
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