Black Holes: Gravity's Relentless Pull

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Black Holes: Gravity's Relentless Pull
OBJECT: Cygnus X-1 (X-Ray binary)
DISTANCE: 8,100 lightyears
BLACK HOLE? Yes (stellar-mass)

This is the brightest X-ray source (indicated as X-1) in the constellation Cygnus (Swan). It consists of a bright blue star and a black hole that orbit around each other. The black hole pulls gas off the surface of this star. This gas heats up and shines in X-rays as it falls towards the black hole.

The X-ray image shows the hot gas around the black hole. The radio image shows that energetic processes accelerate electrons to high speeds. These electrons emit radio waves as they are being expelled in a jet (towards the top right). The visible light image shows a normal bright star at the same position as the X-ray source (the other stars in the image are unrelated and reside in the foreground or background). Analysis of the stellar spectrum reveals that the star revolves around its unseen companion every 5.6 days.

When a star with a birth mass at least 25 times heavier than the Sun dies in a supernova explosion, its core is too massive to withstand its own gravity. It collapses and forms a black hole. We call this a stellar-mass black hole (as opposed to supermassive black holes, which are typically a million times heavier). A stellar-mass black hole is normally invisible to us. But in a binary system, bright X-rays might give away its presence. Cygnus X-1 is an unusual X-ray binary because we can determine the mass of the unseen companion. It's about 10 times more massive than the Sun. In fact, this proves that it must be a black hole. Neutron stars and white dwarfs can also power X-ray binaries, but they cannot be this heavy.

X-Ray image of: Cygnus X-1 (X-Ray binary)
The black hole pulls gas of the star orbiting around it. The gas heats up and emits X-rays (yellow) as it falls into the black hole.