How do black holes form?

Cygnus X-1 was one of the first black hole candidates to be discovered and is still one of the star systems most reliably identified as containing a black hole. It is a double star system, with a massive brilliant young star orbiting a compact object. The compact object has a mass 8.7 times that of our sun. According to astronomers, such a massive object can only be a black hole.

You could make a black hole from anything if you could squeeze it small enough.

That’s because gravity depends on mass, and on how far you are from the center of a mass. By definition, if you’re close to a black hole, its gravity is so powerful that even light – the fastest moving stuff in the universe – is drawn into the hole That’s why black holes are black.

Black holes are part of the natural life and death of stars. A young star burns hydrogen at its core via thermonuclear fusion. Eventually, the star’s core becomes mostly iron, the most stable element in the universe. Fusion stops, as does the heat and outward pressure generated by fusion – which before that had been balancing the tremendous inward pressure of the star’s own gravity.

The core of a massive-enough star crushes inward and becomes a black hole.

Cygnus X-1 is the brightest persistent source of highly energetic x-rays in our sky. Astronomers think that the x-rays are caused by nearby clouds of gas and dust, which ionize as they fall into the black hole, showering the universe with radiation.

There may be countless black holes in the space of our Milky Way galaxy, made from collapsed stars. Scientists also think that black holes as massive as millions, or even billions, of suns form in roughly the same way. Supermassive black holes might lie at the hearts of many galaxies, including our Milky Way.

September 15, 2009

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