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One of my favorite things to do is to introduce someone to astronomy. Most people don’t need an introduction; they’ve been looking at the stars and dreaming of space travel most of their lives. But there are always those with questions like: “How far away is that star?” Or “How big is the Universe?”. Inevitably I get the Black Hole questions. This month, I thought it might be fun to locate some of the various places in our nighttime sky where astronomers believe there are actually black holes. This way, when someone asks you about them, you can point up and say “see that spot in the sky, that’s a black hole” (and yes, then you too can get the blank stare and often heard “WOWwwww…..”)

A black hole is a super dense object that has an intense gravitational pull due to its large mass. So, imagine a star, which is much more massive than our sun, and which has a mass that is large enough to cause a black hole to form. As the star collapses onto itself due to this immense mass, the question then becomes: What keeps this star from collapsing onto itself and becoming a black hole? The answer is that there is also an intense pressure caused by nuclear reactions within the star pushing outward, balancing the inward “push” of mass.

However, when the fuel that feeds this nuclear reaction gets used up this massive star can’t support itself anymore and it collapses to form a black hole – a region of incredibly dense material. It is interesting to note that when a black hole is formed by a collapsing star it is actually impossible to watch the final steps of the formation of the black hole from the Earth. In addition, it is impossible to see any object fall into a black hole. This is not to say that everything appears to freeze just before entering a black hole. As an object falls into a black hole it gets increasingly dimmer and dimmer from the point of view of an outside observer. By the time an object gets to the edge of a black hole, it will be completely black. This effect, called a gravitational redshift, is caused by the immense gravity near the outside of a black hole.

So, this is an interesting problem. How do you prove the existence of something that cannot be observed by definition? There are actually many methods used to see if black holes really exist in our universe. The first method is to look for objects in our universe that have a lot of mass, but are very small. For example we believe that there exists a black hole at the center of M87 in the constellation of Virgo. This object weighs about three billion times more than our sun, but takes up a volume no larger than our solar system.

Also, several thousand light-years away, near the "heart" of Cygnus two stars are locked in a gravitational embrace. One star, a blue supergiant star is about 30 times as massive as the Sun and 400,000 times brighter. The other star is a star that is 5 to 10 times the mass of the Sun, but it's extremely small. This object must be the collapsed core of a star - Its mass is too great to be a white dwarf or a neutron star, though, so it must be a black hole. This stellar system is called Cygnus X-1 (indicating it was the first source of X-rays discovered in the constellation Cygnus). It’s one of the first suspected black holes.

The thought is that the X-rays originate from a disk of gas that's spiraling into the black hole. As the two stars orbit each other at a rate of once every 5.6 days, the black hole's gravitational pull causes the blue supergiant to "bulge" toward the smaller, more dense star. As a result, hot gas flows away from the Blue supergiant toward the black hole. Friction heats the gas to a billion degrees or more, causing it to emit a torrent of X-rays, providing a signpost for our blackhole.

So have a look – and when you see those beginners out there, be sure to show them a blackhole – trust me, they’ll love it !