Astronomers know black holes exist not by seeing them but by detecting X-rays produced when stellar material sucked off an existing star spirals to its doom in the black hole. One of the first suspected black holes detected by X-rays is in the bright constellation Cygnus the Swan, seen in the late autumn evening sky in the west, more than 46 thousand trillion miles away.

Welcome to part three of my trilogy on the birth, life and death of stars.

In last week's Skywatch column, the life of stars, I told how stars cook up energy in their cores with nuclear fusion. Because of intense pressure and the resulting astronomically high temperatures, hydrogen atoms fuse together to form heavier helium atoms. In the process, a little bit of hydrogen is converted into energy, which forces its way to the outer levels of a star.

Simply put, hydrogen is the fuel of a star, and the smaller and less massive stars like our sun sip their supply of hydrogen slowly and live a long time. Our sun's been around for about 5 billion years and should have enough hydrogen fuel in the tank to keep it going for another 5 billion years.

More massive stars are not around for nearly as long. They're literally what you could call hydrogen gas guzzlers. The big guys of the stellar world may only last a few billion years.

Sooner or later, though, all stars begin to run out of hydrogen in their cores and stellar death gets under way. Smaller stars like our sun certainly die a violent death, but the really massive stars go out with a huge bang!

Low mass stars like our sun get really fat before they die and flicker out. In the case of our sun, it will run out of hydrogen in about 5 billion years. In the sun's core, helium builds up as the hydrogen dwindles. As nuclear fusion dies out, internal pressure decreases. The built-up helium in the core starts to contract because of never ending gravitational pressure. This causes the helium core to increase in temperature.Some of the heat escapes beyond the core, reaching the cooler hydrogen layers farther out in the sun. In time the temperature rises high enough in these layers to fire up nuclear fusion. That will cause the sun to bloat out into a red giant star.When this happens to our sun, it will swallow up the planets Mercury and Venus and will just about touch the Earth. When that happens, needless to say, we'll be toast! Even though the sun will have a cooler surface temperature of about 3000 to 4000 degrees Fahrenheit, it will be right on top of us.Also, toward the end of our sun's red giant phase, excess energy "burps" in the outer layers will cause large clouds of gas to blow off and form large rings and shells of gas around what's left of our star.After about a billion or two years of being a red giant, stars like our sun totally run out of all fusion fuel and will shrink into white dwarfs. With no more nuclear reactions within the star to hold it up, gravity collapses the corpse of the once-proud star. In our sun's case, it's believed what's left of the sun's original mass will be squished into a ball about the size of Earth. When that happens our sun will be considered a white dwarf or a "retired star."

Massive stars, at least eight times more massive than our sun, die a spectacular death Just like smaller stars, they also bloat out into red giants, only they become super, super big red giants.Helium atoms inside the stellar core eventually fuse in stages to heavier carbon and oxygen, and eventually an iron core forms. That's the end of the line, though, and a chain of reactions causes all nuclear you-know-what to break loose, and then you have an explosion beyond your wildest dreams. The once super giant red star explodes into a supernova.The supernova flings out material at beyond-incredible speeds of 10,000 miles a second. In the process, heavier material like gold, silver, and many other heavier elements are "cooked" up and become the building blocks of future stars and planets. Out of death comes new celestial life. This is recycling on a cosmic scale.What's left of an exploded star after a supernova can be one of two bizarre objects, depending on the mass of the remaining core. It may become a rapidly rotating neutron star, only about 10 to 15 miles in diameter, and so dense that one tablespoon would weigh over a billion Earth tons. Otherwise, the core left behind may collapse very rapidly, possibly in a matter of hours, to an object so small and so dense that not even light can escape it. When this happens you have a black hole. What goes into a black hole stays in a black hole. Nothing escapes.Astronomers have never actually seen a black hole, but there are ways they can detect their presence. This is especially the case when a black hole is part of multiple star systems. Black holes can literally suck material off existing stars. When that happens, X-rays are produced as stellar material spirals to its doom in the black hole.These X-rays are signals detected by astronomers. One of the first suspected black holes detected by X-rays is found in the bright constellation Cygnus the Swan, shining brightly in our late autumn evening sky in the west. But it can't suck us in; it's over 46 thousand trillion miles away.<B><I>Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis and is author of the book, "Pennsylvania Starwatch", available at bookstores and at his website www.lynchandthestars.com.</B></I>