The Life of a Star


by Janette Vince - Date: 2007-08-28 - Word Count: 945 Share This!

Stars are familiar objects. We see them every day and every night. A single star, our Sun, makes it possible for life on earth to thrive. Stars live so long that they seem eternal to us. But they all had a beginning, and they'll all have an end, too.

Stars begin their lives in immense clouds of gas and dust called nebulae. Nebulae contain mostly hydrogen gas, with a little helium. In most places, a nebula is often so gossamer-thin that if you walked through one, you'd never notice. However, there are some areas in a nebula where gravity causes the gas and dust to coalesce, getting more and more dense and hot.

As more and more material clumps together in these areas, the temperature inside can get extremely hot-around fifteen million degrees. The gravitational pressure is also so immense that the electrons within the atoms of hydrogen and helium are stripped, leaving only the nuclei. As the gravitational force increases, the atomic nuclei fuse. This event is called nuclear fusion. It creates an enormous outpouring of energy strong enough to counteract the crushing gravitational force. The energy pours out into space as light, heat, and electromagnetic radiation. A new star is born, and it begins to shine.

Throughout its life, a star is caught in a balancing act between the crushing forces of gravity and the explosive forces of nuclear fusion within its core. As long as there is material within the star's core that can undergo the process of nuclear fusion, the star will fight gravity and continue to shine. But when the star runs out of fuel, it dies.

Most stars are the size of our sun or smaller. Our sun is a yellow dwarf star, and it's fairly average within the universe. Stars the size of our sun usually emit light in the yellow or orange spectrum, and they live for about ten billion years.

The process of nuclear fusion converts hydrogen into helium. A star the size of our sun turns all its hydrogen into helium, and then starts converting helium into oxygen and carbon as it ages. The oxygen we breathe, and the carbon that provides the most crucial material for life on earth, was created inside a star like our sun.

As a medium-sized star ages, the central core contracts and the outer envelope expands to many times the star's original size. This changes the star from a yellow dwarf to a red giant. This will happen to our sun someday; it will swell until it encompasses even Jupiter's orbit. Red giants are many times larger than our sun, but they are also cooler.

Eventually, the core contracts as much as possible. The atoms within are packed so tightly that they cannot be further contracted. When that happens, a large amount of energy is released, ripping away the outer envelope of gas and forming a nebula of gas and dust.

The only thing left of the star is the tightly-packed core, called a white dwarf star. White dwarfs still emit heat and light, but they no longer undergo nuclear fusion. Eventually, when they radiate all their heat, they will become cold, lifeless black dwarf stars. There are no black dwarf stars today; the universe has not existed long enough for any white dwarf star to lose all of its heat.

Larger stars have a different fate. A star about three times bigger than the sun will become a red supergiant as it ages-a star many times the size of a red giant. It will run out of hydrogen, then out of helium. It will start converting helium into carbon and oxygen, just like a medium-sized star. When it runs out of helium, it will start converting the carbon and oxygen it produced into heavier elements such as sodium, magnesium, sulfur, and iron.

When such a star dies, the iron core contracts in an instant, sending out a massive shock wave. This is a supernova explosion. Supernovas appear in our sky occasionally, many times brighter than the surrounding stars. The outer layers of gas are ripped away and form an enormous nebula. The explosion also ejects heavier elements such as gold, silver, and other metals.

The core, however, is another story. In most large stars, the core contracts so tightly that even the protons in its remaining atoms are ripped away, leaving only the neutrons. This is called a neutron star. Many are only ten miles or so in diameter, but they have an extremely strong magnetic field.

In stars more than three times larger than our sun, the core is so massive that the gravitational forces pulling it together are too strong to stop. All of the star's remaining mass is crushed into a single point, or singularity, no bigger than a single atom. This is how a black hole is born.

A black hole's gravitational pull is so great that nothing, not even light, can escape. Because of this, black holes cannot be directly observed. However, the matter swirling around a black hole can indicate its position. Scientists believe that black holes lurk at the centers of most galaxies, including ours.

Without stars, the universe would be an empty place. Stars are the furnaces that forge most of the material that makes up our world, from the air we breathe to the metals we build with to the matter that makes up our bodies. Next time you see a beautiful gold necklace, handle a cast-iron skillet, or take a breath of cool air, remember-it came from the fiery heart of a star.

About the author

J Vince is managing director of thanksdarling.com - for more articles and for a range of unusual gifts including ‘name a star' visit http://www.thanksdarling.com/categories/out-of-this-world.htm

Related Tags: gifts, star, nuclear, sky, scientists, atomic, explosion, black hole, neutron, galaxies

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