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The life cycle of a star begins in nebulae

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By Mike Lynch
All of the stars we gaze at in our heavenly dome are all going through various stages of life. They are born, live out their lives and die, some more violently than others.
Since stars, including our sun, are basically large balls of mainly hydrogen gas, it only makes sense that stars are born out of loose clouds of hydrogen. All around our home Milky Way Galaxy there are gigantic hydrogen gas clouds, some trillions of miles in diameter.
It's the same story in many of the billions and billions of other galaxies that make up our known universe. These nebulae are the birthing grounds of hundreds and thousands of stars, many of which are born at nearly the same time in these giant stellar nurseries.
So how do these gigantic nebulae give birth to stars? Essentially it comes down to random motion and gravity. Either the gravitational influence from a passing star or group of stars, or the shock waves from an old exploding star in the distance stirs up the nebulae.
As this happens, random pockets of denser gas develop within the nebulae and a stellar birth gets cooking. Since these denser balls of hydrogen are more massive than the surrounding looser nebulae, they start acquiring more and more of a gravitational force that draws in more and more of the surrounding hydrogen. This makes these proto stars even more massive, giving them a stronger gravitational force that allows them to suck in even more of the surrounding gas. It's like a snowball rolling down a hill, becoming exponentially larger.
If these balls of hydrogen gas continue to grow, many of them can become massive enough to become stars. They get "lit up" and start shining like stars when their nuclear fusion furnaces get going deep in their interiors.
That can't happen until the giant hydrogen gas ball becomes so massive that its own gravitational force starts to put a big squeeze on it. Because of this gravitational squeezing, all kinds of tremendous pressure starts building toward the center of the proto star. We're talking billions of pounds per square inch of gravitational pressure.
Just like a giant pressure cooker, this kind of pressure drives the temperature up in the core of the future star to millions of degrees. When a critical level of heat is built up in the proto star, the nuclear fusion furnace gets turned up and the star begins its life of shining.
Most of the time these large nebulae give birth to many stars. Stars usually start life with lots of brothers and sisters in what astronomers call open star clusters. Eventually these clusters are torn apart by the gravitational influence of other neighboring stars, but for millions of years "kid stars" hang together.
Next week in Starwatch I'll have more on the inner workings of nuclear fusion as hydrogen atoms fuse together to form heavier helium atoms with a byproduct of tremendous levels of light and energy.
This coming week, if you're an early riser, see if you can spot the bright planets Jupiter and Venus in the low eastern predawn twilight. Venus is the brighter of the two extremely low planets just above the east-northeast horizon. You'll really need a low flat horizon to see it. On Friday morning the waning crescent moon will be perched just to the upper right of Venus and Jupiter and should make a wonderful site as you start your day.
Mike Lynch is an astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis and is author of the book, "Washington Starwatch," available at bookstores. Check his website,
The Everett Astronomical Society:
Story tags » Star Gazing

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