Last week in Starwatch, in the first part of a trilogy of the birth, life and death of a star, I described how stars are born. Balls of hydrogen gas gravitationally coalesce out of loose clouds of hydrogen referred to as nebulae. Gravity is the tool that makes this happen. The more mass any object has, the more of a gravitational pull it has.
Stars come out of the celestial womb when these balls of gas become massive enough to fire up the nuclear-fusion furnaces at their core. When that happens, incredible amounts of light and radiation are produced and a protostar becomes a star and lives out its celestial life.
What nuclear fusion is and how it operates inside a star’s core is the subject today.
Gravity plays a key role. As I told you last week, gravitational compression builds up the heat tremendously inside a star. In the case of our sun, an average size star in our Milky Way, it has a mass of 2 with 30 zeros after it in kilograms (one kilogram equals just over 2.2 pounds).
That kind of mass results in a tremendous gravitational force and a colossal amount of gravitational pressure in the core of the sun, more than 500 billion pounds per square inch.
With that kind of pressure, the temperature at the core of the sun exceeds 27 million degrees. In more massive stars, there’s even more gravitational pressure and even higher temperatures.
Because of this tremendous heat, hydrogen atoms in the inner core of stars are zipping along at incredible speeds and slamming into each other with incredible violence. By their very nature, hydrogen atoms tend to be electrically repulsive to each other. But the force of these collisions totally overwhelms their atomic independence, so that not only do they bash into each other, they actually fuse together. This is the process of nuclear fusion.
It’s like slamming your hands together with such violence that they merge, or fuse into one big hand. In the core of stars, hydrogen atoms are fusing to form heavier helium atoms. It takes four hydrogen atoms fusing to form one heavier helium atom, and in the process, energy is produced.
That’s the extremely complicated part of nuclear fusion. It all has to do with Albert Einstein’s famous assertion that energy and mass can be interchanged if you have the right conditions — and those conditions are met in the violent cores of stars.
The collective radiation from all of these hydrogen atom collisions — in the form of light, heat, gamma rays and x-rays — slowly makes its way to the outer layer of a star. From there, the radiation travels in all directions at the speed of light, more than 186,000 miles per second.
Essentially, hydrogen is the fuel of a star, helium is the ash, and energy is a byproduct. In just one second, our sun converts almost 700 million tons of hydrogen to helium and energy. In spite of that incredible rate of consumption, our sun won’t run out of hydrogen for about another five billion years.
Since our sun is an average star in terms of its size and mass, the supply of hydrogen atoms at its core will hold out much longer than more massive stars. The really big stellar guys can run through their supply of hydrogen in just a few billion years, while an average star has an active lifetime of well beyond ten billion years.
Eventually though, all stars, including our beloved sun, will run out of hydrogen atoms in their cores, and the death process kicks in. Stellar death will be the subject of next week’s Starwatch, and believe me, stars don’t die quietly. Some really go out with a bang.
Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO radio in Minneapolis and is author of the book, “Washington Starwatch,” available at bookstores and at his Web site www.lynchandthestars.com.
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