Perseus Double Cluster

Last week in Starwatch, I described how stars are born in gigantic families in part one of the trilogy of a star’s birth, life and death. Every phase is controlled mainly by gravity. All stars in the Butler heavens are essentially big balls of hydrogen gas that gravitationally coalesce out of loose clouds of hydrogen referred to as nebulae. As these relatively tiny protostars gravitationally pull in more loose gas from their birth nebulae, they become larger and more massive, which gives them a greater gravitational pull to draw in even more hydrogen. If this gravitational snowball effect continues long enough, a protostar becomes massive enough for the central core's nuclear fusion furnace to ignite, producing incredible amounts of light and radiation. Once that happens, a protostar becomes a bona fide star and lives out its celestial life.

What nuclear fusion is and how it operates inside a star’s core is the subject of this column. It’s a tremendously complicated process. My explanation glosses over many messy and complicated details. So, here it goes.

As with a star’s formation, gravity plays a key role. Gravitational compression builds up the pressure and temperature to incredible levels inside a star. In the case of our sun, an average-sized star in our Milky Way, it has a mass of 2 with 30 zeros after it in kilograms, with one kilogram equaling just over 2.2 pounds. That kind of mass results in a tremendous gravitational force that pushes in on the sun's core to the tune of over 500 billion pounds per square inch. With that kind of pressure, the temperature at the core exceeds 27 million degrees. There’s even more gravitational pressure and higher temperatures in more massive stars.

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 astonishing violence. By their very nature, hydrogen atoms tend to be electrically repulsive to each other. They don’t even like to be close! But because the force of these collisions completely overwhelms their atomic independence, they not only bash into each other, but also fuse to form heavier helium atoms. That’s nuclear fusion in a nutshell. It’s like slamming your hands together with such force that they merge or fuse into one big hand.

Breaking it down a little more, it takes slightly less than four hydrogen atoms to form one helium atom. The tiny extra amount of hydrogen is converted into a tremendous amount of energy. That’s the tricky and highly complicated part of nuclear fusion that has to do with Albert Einstein’s famous assertion that energy and mass can be interchanged if you have the right conditions. Those conditions are met in the violent cores of stars. The collective radiation from 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 that outer layer, the radiation travels away in all directions at the speed of light, over 186,000 miles per second. A balance is struck between gravitational energy pushing in a star and nuclear fusion energy pushing out. The star is said to be in hydrostatic equilibrium as it shines on for in most cases, billions of years! It joins the family of main-sequence stars.

Another way to look at is that hydrogen is a star's fuel, 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. Despite that incredible consumption rate, our sun won’t run out of hydrogen in its core for another five billion years! Since our sun is an average star in terms of its size and mass, the supply of hydrogen atoms in its core will last much longer than in more massive stars. Humongously massive stars can exhaust their hydrogen supply in just a few billion years, while a smaller star can live for well over 10 billion years.

Eventually, all stars, including our beloved sun, will run out of hydrogen in their cores, and the death process will begin. Stellar death is the subject of next week’s Starwatch, and believe me when I tell you that stars don’t die quietly, and some really go out with a bang!

Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul. He is also the author of “Stars: a Month by Month Tour of the Constellations,” published by Adventure Publications and available at bookstores and at adventurepublications.net. Contact him at mikewlynch@comcast.net.

A star