When stars die, they go out with a big bang

  • Fri Jun 22nd, 2012 3:44pm
  • Life

By Mike Lynch

This is part three of my trilogy on the birth, life and death of stars. In last week’s Starwatch, 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. The 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 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 the sun get really fat before they die and flicker out. The helium builds up in the sun’s core as the hydrogen dwindles. As nuclear fusion dies, internal pressure decreases. The built up helium 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, the sun 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, it will be right on top of us.

Also, toward the end of the 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. Astronomers call these planetary nebulae.

Even though they’re dubbed planetary nebulae they have nothing to do with planets. They got that name back in the 1700 and 1800s when those rings and shells through those archaic scopes resembled giant planets.

Planetary nebulae don’t last though, and after about a billion or two years of being a red giant, stars like the sun totally run out of all fusion fuel and will shrink into white dwarfs. It’s believed that the sun’s original mass will be squished into a ball about the size of Earth and it will become a “retired star.”

Massive stars, at least eight times more massive than our sun, die a spectacular death. 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 nuclear chain of reactions causes a massive explosion.

The once super giant red star explodes into a supernova and flings out material at speeds beyond 10,000 miles a second. The heavier material like gold, silver and other heavier elements are “cooked” and become the building blocks of future stars and planets.

Out of death comes new celestial life.

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 more than a billion Earth tons.

Or 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.

Astronomers have never actually seen a black hole, but they can detect their presence. Black holes can literally suck material off existing stars. When that happens, X-rays are produced and these signals can be detected by astronomers.

One of the first suspected black holes is found in the bright constellation Cygnus the Swan, shining brightly in our early summer eastern evening sky.

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, www.lynchandthestars.com.

The Everett Astronomical Society: www.everettastro.org/.