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Starwatch: Size matters in the death of a star

March 25, 2017 GMT

Welcome to part three of my trilogy on the birth, life and death of stars.

In last week’s column, the life of stars, I told you 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, such as our sun, sip their supply of hydrogen slowly and live a long time. Our 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 literally what you could call hydrogen gas-guzzlers. The big guys of the stellar world might 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 underway. Smaller stars such as our sun certainly die a violent death, but the really massive stars go out with a huge bang!

Death of smaller stars

Low-mass stars such as our sun get really fat before they die and flicker out. In the case of our sun, it will run out of hydrogen in about 5 billion years. In the sun’s core, helium builds up as the hydrogen dwindles. As nuclear fusion dies out, internal pressure decreases. The built-up helium in the core 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 to our sun, it will swallow up the planets Mercury and Venus, and will just about touch the Earth. At that point, needless to say, we’ll be toast! Even though the sun will have a cooler surface temperature of about 3,000 to 4,000 degrees, it will be right on top of us.

Also, toward the end of our 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 because back in the 1700s and 1800s, telescopes weren’t quite up to the standards of the Hubble Telescope, and through those archaic scopes stars resembled giant planets.

Planetary nebula don’t last, though, and after about a billion years or so of being a red giant, stars such as our sun totally run out of all fusion fuel and will shrink into white dwarfs. With no more nuclear reactions inside the star to hold it up, gravity collapses the corpse of the once-proud star.

In our sun’s case, it’s believed that whatever is left of the sun’s original mass will be squished into a ball about the size of Earth. When that happens, our sun will be considered a white dwarf, or a “retired star.”

Death of bigger stars

Remember the old saying — the bigger they are, the harder they fall? That’s certainly the case with behemoth stars.

Massive stars, at least eight times more massive than our sun, die a spectacular death. Just like smaller stars, 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 chain of reactions causes all nuclear you-know-what to break loose, and then you have an explosion beyond your wildest dreams.

The once-super giant red star explodes into a supernova. The supernova flings out material at beyond-incredible speeds of 10,000 miles per second. In the process, heavier material such as gold, silver and many other heavier elements are “cooked up” and become the building blocks of future stars and planets. Out of death comes new celestial life. This is recycling on a cosmic scale.

The Crab Nebula in the constellation Taurus the Bull, currently located in the low southwest evening sky, is the remnants of a supernova explosion that our distant ancestors saw back in 1054. Even with a small-to-moderate telescope, you can see the Crab Nebula near one of the horns in Taurus.

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 might 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 mighty 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. Nothing escapes!

Astronomers have never actually seen a black hole, but there are ways they can detect their presence. This is especially the case when a black hole is part of multiple star systems. Black holes literally can suck material off existing stars.

When that happens, X-rays are produced as stellar material spirals to its doom in the black hole. These X-rays are signals that can be detected by astronomers. One of the first suspected black holes that was detected by X-rays is found in the bright constellation Cygnus the Swan, a constellation we’ll see this summer. Don’t worry — it’s more than 46 thousand trillion miles away. You won’t get sucked in!