Leaving behind beautiful supernova

By Tom Burns - Stargazing

Go out after dark and take a look at a giant. His name is Orion. To the ancient Greeks and Romans, he represented a mighty hunter.

He consists of a large, upright rectangle of stars with the three stars of his belt cutting across his midsection.

The bottom-right star of the rectangle is a blue-white beauty called Rigel. Its name derives from the second part of an Arabic phrase (ar-rijl al-jabbār), which means “the giant’s left foot.”

Rigel and the sun have at least one thing in common. They spend most of their lives fusing hydrogen into helium deep in their dense, hot cores. However, Rigel has reached a premature old age. It will soon explode in a massive conflagration astronomers call a type II supernova.

Like all stars, Rigel started its life as part of a diffuse cloud of mostly hydrogen gas. Part of the cloud collapsed into a loose globe of hydrogen gas and dust.

Gravity brought this “starstuff” closer together, with a subsequent increase in heat. When it reached 10 million degrees or so deep in its inner core, it began to convert its hydrogen into helium in a hydrogen-bomb reaction that makes our attempts at such weapons look pathetic by comparison.

That hydrogen-bomb reaction began inside Rigel “only” eight million years ago. If that sounds like a long time to you, consider your star, the sun. It formed similarly five billion years ago and has another five or six billion years to go before it runs out of hydrogen fuel and dies in a far gentler way than Rigel.

During the evolutionary course of a star the size of our sun, the star will fuse all the hydrogen in its dense, hot core into helium. It will then begin to cool down and expand to a red giant so large that, in the case of our sun, it will swell past the orbit of Earth.

Eventually, its thermonuclear reaction will be exhausted completely. It will then collapse into a very dense ball of non-exploding leftover material called a white dwarf. The sun’s white dwarf will probably be about the size of Mars at about 4,000 miles wide.

Rigel started out much fiercer and hotter than our sun because of its enormous mass. Currently, it has about 21 times the “starstuff” of our sun and has a surface temperature of 19,300 degrees Fahrenheit, over 10,000 degrees hotter than the sun.

The result is premature decrepitude. After only eight million years, Rigel has reached its death throes.

Its core is now almost entirely made of helium, which is fusing into carbon and other heavy elements. Just outside its core is a shell of hydrogen gas so hot and dense that it is fusing into helium.

The explosive force of the dual fusions causes Rigel to lose its starstuff at a rate that simply boggles the mind. Our sun’s explosive power causes it to lose mass at about four million tons per second. Rigel is losing its starstuff at a rate estimated at 12 million times that of the sun.

Rigel will eventually go supernova, but astronomers aren’t sure about what happens between its current sorry state and its eventual massive explosion. However, it will probably evolve into a red supergiant before reaching its final cataclysm.

You can see that evolutionary state simply by glancing up at the upper left star of the Orion rectangle. There you will find a red-orange star called Betelgeuse, the “Armpit of the Giant.” Most folks pronounce its name as “beetle juice” (Yes, that’s where the old movie got its name.)

To be fair, the star’s association with an armpit is somewhat suspect. It dates back to a magnificent 1899 study of star designations called “Star Names and Their Meanings” by American naturalist Richard Allen. He believed that the modern name is a corruption of a longer Arabic phrase that means “armpit of the giant one.”

The argument makes good sense. While western astronomy languished during the medieval period, the Arabs did fine astronomical work. They gave names to many naked-eye stars that western astronomers had never named.

However, Paul Kunitzsch, professor of Arabic Studies at the University of Munich, took a look at the original Arabic phrase and concluded that it referred to Orion’s hand and not his underarm.

But Allen’s appellation is the one that stuck — and with good reason. The star does indeed seem to mark Orion’s right shoulder.

Betelgeuse is a stellar type called a red supergiant. Our home star, the sun, is about 900,000 miles in diameter, below average size as stars go. At 640 million miles in diameter, Betelgeuse is so large that it would swallow up our Earth and extend out to the orbit of the next planet, Mars.

Betelgeuse’s motto is “live fast, die young, and leave a beautiful supernova.” It is only 10 million years old, yet it has already reached the end of its life cycle.

How did Betelgeuse reach its sorry state?

Like all stars, it started its life as a loose globe of hydrogen gas and dust and went through the normal evolution of a star — except that it consumed its hydrogen 10,000 times faster than our more conservative sun.

Because of its large mass and the resultant central heat and density in its core, it quickly used up its core hydrogen and became a massive blue-white giant, like Rigel.

As we saw with Rigel, great size and brightness translate into a short life span. After only two million years or so, its center was composed almost entirely of helium. Over the next 10,000 years, it contracted even more and released incredible amounts of energy.

The outer layers of gas surrounding the core became much larger, causing Betelgeuse to turn red, probably from embarrassment. The helium at its core fused into carbon, and heavier elements like calcium and iron formed.

But that’s not the worst of it. Orion’s underarm will annihilate itself in less than one million years.

Fairly soon, Betelgeuse’s center will be composed almost entirely of compressed iron, causing it to contract further. Its gravitational force will become so great that its outer envelope of gases will be forced down upon it. The result is a massive supernova explosion.

When we see such supernovae in other galaxies besides our Milky Way, the outburst is so great that a single star can briefly outshine an entire galaxy of 300 billion stars.

Because Betelgeuse is so close to us (640 light-years, or a mere 3,600 trillion miles), such an explosion may, for weeks or days, be visible during the daytime. At night, it will shine as a single point with the collective brightness of the full moon.

Partly because Rigel is farther away than Betelgeuse, Rigel’s supernova will be far less brilliant but still might be visible during the day. Its supernova will shine for a few days with the brightness of a quarter moon.

Afterward, Betelgeuse and Rigel will disappear from the night sky, leaving Orion without two crucial parts of his anatomy. If our species survives a thousand millennia, future generations may still look at the night sky that humans have looked at since the inception of our species. But they will see a different sky.

In fact, Rigel and Betelgeuse may already be gone. Red supergiants might pop at any time. Betelgeuse is relatively close to us at about 640 light-years away. That means the light from the star you see now has been in transit for 640 years.

Rigel is somewhat farther away and not as far advanced in its evolution. It will almost surely take longer to reach its inevitable supernova. Moreover, Rigel is somewhere around 1,000 light-years away. A millennium will pass before the announcement of its demise gets to your eyeballs.

If Betelgeuse or Rigel exploded any time in their respective interims, they are now gone, but we won’t find out until the information in the form of light reaches us.

Then again, probably not. Humans occupy a thin slice of cosmic history. The chance that anything exciting will happen in our cosmic neighborhood during our lifetimes is, if you will excuse the expression, astronomically small.

Still, every clear night, I glance up at Orion. You never know.


By Tom Burns


Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.