A study in stellar contrasts


Tom Burns - Stargazing



As August turns mercifully into September, the mighty Eagle Aquila soars high in the south just after dark.

Aquila’s two most famous stars are so different from each other that it’s hard to believe they are both called stars.

The first stellar attraction is easy to find. Altair is one of the brightest stars in the sky, but appearances can be deceiving. At only 1 1/2 times as big as the sun and a mere 11 times more luminous than our daystar, it is distinctly average.

Altair’s proximity leads to its brilliance. It’s only 16 light years (96 trillion miles) away, spittin’ distance in the Milky Way galaxy.

Like our sun, Altair will burn its hydrogen fuel for a few billion years before it dies. The product of that violent thermonuclear reaction is the element helium, which sinks to the center of the star, forming a helium core.

That helium core leads inexorably to the demise of all “normal” stars like Altair and our sun. Much of the hydrogen outside the core simply goes to waste. It is shed into space as the core eventually collapses and the star dies.

“A few billion years” sounds like a long time, but it’s a cosmic flash in the pan compared to the longevity of Aquila’s second stellar celebrity. Van Biesbroeck’s Star may last a trillion years, long after the sun and every other star you can see have faded away.

Van B’s Star is the Perry Como of the heavens. It lives a long time because it’s not very large or energetic.

If you’re too young to get the reference to the “Sultan of Snooze,” gentle readers, check out one of the many recordings of his music on the Internet. His rendition of “Don’t Let the Stars Get in Your Eyes” at https://www.youtube.com/watch?v=Dmkg_E2evbg is enough to turn you off to stargazing forever.

Or if you’d like to know how I really feel about him, take a look at the “Anvilania” episode of the Animaniacs https://www.youtube.com/watch?v=ZgqcmzMTtY0. Remember the singer who puts everyone to sleep crooning the Anvilania national anthem? That’s the guy.

Where was I? Oh, yes. Van B’s Star is a red dwarf. Tiny by stellar standards, the star is probably about the size of Earth, or about 1/100 the diameter of the sun. This flea on the back of the Eagle is only 1/500,000 as bright as the sun and 1/5,000,000 as bright as Altair. Even though Van B’s Star is about the same distance as Altair, you’ll need one of the largest telescopes on Earth to glimpse it.

The star’s small size and low gravity mean that it never gets very hot or dense at its core. It probably produces energy in the same way that Altair does – a hydrogen bomb reaction that results in killer helium “ash, which will eventually snuff out the thermonuclear reaction at the center of “normal” Stars. Red dwarfs like Van B’s star are too small for the heavier helium to fall to the center and eventually snuff out the thermonuclear reaction.

As it bubbles and heaves, a red dwarf spreads its helium poison throughout its system. The deadly helium core doesn’t get a chance to form.

Consequently, red dwarfs use their hydrogen very efficiently. They chug along in their Perry-Como-tose way for hundreds of billions, even trillions, of years.

Consider that number. There are red dwarf stars as old as the universe. They could conceivably live until the universe slowly fades away.

Or maybe not. Some astronomers speculate that red dwarfs are a bit too small to hold together. Like all stars, their hydrogen-bomb reaction produces a violent “stellar wind” that pushes some of their hydrogen fuel into space. Red dwarfs may evaporate away in as little as a few billion years.

Either way, lethargy is the key to understanding red dwarfs. They either lack the gravitas to hold themselves together, or they live forever because they putter around in their galactic garages all day.

And yet they are perhaps the most abundant stars in the universe. Our Milky Way Galaxy is composed of 300 or so billion stars. About 3/4’s of them are probably red dwarfs like Van Biesbroeck’s Star. In fact, the closest star to our sun and Earth is a red dwarf. Proxima in the constellation Centaurus is a scant 4.2 light years away, which translates to about 25 trillion miles.

Recently, astronomers discovered a planet orbiting Proxima, and it’s about the same size as planet Earth. But before we get our hopes up about life on that planet, we need to find out more about red-dwarf stars and, come to think of it, about life. More on that next week.

Tom Burns

Stargazing