Baby, You're a Star! Part II

Posted Wed, March 25, 2009 - 10:57 PM
astronomy, science

To continue my previous post, these are a few more interesting celestial "characters".

Sucking the life out of its
companion.

Z Andromedae

A vampire star.

Z Andromedae is a faint star near the square of Pegasus, well below naked-eye visibility at magnitude 11. But every once in a while it decides to blaze out to at least 50 times its original brightness. Astronomers have a good understanding of the reason behind the flareups, and have euphemistically termed Z And a "symbiotic star". However, that's too charitable of a name; one of them is getting a lot more out of the bargain than the other.

Z And is really a binary star system, consisting of a huge, vaporous, cool, red giant star, maybe 50 times the size of the sun. Whirling around it in a close embrace is a tiny, hot (perhaps 50,000°K), blue-white dwarf star that exerts a tremendous attraction on the outer layers of its old, bloated companion. The red star is so large that it's expanded beyond its Roche lobe, so a part of its atmosphere is captured by the smaller star, and spirals into a surrounding disk. Eventually, the stolen gas settles directly onto the blazingly hot surface of the dwarf star and starts to grow into a ever thicker layer. Heated from below and compressed from above, it compacts under its own weight until it eventually reaches a density high enough to cause a runaway thermonuclear explosion.

The hydrogen-rich layer rapidly burns away to helium and the tremendous release of energy blows the byproducts into space at huge speeds, powering the outbursts we can see light-years away and forming a nebula visible through large telescopes. However powerful it seems, though, the outburst only occurs on the surface of the dwarf star, and isn't large enough to disrupt the overall integrity of the star system. Instead, the two of them just fade back into a temporarily quiet state, and the "vampire star" starts over in its next cycle of feeding and eventual release.

Totally ripped.

S Doradus

Big man on campus.

Like Eta Carinae, this is another superlative star. It's massive - more than 100 times heavier than the sun. It's intensely brilliant - possibly even more so than Eta Car, a million times brighter than the sun. It lives in the middle of a brilliant cluster of similar stars, strutting its stuff more than 180,000 light years away in the Large Magellanic Cloud, and in the absence of any concrete evidence for multiplicity, is believed to be the most luminous single star currently known in the entire universe.

But, like a hard core bod on steroids, it can only sustain itself for a short amount of time before collapsing. S Doradus only has a blink of an eye to live, in stellar terms, perhaps as little as two million years. Paradoxically, as it spirals towards its demise, it may get even hotter and brighter as it fuses helium at an increasing rate, then more and more carbon as it attempts to stave off the inevitable. Every increase in intensity is a just a game of diminishing returns - carbon fusion can only last a few million years, neon fusion a few thousand years; oxygen fusion a few years; and silicon fusion a few days; until only inert and unfusable iron is left, and nothing remains to hold the star up from inside. The core starts to collapse under its own tremendous weight.

Don't 'roid up, kids.

But the outer layers of the star are still playing catch-up with the prior stages of fusion, forming a succession of layers like an onion - all of which are unaware of the catastrophe taking place in the core in literally a split second. The core, a ball of iron a thousand times larger than the Earth, starts to collapse at an exponential rate, forcing the atoms into an exotic state characterized by the awesomely sci-fi term "neutron drip". At a certain point, the pressure becomes so intense that the atoms in the core - literally, and counterintuitively - fall apart, and in doing so create one last blast of resistance to the collapse. The result is a lot like dropping a TV off a building, or going over a giant pothole at highway speeds. The sudden shock sends everything flying every which way. A shock wave expands outward from the core and rips apart the oblivious outer layers of the star, stripping them away at speeds of 5 million miles per hour and forming fantastic remnants like the Crab Nebula - the final result of a profligate, self-destructive lifestyle.

Young and on the run.

AE Aurigae

The "Runaway Star".

Unlike the other personalities I've mentioned so far, this one is bright, young, and a little traumatized. It's an example of an "Orion-type" star, of stellar class O or B and probably 5,000 times brighter than our sun. It's a good name to use for classification, since vast quantities of stars just like this one are being churned out in the great star-forming clouds of Orion, which are only the closest representatives of a structure generally quite common in spiral galaxies the universe over. But this star is all by itself, dozens of light years away from Orion. Where did it come from?

The first clues came from a turn-of-the-20th-century survey of the motions of bright stars. Astronomers had discovered, by looking ever more closely at the sky, that the "fixed" stars moved, although so slowly as to be imperceptible, even over a human lifetime. But once that fact was known, they wanted to catalog the motions; and were able to do so by a combination of spectroscopy and proper motion surveys.

They found that AE Aurigae is flying through space ten times faster than the average star, and by looking back along its direction of motion, it seems to have been right near the star-forming region of Orion about two million years ago. That explains a lot, and computer simulations (of multi-body interactions) provide the rest of the story. Here's what we think happened.

About two million years ago, AE Aurigae was born in the region of Orion's sword and happily floated around in the neighborhood, until one day it came a little too close to a couple of bigger and heavier stars locked in a gravitational tug-of-war. Not unlike those of us back here on Earth, the biggest loser in such a situation is usually the little guy. The poor lightweight was caught up in the struggle and rudely swung around, hither and yon, until it was violently kicked out of the system. And for the whole time since, it's kept on running at more than 180,000 miles per hour away from the scene. Since then, we've have found two other stars "running away" from the same area: 51 Arietis and Mu Columbae; both probably victims of the same type of encounter.