Sky Watch: The story of Epsilon Aurigae

Ever since I was a youngster, I've been fascinated by how crime scene investigators can reconstruct a crime with such amazing accuracy from relatively scant evidence. They don't have the luxury of watching the crime occur, and often don't even have an eyewitness. Instead they must work backwards from their own observations and measurements -- using such tools as physics, chemistry and biology -- to reconstruct logically what must have happened to produce the evidence as it now appears.

Astronomers are often faced with the same problem. They must determine complex physical mechanisms by observing and measuring only the tiniest flickers of starlight coming from many trillions of miles away. And it's really quite remarkable the stories that starlight carries to our eyes and telescopes.

One such remarkable story comes from a relatively obscure star in the constellation Auriga, the charioteer, low in the northeastern sky shortly after dark on autumn evenings.

Now I know that beginning stargazers are tempted to search the skies for an outline of a charioteer; that is, after all, the constellation's namesake. But I think you'll find it much easier to trace among its stars a pentagon that emanates from the bright star Capella.

Along Auriga's northwestern-most side, you'll see three relatively faint stars form a tiny triangle. The brightest of these three -- also that closest to Capella -- is known to astronomers as Epsilon Aurigae, or Almaaz.

Viewing this star with the eye or even a small telescope gives no hint as to how unusual it might be. The flickers of starlight tell us that the star is some 12 thousand trillion miles from Earth, so far that its light requires some two millennia to reach us, and suggest that its mass is some 15-20 times larger -- and some 300 times wider -- than the sun.

If this isn't amazing enough, astronomers have long known that, since its brightness dips every 27 years or so, it is being regularly eclipsed by another orbiting nearby. Many such eclipsing binaries exist around the sky, and their eclipses often take a few hours or days. This one, however, lasts two years. Whatever is eclipsing this star must be significantly larger than Almaaz itself -- perhaps a thick, donut-shaped ring of dust as large as the orbit of Saturn around the sun that passes in front of Almaaz from time to time.

During the most recent eclipse of Almaaz in 2009-2011, astronomers compiled more evidence from ground-based and space-borne telescopes and have come up with a couple of competing theories to explain what might be happening.

One is that either Almaaz is, indeed, a massive supergiant star, periodically eclipsed by two tightly bound stars inside a swirling, dusty disc. Another is that it might be a relatively large, but low-mass, dying giant star that is periodically eclipsed by a single star inside a disc, and that it will blast its atmosphere into space sometime in the next few thousand years.

Of course, no one knows for sure what's going on there, and astronomers will be debating the evidence for years to come.

Much like a complex crime scene, the investigation continues.