Pete's Stuff

Baby, You're a Star! Part I

The next time you go outside at night, and it's clear with no moon out, look up. Did you know that each of those little stars you see has a personality? And I'm not talking about that joke calling themselves the "International Star Registry". Most of them look the same at first, but as you look harder you can tell some differences. First off, and most obviously, some are brighter than others. Looking more closely, there's subtle differences in color - some look white, others are bluish, some are yellowish, and some are reddish.

Astronomers go to a much higher level of detail by taking detailed spectra of stars - and even amateurs can do this with a small telescope and inexpensive equipment - and from those spectra they can deduce a number of things about individual stars. Tongue in cheek, here's a few of the more interesting "characters" in the sky above you.

Eta Carinae

Anger management, anyone?

Perched in the southern constellation of "the Keel", Eta Carinae is arguably the most brash, and volatile, star in the sky. Although not visible from much of the northern hemisphere, it's well known for its violent outbursts of activity, spewing out huge quantities of hot gas and wreaking general havoc in its neighborhood. At one point in the 1840s, Eta Carinae threatened to become the brightest star in the sky, if it hadn't been for the nebula surrounding it (or see the hi-res image). Since then, it has settled down, and wasn't visible to the unaided eye throughout the first half of the 20th century. But Hubble images show the results of that last outburst as two spherical lobes surrounding the star on either side.

Ready to blow.
About ten years ago Eta Car suddenly doubled in brightness. It continued to brighten until it came back to naked-eye visibility three years ago - all signs that could point to another imminent explosion.

Astronomers are fairly well agreed that Eta Car is the most likely star to go supernova sometime in the next 100,000 years - and when (if) it does, it'll be one heck of a show. My best estimate based on its distance, possible obscuration, and current understanding of supernova processes, would estimate an apparent magnitude near -8 - far brighter than any planet, nearing the brightness of the moon, and easily casting a shadow while lighting up the night sky of the southern hemisphere.

R Cor Bor.

R Coronae Borealis

Your chain-smoking great aunt.

R Cor Bor is a star in the northern constellation of the Crown, usually visible to the unaided eye from a dark country road. But strangely, every few months it seems to disappear, fading to a level where it's only visible through a medium-sized telescope. Eventually it regains its original brightness, but threatens at any time to blink out, like a snuffed candle.

Despite the oddity of this behavior, astronomers have a pretty good idea what's going on. To quote Burnham [1975]:

The most recent observations suggest that the spectrum is basically that of a supergiant near F7, but peculiar in showing strong absorption bands caused by carbon in the star's atmosphere. According to spectroscopic analysis the composition of the star's atmosphere appears to be about 67% carbon and 33% hydrogen and other elements. These observations have suggested a theory that the light variations are due to the emission of clouds of carbon, the material condensing into a "soot cloud" around the star and effectively obscuring the light. As the cloud clears or is reabsorbed into the star, the light returns to normal.

Or to say it another way, this one is basically the old chain smoker of the celestial sphere: she's lived a long life, but is now nearing the end, occasionally coughing out a large cloud of sooty gunk which hides her face and takes weeks to dissipate.

Always on the move.


A wanderer from far away

Arcturus (Aρκτούρος), the "Guardian of the Bear", is the third brightest star in the sky and a welcome harbinger of spring. Since I was a child I've looked forward to the appearance of this bright star in the east before bedtime, marking the slow ascent (at least in the northern hemisphere) from the depths of winter.

It's also the brightest and most visible representative of the Population II stars, characteristic of the Milky Way's galactic halo. It's an ancient star, evolving only slowly; an orange supergiant estimated at more than 9 billion years old and metal-poor, and resides in a wide, highly inclined orbit around the galactic center, unlike the nearby orbits of the sun and its companion stars, which are confined to the galactic disk. It's quite possible that Arcturus, like other halo stars, is part of the captured remains of a dwarf galaxy shredded by the gravity of the Milky Way many billion years ago, even before the Earth existed.

Because of the high angle of its orbit, Arcturus is cutting a path nearly perpendicular to the plane of the galaxy. It's now almost near its closest approach of 40 light years from the Earth. We're just lucky, to see it now in this short span of prominence. Over the next million years it will move, at first rapidly, but slower with time, from its current position southeast towards the constellation of Virgo. After half that time it will recede so far as to be invisible to the unaided eye, and distant generations will have to find some other way to mark the inevitable changing of the seasons.

But Arcturus will still continue its wandering, swinging wide beneath, and then around to the far side of the galaxy for an incomprehensibly long time before it once again appears in our skies - if it ever does. Although it's possible to tell from the laws of Newtonian mechanics, I don't know if anyone has bothered to extrapolate that far in the future. It's entirely possible that we'll never meet again, and it will disappear forever from our night sky.

Baby, You're a Star! Part II will follow soon.