That’s not supposed to be there

So, a few months back I posted an image of the night sky (duplicated at left) that featured a portion of the constellation Scorpius, and made a remark about two sets of twin stars in the image. I’m only an astronomy buff, and never really bothered memorizing the constellations because I consider them nonsense – the things that they are supposed to represent are ridiculous stretches of the imagination, and I have never made any connection to the figures. Anyway, this is only the excuse for something that I remarked on and failed to notice, which was that Scorpius had an extra star. By the way, Scorpius is the constellation, Scorpio is the astrology sign and Simpson’s character.

Scorpius can be seen at lower right, a string of bright stars curling around in a U-shape just above the horizon haze. The twin stars at the stinger end, upper left of the U, are correct, but the twin stars opposite, the brightest stars in the lower right corner of the image, are not. The uppermost star in that spot doesn’t belong. Well, okay, it does, but in a funny way. It normally isn’t that bright.

Lemme ‘splain. The twin stars at the stinger end are λ (lambda) and υ (upsilon) Scorpii, also known as Shaula and Lesath respectively, and they belong there (don’t get the impression I spit these out casually – I had to look it all up.) Then, there’s two sets of twin stars in Scorpius that only appear as one star to the naked eye and in photos like this. One twin is ζ (zeta) Scorpii 1 and 2, the lower right brightest star. Then there’s the bright star straight above it a short ways, exactly to the right of the stinger stars and forming the base of the tail, and that’s μ (mu) Scorpii 1 and 2. In the image below, which is a full resolution section of the full frame image at top, you can actually see the evidence of these twin stars, with a slight elongation to them because the exposure time is 24 seconds and the earth was turning. I apologize for the cruddy quality, but I was using a high ISO to capture this with as minimal movement as possible, and that means the image quality goes to shit.

The red pointer bars indicate the sneaky little interloper, the extra star that doesn’t belong. The exposure was too long for this to be an airplane or meteor, and it appears in several exposures. In that location, however, is a trio of stars, and the middle one is known as HP 82691, normally a rather dim star that would remain very low key among the brighter constellation stars. But HP 82691 is a Variable, a star that is not fixed in brightness. What I now believe I captured was HP 82691 at a much higher magnitude than average, enough to almost rival the bright pair ζ Scorpii 1 and 2.

How much does HP 82691 vary? I have no idea. Finding information on this star has been difficult, and the best I’ve come up with is “at least 0.2 magnitude” variation – from what I can estimate here, it seems much more than that over the base magnitude of 6.26, perhaps one or two magnitudes brighter. So any astronomers out there who want to chime in with some hard information, including the fact that I’m totally mistaken, feel free. I just stumbled across this anomaly while trying to find some other info about what I captured that night.

As a bit of trivia, there’s a type of variable star called a Cepheid, which has a specific relation between its period of variability and its intrinsic brightness – that is, how bright it actually is, versus how bright it appears to us here on Earth, however far away we happen to be. Henrietta Leavitt pinned down this relation in 1908 (yes, a female astronomer with a major contribution over a hundred years ago,) later refined by Ejnar Hertzsprung. Edwin Hubble, the telescope’s namesake, used this relationship to recognize that a Cepheid star in M31, now otherwise known as the Andromeda Galaxy, was a whole lot further away than previously believed, since he knew how bright it should have been, but didn’t appear to be because of the distance. This little discovery changed the concept of the size of the entire universe from just a bit bigger than our galaxy to, well, unbelievably huge. Our galaxy is simply one of millions.

While we’re at it, clicking on the image at top (or right here) will bring up a much larger version. Almost centered in the frame is the center of our galaxy, pretty much immediately to the right of the big blob of bright Milky Way (I was off a bit in the previous post.) Sitting a bit to the left and above the tail of Scorpius is open star cluster M7, and well above that and just a wee bit to the left, flanking the galactic center, sits a vague brighter blob known as M8, or the Lagoon Nebula. That I captured both of these in this relatively brief exposure is testimony to the clarity of the night, but if you want a better overall view, leave it to the experts. One of these days I’ll get serious about a tracking motor setup and get some better starfield photos on my own, though. A tracking motor turns the camera (or telescope) in the opposite direction of the earth’s rotation, allowing the stars to remain fixed within the frame and thus bringing up more faint details without streaking. Seems simple, but the tracker has to be precisely aligned with the celestial north pole, which isn’t quite Polaris, the north star. I have a decent telescope complete with tracking motor, an eight-inch reflector, but it needs collimating and it’s a bear to haul around and set up. The area I live in has too many trees and is a bit light-polluted, which can be seen in the images, so my astronomy pursuits are haphazard at best.

Image details: Canon 300D (Rebel) with Sigma 24-135 lens at 24mm. 24 second exposure at f2.8, ISO 800. July 3, 2010 at 23:40 EDT. The original image has been color-tweaked and contrast-boosted for web use.

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