Theta Serpentis: A twin star (that's actually a triplet)

Aug 18

Written by:
8/18/2014 2:40 PM  RssIcon

Theta Serpentis is somewhat off the beaten path; over 7 degrees from 3.4 magnitude Delta Aquilae and more than 15 degrees from Altair and Cebalrai, it isn't exactly in a well-traveled area of the Milky Way.  But it does reside along the plane of our galaxy from our perspective, and that makes both the journey to it - plus some other objects nearby - a worthwhile one to make.

Start at Altair, dropping south to Detla Aquilae, and look for Cebalrai in Ophiuchus, a star that has it's own interesting deep sky objects nearby. From there, it's more than a full binoculars or finderscope field of view to reach the star, so using some other stars close by to get our bearings is helpful.

Delta Aquilae to Theta Serpentis star hop graphic

To the east of Delta lies a grouping of six dimmer stars - ranging from 5.8 to 7.3 magnitude. The middle two stars make a helpful "pointer" with Delta then becoming the directional star. It helps that our target star, Theta Serpentis, is a mere one degree north of Delta Aquilae. So for those with accurately aligned equatorial mounts, center Delta, go one degree north, and 7 degrees west. This should place Theta in a wide telescopic field of view.

At a comfortable 22 arc seconds of separation, these stars don't require high magnification or steady seeing to split. They will look quite similar which is unsurprising given that they are each A-spectral class stars, and the data about them bears this out largely as well. Theta Serpentis 1, 2 and C in an eyepieceThough slightly off in magnitude - Theta 1 is 4.6 magnitude while Theta 1 is 5.0 - they are nearly alike in ever other respect. Both are nearly double a solar mass and double the solar radius. While one shines at 18 times solar luminosity (Theta 1) and the other at 13 (Theta 2), their rotational speeds are significantly higher than our Sun. In this respect, the roles are reversed; Theta 2 spins faster, making a complete rotation in just 12 hours, while Theta 1 is slower, taking 17 hours.

These stars take 14,000 years to revolve around each other, and orbit at a distance that is 30 times the Sun / Neptune distance, or 900 AU. But they also have another companion; Theta Serpentis C, a 6.7 magnitude G-class yellow star that sits 7 arc minutes from Theta 2. Helpfully making it easy to find, it will be the closest, next-brightest star in the field of view of Theta 1 and 2. These stars reside 132 light years from Earth, so the light from them has been travelling longer than any human alive today. Why not make the retina of your eye the final destination for a few photons from these stars?

To see even more in this region of the sky, check out the video below that hops over to the lovely open clusters IC 4756 and NGC 6633 from Theta Serpentis. Or, read about the open cluster IC 4756 and how to star hop there.

There's a lot more here on Eyes on the Sky. For example, every week on the homepage there is a new astronomy video about observing objects in the night sky. They're only 5 minutes long - why not check out the latest one right now?


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The Nightlight

This blog includes what to see in the night and daytime skies, thoughts on telescopes, binoculars, and other astronomy observing accessories and equipment, plus my own occasional notes on objects I've seen and observed. Oh, and the random theater or other "my take on life" post. In other words, there is always something interesting. Check it out.