March 31, 2020

AstroEvent: Herschel 3945-A Clone of Albireo.

Probably the most famous star party doubles are Alcor & Mizar in the Big Dipper and Albireo in Cygnus. Both provide a good, wide separation, and in the case of Albireo, a good color contrast. Plus, double stars provide a good “punch through” of the light pollution haze for down-town astronomy, and may save the show when the Moon or planets aren’t above the horizon and even the best nebulae such as M42 evoke remarks like “what, you mean that fuzzy smudge?”

…but did you know there is a binary star known as the “Winter Albireo?” sometimes called 145 Canis Majoris or Herschel 3945, this colorful binary lies roughly midway between Sirius and the variable star UW Canis Majoris. First noted by William Herschel, I think it’s bordering on naked eye visibility at +5th magnitude has something to do with its obscurity. Still, it is worth the hunt; the pair is colorful like Albireo with a sapphire-orange contrast at about 27” arc seconds separation. And like the optical double Albireo, H3945 is a true binary system at 258 light years (component HIP 35213) and 6,523 light years distant (component HIP 35210). Be sure to add this hidden gem to your repertoire this star party season… you’ll be the hit of the show!

The coordinates for H3945 are;

R.A. 7h 17’ 36” Dec. -23° 18’ 55”

The Astro-term for this week is: Position angle. This is one of the visual elements that astronomers use to describe a binary star system; the other is angular separation. The two work like coordinates with the primary (the brighter of the two at the center and the position angle measuring the deviation from the north or south celestial pole. Keep in mind, the position angle and separation of a given pair is only from our vantage point; the actual orbit may be oriented anywhere from edge on to face on to our line of sight. The separation and position angle of a pair also can slowly change as the stars orbit one another. In the case of H3845, the current position angle (P.A.) is 55°, which will slowly change over the centuries as our respective motions about the center of our galaxy alters our terrestrial vantage point.


  1. [...] Struve 3053 in the constellation Cassiopeia. Another star that vies for the title of the “Winter Albireo”, this colorful double would probably also be a star party favorite if it were just a pinch above [...]

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