Corona Borealis and Serpens Caput

by Shaphan Shank | Jun 1, 2024 | 0 comments

A little semicircle of stars known as Corona Borealis sits high overhead in the early summer sky. Corona Borealis, or the Northern Crown, lies just west of the midpoint between Arcturus and Vega, the two brightest stars in the northern sky right now. Corona Borealis doesn’t hold much in the way of deep sky objects, but it does contain several interesting double and variable stars.

Nu Coronae Borealis (abbreviated Nu CrB) is a wide optical double star in the eastern part of Corona Borealis. Optical doubles are those that aren’t physically connected by gravity, but just appear next to each other in the sky. The component stars of Nu CrB are both orange and are similar in brightness. Nu CrB is wide enough that it can be split without any optical aid. However, a binocular will give the best view, increasing the stars’ colors and enhancing the surrounding star field. To find Nu CrB, imagine a line from Alphecca, the brightest star in Corona Borealis, to Iota CrB. Take this line the same distance again past Iota CrB to find Nu CrB.

Our next target star, R CrB, lies within the semicircle of Corona Borealis, halfway between Alphecca and Iota CrB. R CrB is a variable star that spends most of its time at about magnitude 6, which is near the limit of visibility with the unaided eye. At irregular intervals averaging several years, this star suddenly drops in brightness, falling to less than 0.1% of its original brightness. Over the next few months or years, R CrB slowly returns to its normal brightness. Recent observations have shown that the star’s brightness drops are caused by clouds of carbon which the star expels. These clouds block the star’s light for a while, then slowly disperse.

The constellation Serpens Caput lies straight south of Corona Borealis. Serpens Caput is really just half of a larger constellation called Serpens, the Snake. Serpens Caput represents the snake’s head, while Serpens Cauda represents the snake’s tail. The two halves are separated by the constellation Ophiuchus, or the Serpent Bearer.

Serpens Caput holds several double stars, the best of which is Delta Serpentis. This star sits 5° northwest of Unukalhai, the brightest star in Serpens Caput. At 3.9 arcseconds, this pair of white stars is pretty close, so moderate magnification will be needed to split it.

M5 globular star cluster
M5. Photo © Adam Block|Mt Lemmon Skycenter University of AZ.

The finest deep sky object in Serpens Caput is M5, one of the better globular star clusters in the northern sky. M5 is not particularly close to any bright stars, but finding it isn’t too hard. Start by creating an imaginary equilateral triangle with Unukalhai at the northeastern corner, Mu Serpentis at the southeastern corner, and no star marking the western corner. M5 is just north of this imaginary western corner. Binoculars show M5 as a small fuzzy ball. Locating the cluster first with binoculars will help you find it with a telescope. M5 is fairly easy to resolve with a telescope at moderate magnifications.

star map of Corona Borealis and Serpens Caput

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