ζ1 Scorpii

Zeta1 Scorpii is the base of the scorpion's tail. It's a little over three fields southeast of Antares: binoculars.

Despite sharing its name with the close-by zeta2, the two stars do not form a true binary. Nevertheless they form a striking pair, blue and orange.

Zeta1 is on the southern edge of a wonderful star cluster, NGC 6231, a naked eye star cluster of over a hundred stars, well seen in binoculars. Zeta1 is the brightest member of this cluster; zeta2 is not a member.

NGC 6231 is an outstanding open cluster that Messier somehow overlooked. With an apparent magnitude of 2.6, this glittering collection of white stars is often called The Northern Jewel Box. The cluster is 3.2 million years old, which makes it a mere infant by astronomical standards.

On the northern edge of your field of view are mu1 and mu2–two blue subgiants–magnitudes 3.0 and 3.6 which form a naked-eye binary.
      For a long time it was believed the two were gravitationally bound to each other, although at a distance of 0.88 of a light year. It has recently been argued, however, that the distance involved is just too far to be gravitationally significant; it is also argued that the two are moving in slightly different directions. Although it must be pointed out that these directions, 202º and 208º, are closer than those of other stars which are said to be gravitationally bound, i.e. Struve 1998 and Struve 1999.
      The matter hasn't been resolved to everyone's satisfaction.


A naked-eye examination of the skies between Antares and zeta Scorpii will reveal a bright star roughly in the centre of the arch made by these stars. This central point is epsilon Scorpii.

Epsilon marks the base of the tail of the scorpion. Another star, far to the northeast in the same field of view, is far more interesting. This is RR Scorpii, a Mira-type variable which can attain naked-eye visibility (5.9); at its minimum it's close to 12 magnitude. Its deep red colouring makes it easy to find, however; and there is a finders’ chart in Burnham, p.1694, for those too impatient to wait for its maxima, every 281 days.

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