May double star of the month

May double star of the month – mu Crucis (μ Cru)

by Dave Blane

Mu Crucis (μ Cru) (12 54 35.66 -57 10 40.4) is simply one of the most beautiful doubles in the sky and is very suitable for viewing with binoculars or a small telescope.

The pair, whose components are of visual magnitude 3.9 and 5.0, share common proper motions and distances and Hipparcos places both stars at about 412 light years from Earth. If this is so, then the two stars are separated by some 0.0192pc or 0.063ly suggesting the period maybe as long 30 000 years. The second star, μ2 Crucis, is a Gamma Cassiopeia-type (GCAS) variable, changing by 0.19 mag in an unknown period.

This double was discovered by Dunlop in 1826 and listed as Δ126 in his catalogue . The spectral types are both B and Hartung record them as both white whereas Richard Jaworski sees a tinge of yellow in the fainter star. The current separation and position angle are 35″ and 17deg and the brightness of the stars almost make this a southern equivalent of Albireo but without the colours.

Mu Crucis

Image showing mu Cru and other double stars in this area.

The bright components and wide separation of 35” make this system easily to split with modest binoculars and in a small telescope they are a fine sight. So when you are next outside with your binoculars or your telescope, and finished marvelling at the Jewel Box, have a look at this fine double star that is just 4 degrees north of the Jewel Box in the constellation of Crux.


Mu Cru

A beautiful blue pair.