Multiple Star Acrux

Acrux double star system in the Crux constellation
Designation:                    Multiple star Alpha Crucis (or Acrux).
Constellation:Crux (or Southern Cross).
Magnitude: Alpha 1 Crucis:+1.3
Magnitude:  Alpha 2 Crucis:+1.6
Apparent separation: 4.2 arc-seconds.
Distance: 322 light years.
Date: 2020-06-19
Exposure: 24 seconds.
Field of View:1.56° x 1.03° – up is 260 degrees E of N.

The brightest star in the smallest of all constellations, Acrux forms the southernmost base of the Southern Cross and points towards the general direction of the South Celestial Pole.

It appears in the telescope as a double star, with Alpha 1 Crucis and Alpha 2 Crucis easily distinguishable in the above image – but is actually a multiple star system.

Image: The constellation of Crux.
Source: Wikimedia Commons (unattributed).

Wikipedia attributes six stars to the system but “Annals of the Deep Sky (Vol 7)” by Jeff Kanipe indicates there are at least twelve: two 1st magnitude, one 4th, one 5th and eight 10th – 17th magnitude.

Identifying the components in this complex system is not simple. Astrometry.net doesn’t make any attempt at it. Jeff Kanipe’s book includes a diagram (which I cannot reproduce) but it’s not easy to relate it to the stars in the image. Suffice to say they are not necessarily those closest to the two brightest ones.

I was surprised to see that the two main components are both first magnitude stars (+1.3 and +1.6), they don’t seem as equal as that! In fact Alpha 1 is 1.3 times the brightness of Alpha 2.

Telescope Details
SkyWatcher Esprit 120 mm apochromatic 3-element refractor.
840 mm focal length @ f/7 with field flattener.
Baader L-Booster UHC-S light pollution filter 2458276.
SkyWatcher EQ6-R Pro mount.
Camera: ZWO ASI 071 MC Pro (CMOS 28.4mm 16 Mpx)
ZWO ASI120 guide camera, using PHD2 software.

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