The Butterfly Cluster

An open cluster (also known as Messier 6) in Scorpius which is estimated to be  just 50-100 million years old.

The bright star is BM Scorpii, a variable K-type orange supergiant (mag 5.5 – 7). Most of the other members of the cluster are B-type stars.

Object Details:

M6, NGC 6405
Visual magnitude: +4.2
Apparent diameter:  20 arc-min. (about 0.65 LD).
Actual diameter: 9.2 light years.
Distance: 1600 light years.


Exposure: 24 x 1 min, ISO 400.
Date: 2017-10-12.
Location: Leumeah, NSW.
Sky: suburban light pollution with considerable high haze.
Processing: Canon DPP, Deep Sky Stacker and GIMP.
Cropping: no.

Imaging telescope: Skywatcher Esprit 120ED Super APO triplet refractor.
Focal length: 840 mm, focal ratio: f/7.
Imaging camera: Canon EOS 60D.
Guiding telescope: Orion ShortTube 80mm Achromatic doublet refractor.
Focal length: 400 mm, focal ratio: f/5.
Guiding camera: Orion StarShoot.
Guiding control software: PHD2.
Telescope mount: SkyWatcher EQ6-R.
Polar aligning: QHYCCD PoleMaster.
Field flattener: yes; filter: no.

2017-08-17 The Skywatcher
2017-08-17 My Skywatcher ED120 and EQ6-R mount.


Image © R.Powell



  1. I can see the stars form a shape like that of a butterfly! Beautiful!
    Nice presentation of your Skywatcher.
    What means BM, Roger?
    Letter K stands for a spectral type (as stellar classification)?
    Thank you.

    Liked by 1 person

  2. I can find several references to BM Scorpii, Kazia – but nothing to indicate why it is called BM. Most of the brighter stars are designated with Greek letters, then when they run out they are given ordinary numbers. However, astronomers do not always stick their own rules………….

    Spectral types are O, B, A, F, G, K & M. I doubt if anyone knows why we all remember it as “Oh Be A Fine Girl Kiss Me”.

    O is the hottest and M is the coolest – so a K type is cooler than the Sun, which is a G type.

    Liked by 1 person

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