The Butterfly Cluster (cataloged as Messier 6 or M6, and as NGC 6405) is an open cluster of stars in the southern constellation of Scorpius. Its name derives from the vague resemblance of its shape to a butterfly.[5] The Trumpler classification of II 3 r[6] encodes it is rich in stars, ranks II out of IV for disparateness and greatly mixes bright with faint components. It is 3.5° to the northwest of Messier 7, both north of the tail of Scorpius.[7]

Butterfly Cluster
Observation data (J2000.0 epoch)
Right ascension17h 40.1m [1]
Declination−32° 13′[1]
Distance1.59 kly (0.487 kpc)[1]
Apparent magnitude (V)4.2[2]
Apparent dimensions (V)25′[2]
Physical characteristics
Radius6[3] light-years
Estimated age94.2[1] Myr
Other designationsMessier 6, NGC 6405, Cr 341, Mel 178, Lund 769, OCL 1030,[4] ESO 455-SC030
See also: Open cluster, List of open clusters

The first astronomer to record the Butterfly Cluster's existence was Giovanni Battista Hodierna in 1654. However, Robert Burnham Jr. has proposed that the 2nd century astronomer Ptolemy may have seen it with the naked eye while observing its neighbor the Ptolemy Cluster (M7).[7] Credit for the discovery is usually given to Jean-Philippe Loys de Chéseaux in 1746. Charles Messier observed the cluster on May 23, 1764, and added it to his Messier Catalog.[5]

Estimates of the Butterfly Cluster's distance have varied over the years.[8] Wu et al. (2009) found a distance estimate of 1,590 light-years,[1] giving it a spatial dimension of some 12 light years.[3] Modern measurements show its total visual brightness to be magnitude 4.2. The cluster is estimated to be 94.2[1] million years old. Cluster members show a slightly higher abundance of elements heavier than helium compared to the Sun;[9] what astronomers refer to as the metallicity.

120 stars, ranging down to visual magnitude 15.1, have been identified as most likely cluster members.[8] Most of the bright stars in this cluster are hot, blue B-type stars but the brightest member is a K-type orange giant star, BM Scorpii,[10] which contrasts sharply with its blue neighbours in photographs. BM Scorpii, is classed as a semiregular variable star, its brightness varying from magnitude +5.5 to magnitude +7.0. There are also eight candidate chemically peculiar stars.[6][8]

The cluster is located 24.59 ± 0.13 kly (7.54 ± 0.04 kpc)[9] from the Galactic Center and is following an orbit through the Milky Way galaxy with a low eccentricity of 0.03 and an orbital period of 204.2 Myr. At present it is 23 ly (7 pc) below the galactic plane, and it will cross the plane every 29.4 Myr.[1]

As of January 2022, the Butterfly Cluster is one of the few remaining objects within the Messier Catalog to not have been photographed by the Hubble Space Telescope.[11]

See also



  1. ^ a b c d e f g Wu, Zhen-Yu; et al. (November 2009), "The orbits of open clusters in the Galaxy", Monthly Notices of the Royal Astronomical Society, 399 (4): 2146–2164, arXiv:0909.3737, Bibcode:2009MNRAS.399.2146W, doi:10.1111/j.1365-2966.2009.15416.x, S2CID 6066790.
  2. ^ a b Frommert, Hartmut; Kronberg, Christine (August 21, 2007), "Messier 6", SEDS Messier pages, Students for the Exploration and Development of Space (SEDS), retrieved 2018-12-07.
  3. ^ a b From trigonometry: radius = distance × sin( diameter_angle / 2 ) = 1,590 × sin( 25′/2 ) = 6 ly.
  4. ^ "NGC 6405". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2018-12-08.
  5. ^ a b Adam, Len (2018), Imaging the Messier Objects Remotely from Your Laptop, The Patrick Moore Practical Astronomy Series, Springer, p. 73,, ISBN 978-3319653853
  6. ^ a b Paunzen, E.; et al. (July 2006), "CCD photometric search for peculiar stars in open clusters. VII. Berkeley 11, Berkeley 94, Haffner 15, Lyngå 1, NGC 6031, NGC 6405, NGC 6834 and Ruprecht 130", Astronomy and Astrophysics, 454 (1): 171–178, arXiv:astro-ph/0602567, Bibcode:2006A&A...454..171P, doi:10.1051/0004-6361:20054628, S2CID 5131254.
  7. ^ a b Burnham, Robert (1978), Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System, Dover books on astronomy and astrophysics, vol. 3, Courier Corporation, p. 1705, ISBN 978-0486236735.
  8. ^ a b c Kılıçoğlu, T.; et al. (March 2016), "Chemical Composition of Intermediate-mass Star Members of the M6 (NGC 6405) Open Cluster", The Astronomical Journal, 151 (3): 30, arXiv:1510.05385, Bibcode:2016AJ....151...49K, doi:10.3847/0004-6256/151/3/49, S2CID 118553511, 49.
  9. ^ a b Netopil, M.; et al. (January 2016), "On the metallicity of open clusters. III. Homogenised sample", Astronomy & Astrophysics, 585: 17, arXiv:1511.08884, Bibcode:2016A&A...585A.150N, doi:10.1051/0004-6361/201526370, S2CID 118382876, A150.
  10. ^ Eggen, O. J. (February 1973), "Some variables of spectral type K", Publications of the Astronomical Society of the Pacific, 85: 42, Bibcode:1973PASP...85...42E, doi:10.1086/129403, S2CID 120272647.
  11. ^ Garner, Rob (28 August 2017). "Explore - The Night Sky | Hubble's Messier Catalog". Retrieved 23 January 2022.