Theta Muscae

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Theta Muscae sits above the southern constellation of Muscae.
Theta Muscae
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Musca
Right ascension 13h 08m 07.15286s [1]
Declination −65° 18′ 21.6819″ [1]
Spectral type WC5/6, O6/7V, O9.5/B0Iab[2]

Theta Muscae (θ Mus) is a star in the southern constellation Musca ("the Fly") with an apparent magnitude of 5.66. In reality, it is a remote triple star system, the primary component of which is a carbon-sequence Wolf–Rayet star. This is a variety of highly-luminous hot blue star that has blown off its hydrogen envelope and is emitting heavier elements, in this case carbon, amid a strong stellar wind. Theta Muscae is the second-brightest such star in the sky after Regor in Vela. θ Mus is beyond the reach of parallax, but has been estimated as around 7,500 light-years (470 million astronomical units) from Earth.[3] While cataloging the stars in the far-southern sky, French explorer and astronomer Nicolas Louis de Lacaille gave the star its Bayer designation in 1756.[4]

Optical binary

To small telescopes, Theta Muscae appears as a double star, with a blue-cream brighter star and a blue-white companion of magnitude 7.3 some 5.3 arcseconds away.[5] The companion, Theta Muscae B, is not part of the triple system but an optical double which happens to be along the same line of sight.[3]

Triple system

The triple star is composed of two parts: a spectroscopic binary system composed of the Wolf–Rayet star (spectral type: WC5 or 6) and an O-type main sequence star (spectral type: O6 or O7) that orbit each other every 19 days and a blue supergiant (spectral type: O9.5/B0Iab) set about 46 milliarcseconds apart from them. If the system's estimated distance from Earth is accurate, the binary stars are about 0.5 AU apart and the supergiant about 100 AU apart from them.[2] All three are highly luminous: combined, they are likely to be over a million times as luminous as the Sun.[3] The stellar winds of the Wolf–Rayet star and blue supergiant are so powerful that they form a shock front where they meet.[6] The front produces X-rays.[3]


A surrounding emission nebula is now thought to be a supernova remnant.[7]


  1. ^ a b SIMBAD, Theta Muscae (accessed 21 November 2013)
  2. ^ a b Sugawara, Y.; Tsuboi, Y.; Maeda, Y. (2008). "Redshifted emission lines and radiative recombination continuum from the Wolf–Rayet binary θ Muscae: evidence for a triplet system?". Astronomy & Astrophysics 490: 259–64. arXiv:0810.1208. Bibcode:2008A&A...490..259S. doi:10.1051/0004-6361:20079302. 
  3. ^ a b c d Kaler, Jim. "Theta Muscae". Stars. University of Illinois. Retrieved 21 November 2013. 
  4. ^ Wagman, Morton (2003). Lost Stars: Lost, Missing and Troublesome Stars from the Catalogues of Johannes Bayer, Nicholas Louis de Lacaille, John Flamsteed, and Sundry Others. Blacksburg, VA: The McDonald & Woodward Publishing Company. pp. 213–14. ISBN 978-0-939923-78-6. 
  5. ^ Streicher, Magda (April 2006). "Musca—The Heavenly Fly". Deepsky Delights. The Astronomical Society of Southern Africa. pp. 56–59. Retrieved 21 November 2013. 
  6. ^ Hill, G. M.; Moffat, A. F. J.; St-Louis, N. (2002). "Modelling the colliding-winds spectra of the 19-d WR + OB binary in the massive triple system Theta Muscae". Monthly Notice of the Royal Astronomical Society 335 (4): 1069–78. Bibcode:2002MNRAS.335.1069H. doi:10.1046/j.1365-8711.2002.05694.x. 
  7. ^ Stupar, M.; Parker, Q.A.; Filipovic, M.D. (2010). "The optical emission nebulae in the vicinity of WR 48 (Θ Mus); True Wolf–Rayet ejecta or unconnected supernova remnant?". Monthly Notices of the Royal Astronomical Society 401 (3): 1760–69. arXiv:0910.1546. Bibcode:2010MNRAS.401.1760S. doi:10.1111/j.1365-2966.2009.15814.x.