Theta Muscae

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Theta Muscae
Musca constellation map.svg
Red circle.svg
Location of θ Mus (circled)
Observation data
Epoch 2000      Equinox 2000
Constellation Musca
Right ascension 13h 08m 07.15286s[1]
Declination −65° 18′ 21.6819″[1]
Apparent magnitude (V) 5.53[2] (5.662 + 7.555[3])
θ Mus A
Spectral type WC5/6 + O6/7V + O9.5/B0Iab[4]
U−B color index −0.91[5]
B−V color index −0.43[3]
Variable type Eclipsing + WR[6]
Spectral type O9III[7]
U−B color index −0.90[5]
B−V color index −0.055[3]
Radial velocity (Rv)−28.4[8] km/s
Proper motion (μ) RA: -2.10[1] mas/yr
Dec.: −11.52[1] mas/yr
Parallax (π)0.26 ± 0.48[1] mas
Distance7,400[4] ly
(2,270[4] pc)
Absolute magnitude (MV)−6.2[9]
Mass<11.5[10] M
Luminosity230,000[10] L
Other designations
Theta Muscae, CD−64° 699, HR 4952, HD 113904, HIP 64094, GSC 08997-02337, SAO 252162, PPM 359890, GC 17788, UCAC3 50-186265, IRAS 15465+2818, WR 48
Database references

Theta Muscae (θ Muscae) is a multiple star system in the southern constellation Musca ("the Fly") with an apparent magnitude of 5.5. It is the second-brightest Wolf–Rayet star in the sky, although much of the visual brightness comes from the massive companions and it is not one of the closest of its type.


Theta Muscae 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 Gamma Velorum in Vela. θ Mus is beyond the current reach of useful visual parallax measurements, but has been estimated as around 7,400 light-years (460 million astronomical units) from Earth. 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.[11]

Optical binary[edit]

To small telescopes, Theta Muscae appears as a double star, with a blue-cream brighter star and an O9III companion of magnitude 7.3 some 5.3 arcseconds away. The primary θ Muscae A is a massive triple star system.

The companion θ Muscae B is not part of the triple system but an optical double which happens to be along the same line of sight. It is a luminous O class giant star. It is suspected of itself being a spectroscopic binary with the companion being much fainter.[12]

Triple system[edit]

The triple star θ Muscae A 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. Although the Wolf–Rayet star dominates the spectrum, it is visually only about a quarter of the brightness of the supergiant companion.[4] All three are highly luminous: combined, they are likely to be over a million times as luminous as the Sun. The stellar winds of the Wolf–Rayet star and its close companion are so powerful that they form a shock front where they meet.[13] The front produces X-rays.[14]


A surrounding emission nebula is now thought to be a supernova remnant, not directly connected to θ Mus.[14]


  1. ^ a b c d e Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357.
  2. ^ Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237: 0. Bibcode:2002yCat.2237....0D.
  3. ^ a b c Høg, E.; Fabricius, C.; Makarov, V. V.; Urban, S.; Corbin, T.; Wycoff, G.; Bastian, U.; Schwekendiek, P.; Wicenec, A. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27. Bibcode:2000A&A...355L..27H. doi:10.1888/0333750888/2862.
  4. ^ a b c d 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.
  5. ^ a b Moffat, A. F. J.; Seggewiss, W. (1977). "The Wolf-Rayet binary theta Muscae". Astronomy and Astrophysics. 54: 607. Bibcode:1977A&A....54..607M.
  6. ^ Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/gcvs. Originally published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
  7. ^ Sota, A.; Maíz Apellániz, J.; Morrell, N. I.; Barbá, R. H.; Walborn, N. R.; Gamen, R. C.; Arias, J. I.; Alfaro, E. J. (2014). "The Galactic O-Star Spectroscopic Survey (GOSSS). II. Bright Southern Stars". The Astrophysical Journal Supplement. 211: 10. arXiv:1312.6222. Bibcode:2014ApJS..211...10S. doi:10.1088/0067-0049/211/1/10.
  8. ^ Wilson, Ralph Elmer (1953), "General Catalogue of Stellar Radial Velocities", Washington, Washington: Carnegie Institution of Washington, Bibcode:1953GCRV..C......0W.
  9. ^ Hidayat, Bambang; Admiranto, A. Gunawan; Van Der Hucht, Karel A. (1984). "Wolf-Rayet binaries: Evolutionary causes for their distribution in the Galaxy". Astrophysics and Space Science. 99 (1–2): 175–190. Bibcode:1984Ap&SS..99..175H. doi:10.1007/BF00650243. ISSN 0004-640X.
  10. ^ a b Nugis, T.; Lamers, H. J. G. L. M. (2000). "Mass-loss rates of Wolf-Rayet stars as a function of stellar parameters". Astronomy and Astrophysics. 360: 227. Bibcode:2000A&A...360..227N.
  11. ^ 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.
  12. ^ Sota, A.; Maíz Apellániz, J.; Walborn, N. R.; Alfaro, E. J.; Barbá, R. H.; Morrell, N. I.; Gamen, R. C.; Arias, J. I. (2011). "THE GALACTIC O-STAR SPECTROSCOPIC SURVEY. I. CLASSIFICATION SYSTEM AND BRIGHT NORTHERN STARS IN THE BLUE-VIOLET ATR∼ 2500". The Astrophysical Journal Supplement Series. 193 (2): 24. arXiv:1101.4002. Bibcode:2011ApJS..193...24S. doi:10.1088/0067-0049/193/2/24. ISSN 0067-0049.
  13. ^ 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 Notices of the Royal Astronomical Society. 335 (4): 1069–78. Bibcode:2002MNRAS.335.1069H. doi:10.1046/j.1365-8711.2002.05694.x.
  14. ^ a b 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.