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WR 2
Observation data; Epoch J2000 Equinox J2000
Constellation	Cassiopeia
Right ascension	01h 05m 23.01436s
Declination	+60° 25′ 18.9652″
Apparent magnitude (V)	11.33
Characteristics
Evolutionary stage	Wolf-Rayet
Spectral type	WN2-w
B−V color index	+0.16
Astrometry
Proper motion (μ)	RA: −12.80 mas/yr ; Dec.: −7.18 mas/yr
Parallax (π)	4.46 ± 3.31 mas
Distance	2,510 pc
Absolute magnitude (MV)	−2.43
Details
Mass	16 M☉
Radius	0.89 R☉
Luminosity	282,000 L☉
Temperature	141,000 K
Rotational velocity (v sin i)	500 km/s
Age	4.0 Myr
Other designations
	HD 6327, WR 2, HIP 5100
Database references
SIMBAD	data

WR 2 is a Wolf-Rayet star located around 8,000 light years away from Earth in the constellation of Cassiopeia, in the stellar association Cassiopeia OB1.^[7] It is smaller than the Sun, but due to a temperature over 140,000 K it is 282,000 times as luminous as the Sun. With a radius of 89% that of the Sun, it is the smallest known WN star in the Milky Way.^[3]

WR 2 is considered to be a member of the nitrogen sequence of WR stars, but completely lacks lines of N_III, N_IV, N_V, and He_I. Its spectrum is dominated by broad rounded emission lines of He_II, leading to the classification of WN2-b (for broad).^[8] It is now given the spectral type of WN2-w (for weak), due to the relative strength of the continuum and lack of extremely intense emission lines. It is the only galactic WN2 star known.^[3] Weak-lined Wolf-Rayet stars often have hot luminous companions which dilute the emission. WR 2 does have a companion, but it is much fainter than the primary and not thought to be the cause of the weak-lined spectrum.^[5]

WR 2 is the smallest and hottest WN star known in the galaxy. Its unusual rounded emission lines are thought to be due to extremely fast rotation, although the exact rotation rate is not known. Estimates range from 500 km/s to approximately the breakup rate for the star of 1,900 km/s.^[9]^[3]^[5] The high temperature also leads to a very fast stellar wind of 1,800 km/s,^[3] although the overall rate of mass loss is one of the lowest for any Wolf-Rayet star.^[10] The combination of a massive Wolf-Rayet star and rapid rotation is likely to result in a gamma-ray burst when the star explodes as a supernova.^[9]

X-rays have been detected from WR 2 although they may not be due to colliding winds as is common for massive stars.^[10]

References

^ ^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. S2CID 18759600.
^ ^a ^b 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.
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j Hamann, W. -R.; Gräfener, G.; Liermann, A. (2006). "The Galactic WN stars". Astronomy and Astrophysics. 457 (3): 1015. arXiv:astro-ph/0608078. Bibcode:2006A&A...457.1015H. doi:10.1051/0004-6361:20065052. S2CID 18714731.
^ Van Der Hucht, K. A. (2001). "The VIIth catalogue of galactic Wolf–Rayet stars". New Astronomy Reviews. 45 (3): 135–232. Bibcode:2001NewAR..45..135V. doi:10.1016/S1387-6473(00)00112-3.
^ ^a ^b ^c Shenar, T.; Hamann, W.-R.; Todt, H. (2014). "The impact of rotation on the line profiles of Wolf-Rayet stars". Astronomy & Astrophysics. 562: A118. arXiv:1401.2159. Bibcode:2014A&A...562A.118S. doi:10.1051/0004-6361/201322496. S2CID 21732510.
^ Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (2011). "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun". Monthly Notices of the Royal Astronomical Society. 410 (1): 190–200. arXiv:1007.4883. Bibcode:2011MNRAS.410..190T. doi:10.1111/j.1365-2966.2010.17434.x. S2CID 118629873.
^ Lundstrom, I.; Stenholm, B. (1984-10-01). "Wolf-Rayet stars in open clusters and associations". Astronomy and Astrophysics Supplement Series. 58: 163–192. Bibcode:1984A&AS...58..163L. ISSN 0365-0138.
^ Hiltner, W. A.; Schild, R. E. (1966). "Spectral Classification of Wolf-Rayet Stars". Astrophysical Journal. 143: 770. Bibcode:1966ApJ...143..770H. doi:10.1086/148556.
^ ^a ^b Sander, A.; Hamann, W.-R.; Todt, H. (2012). "The Galactic WC stars. Stellar parameters from spectral analyses indicate a new evolutionary sequence". Astronomy & Astrophysics. 540: A144. arXiv:1201.6354. Bibcode:2012A&A...540A.144S. doi:10.1051/0004-6361/201117830. S2CID 119182468.
^ ^a ^b Skinner, Stephen L.; Zhekov, Svetozar A.; Güdel, Manuel; Schmutz, Werner; Sokal, Kimberly R. (2012). "New X-Ray Detections of WNL Stars". The Astronomical Journal. 143 (5): 116. arXiv:1203.5098. Bibcode:2012AJ....143..116S. doi:10.1088/0004-6256/143/5/116. S2CID 119281411.