VY Canis Majoris
Epoch J2000 Equinox J2000
|Right ascension||07h 22m 58.32877s|
|Declination||-25° 46′ 03.2355″|
|Apparent magnitude (V)||6.5 to 9.6
|Spectral type||M3-M5e Ia|
|B−V color index||2.24|
|Radial velocity (Rv)||49 ± 10 km/s|
|Proper motion (μ)||RA: 9.84 mas/yr
Dec.: 0.75 mas/yr
|Parallax (π)||0.83 ± 0.1 mas|
(1,170  pc)
|Mass||17 ± 8 M☉|
|Radius||1420 ± 120 R☉|
|Surface gravity (log g)||-0.6|
VY Canis Majoris (VY CMa) is a red hypergiant in the constellation Canis Major. It is one of the largest known stars by radius and also one of the most luminous of its type. It is approximately 1,420 ± 120 solar radii (6.6 astronomical units), about 1,975,000,000 kilometres (1.227×109 mi) in diameter, and about 1.2 kiloparsecs (3,900 light-years) distant from Earth. VY CMa is a single star categorized as a semiregular variable and has an estimated period of 2,000 days. It has an average density of 5 to 10 mg/m3. If placed at the center of the Solar System, VY Canis Majoris's surface would extend beyond the orbit of Jupiter, although there is still considerable variation in estimates of the radius, with some making it larger than the orbit of Saturn.
Nature of VY Canis Majoris 
The first known recorded observation of VY Canis Majoris is in the star catalogue of Jérôme Lalande, on 7 March 1801, which lists VY CMa as a 7th magnitude star. Further 19th-century studies of its apparent magnitude demonstrate that the star has been fading since 1850.
Since 1847, VY CMa has been known to be a crimson star. During the 19th century, observers measured at least six discrete components to VY CMa, suggesting the possibility that it is a multiple star. These discrete components are now known to be bright areas in the surrounding nebula. Visual observations in 1957 and high-resolution imaging in 1998 showed that VY CMa does not have a companion star.
VY CMa is a high-luminosity M star with an effective temperature of about 3,000 K, placing it at the upper-right hand corner of the Hertzsprung–Russell diagram and meaning it is a highly evolved star. During its main sequence, it would have been an O star with a mass of 15 to 35 M☉.
Measuring the distance 
Stellar distances can be calculated by measuring parallaxes as the Earth orbits around the Sun. However, VY CMa has a tiny parallax with a high margin of error, which makes it unreliable to calculate its distance using this method.
In 1976, Charles J. Lada and Mark J. Reid published the discovery of a bright-rimmed molecular cloud 15 minutes of arc east of VY CMa. At the edge of the cloud bordered by the bright rim, an abrupt decrease in the CO emission and an increase in brightness of the 12CO emission were observed, indicating possible destruction of molecular material and enhanced heating at the cloud-rim interface, respectively. Lada and Reid assumed the distance of the molecular cloud is approximately equal to that of the stars, which are members of open cluster NGC 2362, that ionize the rim. NGC 2362 has a distance of 1.5 ± 0.5 kiloparsecs as determined from its color-magnitude diagram. VY CMa is projected onto the tip of the cloud rim, suggesting its association with the molecular cloud. In addition to that, the velocity of the molecular cloud is very close to the velocity of the star. This further indicates the association of the star with the molecular cloud, and consequently with NGC 2362, which means VY CMa is also at a distance of 1.5 kpc. A more recent measurement of the distance to NGC 2362 gives 1.2 kpc.
University of Minnesota Professor Roberta M. Humphreys originally estimated the radius of VY CMa at 1,800 to 2,100 solar radii, which would make it the largest known star by radius. However, a more recent VLTI direct measurement gives a radius of 1420 ± 120 solar radii.
In 2006, Humphreys used the spectral energy distribution distance of VY Canis Majoris to calculate its luminosity. Since most of the radiation coming from the star is reprocessed by the dust in the surrounding cloud, she integrated the total fluxes over the entire nebula and showed that VY Canis Majoris has a luminosity of 5.6×105 L☉.
More recent estimates of the luminosity using a variety of methods give lower values of around 3×105 L☉.
Circumstellar nebula 
As mentioned above, VY Canis Majoris is surrounded by an extensive nebula that shows condensations that were taken as companion stars and that has been extensively studied with the aid of the Hubble Space Telescope, showing a complex structure with filaments and arcs caused by past eruptions, with a structure similar of the one surrounding the yellow hypergiant IRC+10420, something that have led some astronomers to suggest that VY Canis Majoris will become an object similar to the former, and later a Wolf-Rayet star.
There have been conflicting opinions of the properties of VY CMa. In one view, the star is a very large and very luminous red hypergiant. The various larger estimates of the size and luminosity fall outside the bounds of current stellar theory, both beyond the maximum predicted size of any star and far cooler than a star of its luminosity can become. In another opinion (such as Massey, Levesque, and Plez's study), the star is a normal red supergiant, with a radius around 600 solar radii and falling comfortably inside models of stellar structure and evolution. More recent papers produce intermediate values for radius and luminosity, falling at the very extreme for the expected size and luminosity of red supergiants (or hypergiant based on its emission spectrum and high mass loss rate).
VY Canis Majoris also illustrates the conceptual problem of defining the "surface" (and radius) of very large stars. With an average density of 0.000005 to 0.000010 kg/m3, the star is a thousand times less dense than the atmosphere of the Earth (air) at sea level. It is also undergoing strong mass loss with the outer layers of the star no longer gravitationally bound. The definition of the boundary of such stars is based on the Rosseland Radius, the location at which the opacity is one (or sometimes a different value such as 2/3). In cases such as VY CMa, the radius may be defined on a different opacity value or on an opacity at a particular wavelength.
See also 
- Orders of magnitude (length)
- List of largest stars
- List of most luminous stars
- R136a1, the most massive Star
- "SIMBAD basic query result: VY Canis Majoris". SIMBAD, Centre de Données astronomiques de Strasbourg. Retrieved 23 January 2013.
- "GCVS Query=VY CMa". General Catalogue of Variable Stars @ Sternberg Astronomical Institute, Moscow, Russia. Retrieved 2010-11-24.
- "Hipparchos catalogue: query form". CASU Astronomical Data Centre. Cambridge Astronomical Survey Unit. 2006. Retrieved 10 March 2009.
- "SIMBAD basic query result: VY Canis Majoris". SIMBAD, Centre de Données astronomiques de Strasbourg. Retrieved 7 March 2009.
- Lipscy, S. J.; Jura, M.; Reid, M. J. (10 June 2005). "Radio photosphere and mass-loss envelope of VY Canis Majoris". The Astrophysical Journal (The American Astronomical Society) 626 (1): 439–445. arXiv:astro-ph/0502586. Bibcode:2005ApJ...626..439L. doi:10.1086/429900.
- Monnier, J. D.; Geballe, T. R.; Danchi, W. C. (1 August 1998). "Temporal variations of midinfrared spectra in late-type stars". The Astrophysical Journal (American Astronomical Society) 502 (2): 833–846. arXiv:astro-ph/9803027. Bibcode:1998ApJ...502..833M. doi:10.1086/305945.
- "Distance and Kinematics of the Red Hypergiant VY CM". Retrieved 19 May 2012.
- Wittkowski, M.; Hauschildt; Arroyo-Torres, B.; Marcaide, J.M. (5 April 2012). "Fundamental properties and atmospheric structure of the red supergiant VY CMa based on VLTI/AMBER spectro-interferometry". Astronomy & Astrophysics 540: L12. arXiv:astro-ph/1203.5194. Bibcode:2012A&A...540L..12W. doi:10.1051/0004-6361/201219126. Unknown parameter
|first 2=ignored (help)
- Y. K. Choi; Hirota; Honma; Kobayashi; Bushimata; Imai; Iwadate; Jike et al. (2008). "Distance to VY VMa with VERA". Publications of the Astronomical Society of Japan (Publications Astronomical Society of Japan) 60: 1007. arXiv:0808.0641. Bibcode:2008PASJ...60.1007C. More than one of
|last2=specified (help); More than one of
|last3=specified (help); More than one of
|last4=specified (help); More than one of
|last5=specified (help); More than one of
|last6=specified (help); More than one of
|last7=specified (help); More than one of
|last8=specified (help); More than one of
- Massey, Philip; Levesque, Emily M.; Plez, Bertrand (1 August 2006). "Bringing VY Canis Majoris down to size: an improved determination of its effective temperature". The Astrophysical Journal 646 (2): 1203–1208. arXiv:astro-ph/0604253. Bibcode:2006ApJ...646.1203M. doi:10.1086/505025.
- Robinson, L. J. (7 December 1971). "Three somewhat overlooked facets of VY Canis Majoris". Commission 27 of the I. A. U., Information Bulletin on Variable Stars (Konkoly Observatory, Budapest) (599).
- Wittkowski, M.; Langer, N.; Weigelt, G. (27 October 1998). "Diffraction-limited speckle-masking interferometry of the red supergiant VY CMa". Astronomy and Astrophysics (European Southern Observatory) 340: 39–42.
- Lada, Charles J.; Reid, Mark J. (1 January 1978). "CO observations of a molecular cloud complex associated with the bright rim near VY Canis Majoris". The Astrophysical Journal (American Astronomical Society) 219: 95–104. Bibcode:1978ApJ...219...95L. doi:10.1086/155758.
- Pogge, Richard W. (31 October 2006). "Stellar distances". Astronomy 162: Introduction to Stars, Galaxies and the Universe. Ohio State University. Retrieved 20 March 2009.
- Lada, C. J.; Reid, M. (March 1976). "The discovery of a molecular cloud associated with VY CMa". Bulletin of the American Astronomical Society (American Astronomical Society) 8: 322.
- Mel'nik, A.M.; Dambis, A.K. Monthly Notices of the Royal Astronomical Society 400: 518.
- Humphreys, Roberta (13 October 2006). "VY Canis Majoris: the astrophysical basis of its luminosity". arXiV. arXiv:astro-ph/0610433.
- "Spectrum of massive star VY Canis Majoris". ESA Herschel. Retrieved 2011-11-15.
- Smith, Nathan; Humphreys, Roberta M.; Davidson, Kriz; Gehrz, Robert D.; Schuster, M. T.; Krautter, Joachim (February, 2001). "The Asymmetric Nebula Surrounding the Extreme Red Supergiant Vy Canis Majoris". The Astronomical Journal 121 (2): 1111–1125. Bibcode:2001AJ....121.1111S. doi:10.1086/318748.
- HubbleSite: Astronomers Map a Hypergiant Star's Massive Outbursts
- Bibcode: 1991A&A...246..374B
Further reading 
- Kastner, Joel H.; Weintraub, David A. (1998). "Hubble Space Telescope Imaging of the Mass-losing Supergiant VY Canis Majoris". Astronomical Journal 115 (4): 1592–1598. Bibcode:1998AJ....115.1592K. doi:10.1086/300297.
|Wikimedia Commons has media related to: VY Canis Majoris|
- Astronomers Map a Hypergiant Star's Massive Outbursts, HubbleSite NewsCenter, 2007-01-08
- "What is the Biggest Star in the Universe?", Universe Today, 2008-04-06
- Jonathan Amos (27 November 2009). "Herschel telescope 'fingerprints' colossal star". BBC News. Retrieved 30 November 2009. "The death throes of one of the biggest stars known to science have been spied by Europe's Herschel space telescope."
- American Association of Variable Star Observers