VY Canis Majoris

VY Canis Majoris

Actual image of VY Canis Majoris (brightest star in the image) and its surrounding molecular cloud complex in the open cluster NGC 2362 as seen from the Rutherfurd Observatory of the Columbia University in New York, United States on the evening of September 7, 2014, Sunday.
Observation data Epoch J2000      Equinox J2000
Constellation	Canis Major
Right ascension	07h 22m 58.32877s[1]
Declination	−25° 46′ 03.2355″[1]
Apparent magnitude (V)	6.5 to 9.6[2] 7.9607[3]
Characteristics
Spectral type	M3[4]-M5e Ia[5]
B−V color index	2.24[4]
Variable type	Semiregular[6]
Astrometry
Radial velocity (Rv)	49 ± 10[4] km/s
Proper motion (μ)	RA: 9.84[4] mas/yr Dec.: 0.75[4] mas/yr
Parallax (π)	0.83 ± 0.1[7] mas
Distance	~3,840 ly (1,170[8] [9] pc)
Details
Mass	17 ± 8[8] M☉
Radius	1420 ± 120[8] R☉
Luminosity	~270,000[8] L☉
Surface gravity (log g)	-0.6[8] cgs
Temperature	~3,490[8] K
Other designations
VY CMa, HD 58061,[5] HIP 35793[3]

VY Canis Majoris (VY CMa) is a red hypergiant star located in the constellation Canis Major. It is one of the largest stars (at one time the largest known) and also one of the most luminous of its type, and has a radius of approximately 1,420 ± 120 solar radii^[8] (equal to a diameter of 13.2 astronomical units, or about 1,976,640,000 km), and is located about 1.2 kiloparsecs (3,900 light-years) from Earth. VY Canis Majoris is a single star categorized as a semiregular variable with 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.^[10]

Nature

The first known recorded observation of VY Canis Majoris is in the star catalogue of Jérôme Lalande, on 7 March 1801, which lists it as a 7th magnitude star. Further 19th-century studies of its apparent magnitude demonstrate that the star has been fading since 1850.^[11] Since 1847, VY Canis Majoris has been described as a crimson star.^[11] During the 19th century, observers measured at least six discrete components, suggesting that it might be 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 there are no companion stars.^[11][12]

VY Canis Majoris is a high-luminosity M star with an effective temperature of about 3,500 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^[13] with a mass of 15 to 35 M_☉.^[8]

Distance

Stellar distances can be calculated by measuring parallaxes as the Earth orbits around the Sun. However, VY Canis Majoris has a tiny parallax with a high margin of error due to its great distance, which makes it unreliable to calculate the distance precisely using this method.^[14]

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 Canis Majoris. 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 12
CO 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 the 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.^[13] 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 Canis Majoris is at the same distance.^[15] A more recent measurement of the distance to NGC 2362 gives 1.2 kiloparsecs.^[16]

Recent VLBI direct measurements of the parallax of VY Canis Majoris^[7][9] give a distance of 1.1–1.2 kiloparsecs.

Size

Right to left: VY Canis Majoris compared to Betelgeuse, Rho Cassiopeiae, the Pistol Star, and the Sun (too small to be visible in this thumbnail). The orbits of Jupiter and Neptune are also shown.

Relative sizes of some planets in the Solar System and several well-known stars, including VY Canis Majoris
1. Mercury < Mars < Venus < Earth
2. Earth < Neptune < Uranus < Saturn < Jupiter
3. Jupiter < Proxima Centauri < Sun < Sirius
4. Sirius < Pollux < Arcturus < Aldebaran
5. Aldebaran < Rigel < Antares < Betelgeuse
6. Betelgeuse < VY Canis Majoris < NML Cygni < UY Scuti.

VY Canis Majoris is a Class M hypergiant. Its true size is debated, as it ejects much of its mass off its atmosphere in wild flares that stretch well beyond the star itself. Artist's impression.

University of Minnesota Professor Roberta M. Humphreys originally estimated the radius of VY CMa at 1,800-2,100 solar radii,^[17] which made it the largest star then known. However a more recent and accurate VLTI measurement^[8] gives the star a radius of 1420 ± 120 solar radii.

Luminosity

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 7005560000000000000♠5.6×10⁵ L_☉.^[17][18]

More recent estimates of the luminosity using a variety of methods give lower values of around 7005300000000000000♠3×10⁵ L_☉.

Circumstellar nebula

VY Canis Majoris is surrounded by an extensive nebula that shows condensations that were taken as companion stars^[12] 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 has led some astronomers to suggest that VY Canis Majoris will become an object similar to the former,^[19] and later a Wolf-Rayet star.^[12]

Controversy

VY Canis Majoris ejects huge amounts of gas during its outbursts.^[20]

There have been conflicting opinions of the properties of VY CMa. In one view,^[17] 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),^[10] 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^[7][8] 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 problem of defining the "surface" (and so the radius) of very large stars. With an average density of 0.000005 to 0.000010 kg/m³, the star is a hundred 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 optical depth is one (or sometimes a different value such as 2/3).^[21] In cases such as VY CMa, the radius may be defined on a different opacity value or on an opacity at a particular wavelength.^[8]

See also

• Orders of magnitude (length)
• List of largest known stars
• List of most luminous stars
• WASP-17b
• R136a1
• VV Cephei A
• NML Cygni
• UY Scuti
• Westerlund 1-26
• WOH G64

References

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. 

External links

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?", Fraser Cain, Universe Today, published 2008-04-06, updated May 13, 2013

Coordinates: 07^h 22^m 58.33^s, −25° 46′ 03.17″

• Amos, Jonathan (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
• Remote Sensing Tutorial Page A-5 archive

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