 Size comparison between the Sun and VY Canis Majoris | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Canis Major |
| Right ascension | 07h 22m 58.33s[1] |
| Declination | −25° 46′ 03.17″[1] |
| Apparent magnitude (V) | 7.9607[2] |
| Characteristics | |
| Spectral type | M3[1]-M5e Ia[3] |
| B-V color index | 2.24[1] |
| Variable type | Semiregular[4] |
| Astrometry | |
| Radial velocity (Rv) | 49 ± 10[1] km/s |
| Proper motion (μ) | RA: 9.84[1] mas/yr Dec.: 0.75[1] mas/yr |
| Parallax (π) | 1.78 ± 3.54[1] mas |
| Distance | ~4,900 ly (~1,500[5] pc) |
| Details | |
| Mass | ~15[6]-25[7] M☉ |
| Radius | ~1,800-2,100[8] R☉ |
| Luminosity | ~2-5.6×105[9][10] L☉ |
| Temperature | ~3000[10] K |
| Other designations | |
VY Canis Majoris (VY CMa) is a red hypergiant star located in the constellation Canis Major. With a size between 1,800 and 2,100 solar radii, it is the largest known star and also one of the most luminous known. It is located about 1.5 kiloparsecs (4.6×1016 km) or about 4,900 light years away from Earth. Unlike most stars, which occur in either binary or multiple star systems, VY CMa is a single star (i.e. does not have any stellar companions). It is categorized as a semiregular variable and has an estimated period of 2,200 days.[4]
Nature of VY Canis Majoris
The first known record of VY Canis Majoris is in the star catalogue of Jérôme Lalande, on March 7, 1801. The catalogue listed VY CMa as a 7th magnitude star. Further studies on its apparent magnitude during the 19th century showed that the star has been fading since 1850.[11]
Since 1847, VY CMa has been known to be a red star.[11]
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.[6][11]
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 suggesting that it is a highly evolved star. During its main sequence, it would have been an O star[10] with a mass of about 30 to 40 M☉.[6]
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.[12]
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 kpc as determined from its color-magnitude diagram.[10]
VY CMa is projected onto the tip of the 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.[5]
Size
University of Minnesota professor Roberta M. Humphreys[13] estimates the radius of VY CMa at 1,800 to 2,100 solar radii.[8] To illustrate, if Earth's Sun were replaced by VY Canis Majoris, its diameter might extend to the orbit of Saturn. Assuming the upper size limit of 2100 solar radii, light would take more than 8 hours to travel around the star's circumference, compared to 14.5 seconds for the sun. It would take over 7,000,000,000,000,000 (7 quadrillion) Earths to fill the volume of VY Canis Majoris. [14]
Luminosity
In 2006, Humphreys used the spectral energy distribution and distance of VY CMa 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 CMa has a luminosity of 4.3×105 L☉[8].
Controversy
There are two controversial opinions of the properties of VY CMa. In one view (such as Roberta M. Humphreys' study[8]), the star is a very large and very luminous red hypergiant. In another opinion (such as Massey, Levesque, & Plez's study[16]), the star is a normal red supergiant, with a radius around 600 solar radii. In this case, its surface would extend well past the orbit of Mars.
Like its size, the luminosity of VY CMa is also the subject of doubt. Humphreys contests that visual photometry is not sufficient for stars with enough circumstellar dust to reprocess the visual and red fluxes into the thermal infrared.[8]
See also
References
- ^ a b c d e f g h "SIMBAD basic query result: VY Canis Majoris". SIMBAD, Centre de Données astronomiques de Strasbourg. http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=VY+canis+majoris&submit=SIMBAD+search. Retrieved on March 7, 2009.
- ^ a b "Hipparchos catalogue: query form". CASU Astronomical Data Centre. Cambridge Astronomical Survey Unit. 2006. http://archive.ast.cam.ac.uk/hipp/hipparcos.html. Retrieved on March 10, 2009.
- ^ a b Lipscy, S. J.; Jura, M.; Reid, M. J. (June 10, 2005). "Radio photosphere and mass-loss envelope of VY Canis Majoris". The Astrophysical Journal (The American Astronomical Society) 626: 439-445.
- ^ a b Monnier, J. D.; Geballe, T. R.; Danchi, W. C. (August 1, 1998). "Temporal variations of midinfrared spectra in late-type stars". The Astrophysical Journal (American Astronomical Society) 502: 833-846.
- ^ a b 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.
- ^ a b c Wittkowski, M.; Langer, N.; Weigelt, G. (October 27, 1998). "Diffraction-limited speckle-masking interferometry of the red supergiant VY CMa". Astronomy and Astrophysics (European Southern Observatory) 340: 39-42.
- ^ Monnier, J. D.; Danchi, W. C.; Hale, D. S.; Lipman, E. A.; Tuthill, P. G.; Townes, C. H. (November 10, 2000). "Mid-infrared interferometry on spectral lines. II. Continuum (dust) emission around IRC +10216 and VY Canis Majoris". The Astrophysical Journal (The American Astronomical Society) 543: 861-867.
- ^ a b c d e Humphreys, Roberta (October 13, 2006). "VY Canis Majoris: the astrophysical basis of its luminosity". arXiV. Retrieved on May 15, 2007.
- ^ Monnier, J. D.; Tuthill, P. G.; Lopez, B.; Cruzalebes, P.; Danchi, W. C.; Haniff, C. A. (February 10, 1999). "The last gasps of VY Canis Majoris: aperture synthesis and adaptive optics imagery". The Astrophysical Journal (American Astronomical Society) 512: 351-361. doi:.
- ^ a b c d Lada, Charles J.; Reid, Mark J. (January 1, 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.
- ^ a b c Robinson, L. J. (December 7, 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).
- ^ Pogge, Richard W. (October 31, 2006). "Stellar distances". Astronomy 162: Introduction to Stars, Galaxies and the Universe. Ohio State University. http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit1/distances.html. Retrieved on March 20, 2009.
- ^ Roberta Humphreys, physics.umn.edu, http://www.physics.umn.edu/people/roberta.html, retrieved on 2009-05-17
- ^ Volume with radius 9.58 AU=1.23*10^37 m3; Earth volume 1.08*10^21 m3; ratio is 1.14*10^16 or 11.4 quadrillion.
- ^ HubbleSite: Astronomers Map a Hypergiant Star's Massive Outbursts
- ^ Massey, Philip; Levesque, Emily M.; Plez, Bertrand (August 1, 2006). "Bringing VY Canis Majoris down to size: an improved determination of its effective temperature". The Astrophysical Journal (American Astronomical Society) 646: 1203-1208. doi:.
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