VY Canis Majoris

Coordinates: Sky map 07h 22m 58.33s, −25° 46′ 03.17″
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VY Canis Majoris
Location of VY CMa (circled in red)
Observation data
J2000.0
Constellation Canis Major
Right ascension 07h 22m 58.32877s[1]
Declination −25° 46′ 03.2355″[1]
Apparent magnitude (V) 6.5–9.6[2]
Characteristics
Evolutionary stage
Red hypergiant
Spectral type M3–M4.5[3] (M2.5[4]–M5e Ia[5])
Apparent magnitude (U) 12.01[6]
Apparent magnitude (B) 10.19[6]
Apparent magnitude (V) 7.95[6]
Apparent magnitude (J) 1.98[6]
Apparent magnitude (H) 0.44[6]
Apparent magnitude (K) -0.72[6]
U−B color index +2.32[7]
B−V color index +2.057[1]
V−R color index +2.20[7]
Variable type
SRc[2] or Lc[8]
Distance
~3,820+260
−230 ly
(1,170+80
−70[3] pc)
Details
Myr
AAVSO 0718-25, IRAS 07209-2540, IRC −30087, RAFGL 1111, SAO 173571, WDS J07230-2546AB, 2MASS
 J07225830-2546030
Database references
SIMBADdata

VY Canis Majoris (abbreviated to VY CMa) is an extreme oxygen-rich (O-rich)

southern constellation of Canis Major. It is one of the largest known stars, one of the most luminous and massive red supergiants, and one of the most luminous stars in the Milky Way
.

No evidence has been found that it is part of a multiple star system. Its great

arcminutes (′) or 681 ly (209 pc).[16][17]

The radius of VY CMa is very roughly 1,420 times that of the Sun (R☉), which is close to the modelled maximum, the Hayashi limit, a volume nearly 3 billion times that of the Sun.[3] Taking this mid-point estimate as correct, an object travelling at the speed of light would take 6 hours to go around its surface, compared to 14.5 seconds for the Sun.[18] If this star replaced the Sun its surface would, per this approximation, be beyond the orbit of Jupiter.[3]

Observational history

Portrait in bust of Jérôme Lalande in 1802

The first known-recorded observation of VY Canis Majoris is in the star catalogue of the French astronomer Jérôme Lalande in 1801,[b] which lists it as a 7th order of magnitude star. Further quite frequent studies of its apparent magnitude imply the light of the star as viewed from Earth has faded since 1850, which could be due to emission changes or a denser part of its surrounds becoming interposed (extinction).[19] Since 1847, VY Canis Majoris has been described as a

companion stars.[19][20]

Giving spectral lines in brackets, the star is a strong emitter of OH (1612 MHz), H
2O (22235.08 MHz), and SiO (43122 MHz) masers, which has been proven to be typical of an OH/IR star.[21][22][23] Molecules, such as HCN, NaCl, PN, CH, CO, CH
3OH, TiO, and TiO
2 have been detected.[3][24][25][26][27][excessive citations]

The variation in the star's brightness was first described in 1931, when it was listed (in German) as a

variable star designation VY Canis Majoris in 1939, the 43rd variable star of the constellation Canis Major.[29]

Map of the giant molecular cloud Sharpless 310 and its surroundings

Combining data from the mentioned telescope with others from the

Keck in Hawaii it was possible to make a three-dimensional reconstruction of the envelope of the star. This reconstruction showed that the star's mass loss is much more complex than expected for any red supergiant or hypergiant. It became clear that the bows and nodules appeared at different times; the jets are randomly oriented, which prompts suspicion they derive from explosions of active parts of the photosphere. The spectroscopy proves the jets move away from the star at different speeds, confirming multiple events and directions as with coronal mass ejections.[30] Multiple asymmetric mass loss events and the ejection of the outermost material are deduced to have occurred within the last 500 to 1,000 years, while that of a knot near the star would be less than 100 years. The mass loss is due to strong convection in the tenuous outer layers of the star, associated with magnetic fields. Ejections are analogous to—but much larger than—coronal ejections of the Sun.[9][30][31]

Distance

τ Canis Majoris.
(ESO/Digitized Sky Survey 2
)
VLBA used to derive VY CMa's 2011 distance estimate

In 1976, Lada and Reid

light-years (ly) away as determined from its color-magnitude diagram.[32] This star is projected onto the tip of the cloud rim, strongly suggesting its association. Furthermore, all the vectors of velocity of Sh2-310 are very close to those of the star. There is thus a near-certain physical association of the star with Sh2-310 and with NGC 2362 in all standard models.[33]

Melnik and others later prefer a range centred on 1.2 kiloparsecs (about 3,900 light-years).[34]

Distances can be calculated by measuring

milliarcseconds (mas), in which the "central" figure equates to 562 pc (1,832 ly).[35] Parallax can be measured to high accuracy from the observation of masers using a long baseline interferometry. In 2008, such observations of H
2O masers using VERA interferometry from the National Astronomical Observatory of Japan gave a parallax of 0.88±0.08 mas, corresponding to a distance of 1.14+0.11
−0.09 kpc (about 3,720+360
−300 ly).[36] In 2012, observations of SiO masers using very-long-baseline interferometry (VLBI) from Very Long Baseline Array (VLBA) independently derived a parallax of 0.83±0.08 mas, corresponding to a distance of 1.20+0.13
−0.10 kpc (about 3,910+423
−326 ly).[10] These imply the cloud (Sh2-310) is less remote than thought or that the star is a foreground object.[16]

The

data release 2 value of −5.92±0.83 mas for this star is not meaningful.[37]

Variability

A visual band light curve for VY Canis Majoris, from AAVSO data[38]

VY Canis Majoris is a

semiregular variable of sub-type SRc, indicating a cool supergiant,[2] although it is classed as a type LC slow irregular variable star in the American Association of Variable Star Observers (AAVSO) Variable Star Index.[8] Other periods of 1,600[39] and 2,200[20]
 days have been derived.

VY CMa is sometimes considered as the prototype for a class of heavily mass-losing OH/IR supergiants, distinct from the more common asymptotic giant branch OH/IR stars.[40]

Spectrum

The spectrum of VY Canis Majoris is that of a high-luminosity

bright supergiant (Ia), with a compromise being given: as M5eIbp. Old classifications were confused by the interpretation of surrounding nebulosity as companion stars.[41]

The present spectral classification system is inadequate to this star's complexities. The class depends on which of its complex spectral features are stressed. Further, key facets vary over time as to this star. It is cooler and thus redder than M2, and is usually classified between M3 and M5. A class as extreme as M2.5 appeared in a study of 2006.[4] The luminosity class is likewise confused and often given only as I, partly because luminosity classes are poorly defined in the red and infrared portions of the spectrum. One study though, gives a luminosity class of Ia+ which means a hypergiant or extremely luminous supergiant.[42]

Physical properties

VY Canis Majoris compared to the Sun and the Earth's orbit
VV Cephei A
< VY Canis Majoris.

A very large and luminous star, VY CMa is among the most extreme stars in the Milky Way and has an effective temperature below 4,000 K (3,730 °C; 6,740 °F). It occupies the upper-right hand corner of the HR diagram although its exact luminosity and temperature are uncertain. Most of the properties of the star depend directly on its distance.

Luminosity

The

Humphreys–Davidson limit. One study gave nearly one million L☉ at a distance of 2.1 kpc (6,800 ly).[44] In 2006 a luminosity of 430,000 L☉ was calculated by integrating the total fluxes over the entire nebula, since most of the radiation coming from the star is reprocessed by the dust in the surrounding cloud.[31] More recent estimates of the luminosity extrapolate values below 350,000 L☉ based on distances below 1.2 kpc.[3][36][45]

Most of the output of VY CMa is emitted as infrared radiation, with a maximum emission at 5–10 μm, which is in part caused by reprocessing of the radiation by the circumstellar nebula.[9][31] Many older luminosity estimates are consistent with current ones if they are re-scaled to the distance of 1.2 kpc.[36] Despite being one of the most luminous stars in the Milky Way, much of the visible light of VY CMa is absorbed by the circumstellar envelope, so the star needs a telescope to be observed. Removing its envelope, the star would be one for the naked eye.[24]

Mass

Since this star has no companion star, its mass cannot be measured directly through gravitational interactions. Comparison of the effective temperature and bolometric luminosity compared to evolutionary tracks for massive stars suggest its initial mass was 25±10 M☉ for a rotating star but current mass 15 M☉—or 32 M☉ at first if non-rotating falling to present-day 19 M☉,[3] and an age of 8.2 million years (Myr).[10] Older studies have found much higher initial masses (thus also higher current masses) or a progenitor mass of 40–60 M☉ based on old luminosity estimates.[14][46]

Mass loss

SPHERE
instrument. The star itself is hidden behind a dark disk. Crosses are artifacts (lens effects) due to the characteristics of the instrument.

VY CMa has a strong stellar wind and is losing much material due to its high luminosity and quite low surface gravity. It has an average mass loss rate of 6×10−4 M☉ per year, among the highest known and unusually high even for a red supergiant, as evidenced by its extensive envelope.[47][39] It is thus an exponent for the understanding of high-mass loss episodes near the end of massive star evolution.[48] The mass loss rate probably exceeded 10−3 M☉/yr during the greatest mass loss events.[47]

The star has produced large, probably convection-driven, mass-loss events 70, 120, 200, and 250 years ago. The clump shed by the star between 1985 and 1995 is the source of its hydroxyl maser emission.[49]

Temperature

The effective temperature of this star is uncertain. Some signature changes in its spectrum correspond to temperature variations. Early estimates of the mean temperature assumed values below 3,000 K based on a spectral class of M5.[43][44] In 2006, its temperature was calculated to be as high as 3,650±25 K, corresponding to a spectral class of M2.5,[4] yet this star is usually considered as an M4 to M5 star. Adopting the latter classes with the temperature scale proposed by Emily Levesque gives a range of between 3,450 and 3,535 K.[50]

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.

The calculation of the radius of VY CMa is complicated by the extensive circumstellar envelope of the star. VY CMa is also a pulsating star, so its size changes with time. Earlier direct measurements of the radius at infrared (K-band = 2.2 µm) wavelength gave an angular diameter of 18.7±0.5 mas, corresponding to radii above 3,000 R☉ (2.1×109 km; 14 au; 1.3×109 mi) at an assumed distance of 1.5 kpc, considerably larger than expected for any red supergiant or red hypergiant.[43] However, this is probably larger than the actual size of the underlying star and the angular diameter estimate appears exceedingly large due to interference by the circumstellar envelope.[31][9][3] In 2006–2007 radii of 1,800–2,100 R☉ have been derived from the estimated luminosity of 430,000 L☉ and temperatures of 3,450–3,535 K.[31][9]

On 6 and 7 March 2011, VY CMa was observed at near-infrared wavelengths using interferometry at the Very Large Telescope. The size of the star was calculated using the Rosseland Radius, the location at which the optical depth is 2⁄3,[51] with two modern distances of 1.14+0.11
−0.09 and 1.20+0.13
−0.10 kpc.[36][10] Its angular diameter was directly measured at 11.3±0.3 mas, which corresponds to a radius of 1,420±120 R☉ at a distance of 1.17+0.08
−0.07 kpc. The high spectral resolution of these observations allowed the effects of contamination by circumstellar layers to be minimised. An effective temperature of 3,490±90 K, corresponding to a spectral class of M4, was then derived from the radius and a luminosity of 270,000±40,000 L☉ which is based on the distance and a measured flux of (6.3±0.3)×10−13 W/cm2.[3] In late 2013, a radius of 2,069 R☉ was determined, based on a rather cool adopted temperature of 2,800 K and a luminosity of 237,000 L☉.[52]

Most radius estimates of the VY CMa are considered as the size for the optical photosphere while the size of the star for the radio photosphere is calculated to be twice that of the size of the star for the optical photosphere.[5] Despite the mass and very large size (though some estimates give smaller sizes), VY CMa has an average density of 5.33 to 8.38 mg/m3 (0.00000533 to 0.00000838 kg/m3), it is over 100,000 times less dense than Earth's atmosphere at sea level (1.2 kg/m3).

Largest star

VY Canis Majoris (brightest star in the image) and its surrounding molecular cloud complex
(Rutherfurd Observatory/Columbia University)

VY Canis Majoris has been known to be an extreme object since the middle of the 20th century, although its true nature was uncertain.[41][53] In the late 20th century, it was accepted that it was a post-main sequence red supergiant. Its angular diameter had been measured and found to be significantly different depending on the observed wavelength. The first meaningful estimates of its properties showed a very large star.[54][55]

Early direct measurements of the radius at infrared (K-band = 2.2 µm) wavelength gave an angular diameter of 18.7±0.5 mas, corresponding to radii above 3,000 R☉ (2.1×109 km; 14 au; 1.3×109 mi) at a still very plausible distance of 1.5 kiloparsecs; a radius dwarfing other known red hypergiants.[43] However, this is probably larger than the actual size of the underlying star—this angular diameter estimate is heightened from interference by the envelope.[3][9][31] In 2006–07, radius between 1,800–2,100 R☉ has been derived from the preferring luminosity of 430,000 L☉ and the still-preferred temperature range of 3,450–3,535 kelvin.[9][31]

In contrast to prevailing opinion, a 2006 study, ignoring the effects of the circumstellar envelope in the observed flux of the star, derived a luminosity of 60,000 L☉, suggesting an initial mass of 15 M☉ and radius of 600 R☉ based on an assumed effective temperature of 3,650 K and distance of 1.5 kpc. On this basis they considered VY CMa and another notable extreme cool hypergiant star, NML Cygni, as normal early-type red supergiants.[4][56] They assert that earlier very high luminosities of 500,000 L☉ and very large radii of 2,800–3,230 R☉[14][57] (or even 4,000 R☉[20]) were based on effective temperatures below 3,000 K that were unreasonably low.[4]

Almost immediately another paper published a size estimate of 1,800–2,100 R☉ and concluded that VY CMa is a true hypergiant. This uses the later well-reviewed effective temperature 3450–3535 kelvin, and a luminosity of 430,000 L☉ based on SED integration and a distance of 1.5 kpc.[31]

In 2011,[d] the star was studied at near-infrared wavelengths using interferometry at the Very Large Telescope. The size of the star was published at its Rosseland Radius, outside of which optical depth falls below 2⁄3,[51] given the mean of two most modern, similar but distinct distances.[e][10][36] Its angular diameter was directly measured at 11.3±0.3 mas, thus radius of 1,420±120 R☉ given a distance of 1.17+0.08
−0.07 kpc. The high spectral resolution of these observations allowed the effects of contamination by circumstellar layers to be minimised. An effective temperature of 3,490±90 K, corresponding to a spectral class of M4, was then derived from the radius and a luminosity of 270,000±40,000 L☉ which is based on the distance and a measured flux of (6.3±0.3)×10−13 W/cm2.[3]

Most such radius estimates are considered as the size for the mean limit of the optical photosphere while the size of the star for the radio photosphere is calculated to be twice that.[5] Despite the mass and very large size (though some estimates give smaller sizes), VY CMa has an average density of 5.33 to 8.38 mg/m3 (0.00000533 to 0.00000838 kg/m3). It is over 100,000 times less dense than Earth's atmosphere at sea level (1.2 kg/m3).

In 2012, the size was calculated more accurately to be somewhat lower, for example 1,420 R☉,[3] which leaves larger sizes published and in-date for other galactic and extragalactic red supergiants (and hypergiants) such as Westerlund 1 W26 and WOH G64. Despite this, VY Canis Majoris is still often described as the largest known star, sometimes with caveats to account for the highly uncertain sizes of all these stars.[58][f] A 2013 estimate based on the Wittkowski radius and the Monnier radius put mean size at 2,000 R☉,[27] and later that year, Matsuura and others put forward a competing method of finding radius within the envelope, putting the star at 2,069 R☉, based on a cool-end of estimates adopted temperature of 2,800 K and a luminosity of 237,000 L☉.[52] However, these values are not consistent with its spectral types, leaving the 2012 values in better match.

Surroundings

WFPC2/HST image showing the asymmetric nebula surrounding VY CMa, which is the central star

VY Canis Majoris is surrounded by an extensive and dense asymmetric red

″ across, corresponding to 140 AU (0.0022 ly) for a star 1,200 parsecs away, whereas that of the outer one is at 10″, corresponding to 12,000 AU (0.19 ly).[47] This nebula is so bright that it was discovered in a dry night sky in 1917 with an 18 cm telescope, and its condensations were once regarded as companion stars.[20] It has been extensively studied with the aid of the Hubble Space Telescope (HST), showing that the nebula has a complex structure that includes filaments and arcs, which were caused by past eruptions; the structure is akin to that around post-red supergiant (Post-RSG) or yellow hypergiant (YHG) IRC +10420. The similarity has led at least two professional articles to propose a model that the star might evolve blueward on the Hertzsprung–Russell diagram (HR diagram) to become a yellow hypergiant, then a luminous blue variable (LBV), and finally a Wolf–Rayet star (WR star).[14][20]

Evolution

VY Canis Majoris is a highly

circumstellar disk.[15] It has probably evolved from a hot, dense O9 main sequence star of 5–20 R☉ (solar radii).[30][32][59] The star has evolved rapidly because of its high mass. The time spent to the red hypergiant phase is estimated to be between 100,000 and 500,000 years, and thus VY CMa most likely left its main sequence phase more than a million years ago.[10][30]

The future evolution of VY CMa is uncertain, but like the most cool supergiants, the star will certainly explode as a supernova. It has begun to fuse helium into carbon en masse.[g] Like Betelgeuse, it is losing mass and is expected to explode as a supernova within the next 100,000 years — it will probably revert to a higher temperature beforehand.[3][58][60] The star is very unstable, having a prodigious mass loss such as in ejections.

VY Canis Majoris is a candidate for a star in a second red supergiant phase, but this is mostly speculative and unconfirmed.[61]

From this star CO emission is coincident with the bright KI shell in its asymmetric nebula.

The star will produce either:

The explosion could be associated with gamma-ray bursts (GRB), and it will produce a shock wave of a speed of a few thousand kilometers per second that could hit the surrounding envelope of material, causing strong emission for many years after the explosion. For a star so large, the remnant would be probably a black hole rather than a neutron star.[60]

Notes

  1. ^ The given stellar metallicity is given as the solar metallicity ([Fe/H] = approx. +0.0 dex).
  2. ^ on 7 March
  3. ^ Charles J. Lada and Mark J. Reid
  4. ^ On 6 and 7 March
  5. ^ 1.14+0.11
    −0.09     and 1.20+0.13
    −0.10     kpc    , see above at Distance
  6. ^ Alcolea et al 2013 refer to VY CMa as having the highest radius "among well-characterised stars in our galaxy", referring to the Wittkowski et al. 2012 value of 1,420 R☉ which is based on the distances from Choi et al. 2008 and Zhang et al. 2012 plus an angular diameter. Several red supergiants (or hypergiants) are possibly larger, although they could have less accurate radius estimates.
  7. ^ a main sequence star fuses hydrogen into helium.

References and footnotes

  1. ^
    S2CID 18759600
    .
  2. ^ a b c d "GCVS Query=VY CMa". General Catalogue of Variable Stars @ Sternberg Astronomical Institute, Moscow, Russia. Retrieved 24 November 2010.
  3. ^
    S2CID 54044968
    .
  4. ^
    S2CID 14314968
    .
  5. ^
    S2CID 14878122
    .
  6. ^
    Bibcode:2002yCat.2237....0D
    .
  7. ^
    doi:10.1086/180366
    .
  8. ^ a b c "VSX: Detail for VY CMa". American Association of Variable Star Observers. Retrieved 20 July 2018.
  9. ^
    S2CID 119009102
    .
  10. ^
    S2CID 121202336
    .
  11. S2CID 119044678
    .
  12. ^
    S2CID 85459804
    .
  13. S2CID 210714093
    .
  14. ^
    doi:10.1086/318748
    .
  15. ^
    doi:10.1086/150688
    .
  16. ^ a b "Result for Sh-2 310". Galaxy Map. Archived from the original on 13 April 2009. Retrieved 20 August 2018.
  17. doi:10.1086/190049
    .
  18. ^ "Solar System Exploration: Planets: Sun: Facts & Figures". NASA. Archived from the original on 2 January 2008. Retrieved 15 January 2016.
  19. ^
    Bibcode:1971IBVS..599....1R
    .
  20. ^
    Bibcode:1998A&A...340L..39W
    .
  21. S2CID 29999031
    .
  22. doi:10.1086/180306
    .
  23. doi:10.1086/153517
    .
  24. ^ a b David Darling. "VY Canis Majoris". Retrieved 9 July 2018.
  25. ^ "VY Canis Majoris". American Association of Variable Star Observers. 13 April 2010.
  26. S2CID 56413042
    .
  27. ^
    S2CID 59038056
    .
  28. doi:10.1002/asna.19312420702
    .
  29. doi:10.1002/asna.19392681102
    .
  30. ^ a b c d "Astronomers Map a Hypergiant Star's Massive Outbursts". HubbleSite. 8 January 2007. Retrieved 9 July 2018.
  31. ^
    Bibcode:2006AAS...20910109G
    .
  32. ^
    doi:10.1086/155758
    .
  33. Bibcode:1976BAAS....8R.322L
    .
  34. S2CID 11885068
    .
  35. Bibcode:1997A&A...323L..49P
    .
  36. ^
    S2CID 15042252
    .
  37. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR
    .
  38. ^ "Download Data". aavso.org. AAVSO. Retrieved 1 October 2021.
  39. ^
    S2CID 55162530
    .
  40. Bibcode:1996hst..prop.6416K
    .
  41. ^
    doi:10.1086/127278
    .
  42. Bibcode:2014yCat....1.2023S
    .
  43. ^
    S2CID 7851916
    .
  44. ^
    Bibcode:1996A&A...314..896L
    .
  45. S2CID 119276502
    .
  46. doi:10.1086/191820
    .
  47. ^
    S2CID 119281306
    .
  48. doi:10.1086/426565
    .
  49. S2CID 229188960
  50. S2CID 15109583
    .
  51. ^
    Bibcode:1991A&A...246..374B
    .
  52. ^
    S2CID 53393704
    .
  53. doi:10.1086/150224
    .
  54. doi:10.1086/191488
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  55. doi:10.1086/131948
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  56. S2CID 15766762
    .
  57. S2CID 14352419
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  58. ^
    S2CID 55758451
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  59. Bibcode:1978Obs....98..224W
    .
  60. ^
    S2CID 19019913
    .
  61. S2CID 125806208
    .

Further reading

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