VY Canis Majoris
VY Canis Majoris is an extreme oxygen-rich red hypergiant or red supergiant and pulsating variable star from the Solar System in the slightly 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 infrared excess makes it one of the brightest objects in the local part of the galaxy at wavelengths of 5 to 20 microns and indicates a dust shell or heated disk. It is about times the mass of the Sun. It is surrounded by a complex asymmetric circumstellar envelope caused by its mass loss. It produces strong molecular maser emission and was one of the first radio masers discovered. VY CMa is embedded in the large molecular cloud Sh 2-310, a large, quite local star-forming H II region—its diameter: 480 arcminutes or. It has been described as 'Betelgeuse on steroids'.
The radius of VY CMa is estimated at 1,420 times that of the Sun, which is close to the modelled maximum, the Hayashi limit, corresponding to a volume almost 3 billion times that of the Sun. At this radius, 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. If this star replaced the Sun its surface would extend beyond the orbit of Jupiter.
Observational history
The first known-recorded observation of VY Canis Majoris is in the star catalogue of the French astronomer Jérôme Lalande in 1801, 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.Since 1847, VY Canis Majoris has been described as a crimson star. During the 19th century, observers measured at least six discrete components, suggesting that it might be a multiple star. These are now known to be bright zones in the host nebula. Observations in 1957 and high-resolution imaging in 1998 all but rule out any companion stars.
Giving spectral lines in brackets, the star is a strong emitter of OH, , and masers, which has been proven to be typical of an OH/IR star. Molecules, such as,,,,,,, and have been detected.
The variation in the star's brightness was first described in 1931, when it was listed as a long-period variable with a photographic magnitude range of 9.5 to 11.5. It was given the variable star designation VY Canis Majoris in 1939, the 43rd variable star of the constellation Canis Major.
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. 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.
Distance
In 1976, Lada and Reid published observations of the bright-rimmed molecular cloud Sh 2-310, which is 15″ east of the star. They assumed the cloud is associated with the open cluster NGC 2362 based on the cloud rim's ionization. NGC 2362 could be anywhere in the ranges of or away as determined from its color-magnitude diagram. VY CMa is projected onto the tip of the cloud rim, strongly suggesting its association. Furthermore, all the vectors of velocity of Sh 2-310 are very close to those of the star. There is thus a near-certain physical association of the star with Sh 2-310 and with NGC 2362 in all standard models. Melnik and others later prefer a range centred on 1.2 kiloparsecs.Distance can be more directly measured via stellar parallax; however, this star has a small parallax due to its distance, and standard visual observations have a margin of error too large for a hypergiant star with an extended CSE to be useful. For example, the Hipparcos Catalogue of 1997 gives a purely notional parallax of , and Gaia DR2 gives a purely notional parallax of.
In some cases, radio parallax can be useful. For VY CMa this was achieved in 2008, with observations of masers using VERA interferometry from the National Astronomical Observatory of Japan giving a parallax of, corresponding to a distance of . In 2012, observations of masers using very-long-baseline interferometry from Very Long Baseline Array independently derived a parallax of, corresponding to a distance of . These imply the cloud is less remote than thought or that VY CMa is a foreground object.
Spectrum
The spectrum of VY Canis Majoris is that of a high-luminosity M-class star. The hydrogen lines, however, have P Cygni profiles fit for luminous blue variables. The spectrum is dominated by TiO bands whose strengths suggest a classification of M5, while another class as late as M7.0 has been based on the VO spectrum, highly unusual for a supergiant and similar to other very late-type OH/IR red supergiants such as MY Cephei, S Persei, VX Sagittarii, and NML Cygni. Such stars were also referred to as "S Per et al.". The H-alpha line is not visible yet and there are unusual emission lines of neutral elements such as sodium and calcium. The luminosity class, as determined from different spectral features, varies from bright giant to bright supergiant, with a compromise being given: as M5eIbp. Old classifications were confused by the interpretation of surrounding nebulosity as companion stars.The present spectral classification system is inadequate for this star's complexities. The class depends on which of its complex spectral features are stressed. Further, key facets vary over time for 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 from 2006. 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.
Variability
VY Canis Majoris is a variable star that varies from an apparent visual magnitude of 9.6 at minimum brightness to a magnitude of 6.5 at maximum with an estimated pulsational period of 956 days. In the General Catalogue of Variable Stars it is classed a semiregular variable of sub-type SRc, indicating a cool supergiant, although it is classed as a type LC slow irregular variable star in the American Association of Variable Star Observers Variable Star Index. Other periods of 1,600 and 2,200 days have been derived.VY CMa is also sometimes considered the prototype for a class of heavily mass-losing OH/IR supergiants, distinct from the more common asymptotic giant branch OH/IR stars.
Physical properties
A very large and luminous star, VY Canis Majoris has been known to be an extreme object since the middle of the 20th century, although its true nature was uncertain. Its most analogous star is NML Cygni, another notable but less studied extreme cool hypergiant star within the Cygnus constellation.In the late 20th century, it was accepted that the star was a post-main-sequence red supergiant, occupying the upper-right-hand corner of the Hertzsprung–Russell diagram despite the uncertainty of its exact luminosity and temperature. Its angular diameter was measured and found to be significantly different depending on the observed wavelength. Most of the properties of the star depend directly on its distance, but the first meaningful estimates of its properties showed a very large star.
Luminosity
The bolometric luminosity of VY CMa can be calculated from spectral energy distribution or bolometric flux, which can be determined from photometry in several visible and infrared bands. Earlier calculations of the luminosity based on an assumed distance of gave luminosities between 200,000 and 560,000 times the Sun's luminosity, considerably very close or beyond the empirical Humphreys–Davidson limit. One study gave nearly at a distance of. In 2006 a luminosity of 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.Modern estimates of the luminosity extrapolate values below based on distances below 1.2 kpc, with a 2011 value calculated to be based on a 2001 photometry. More recently, a lower luminosity of was derived in 2020 based on more recent photometry at more wavelengths to estimate the bolometric flux. Many older luminosity estimates are consistent with current ones if they are rescaled to the distance of 1.2 kpc.
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. Most of the output of VY CMa is emitted as infrared radiation, with a maximum emission at, which is in part caused by reprocessing of the radiation by the circumstellar nebula.
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 suggests:- if a rotating star, an initial mass of but current mass and an age of 8.2 million years ; or
- if non-rotating, initially, falling to present-day.