VY Canis Majoris

VY Canis Majoris (abbreviated to VY CMa) is an extreme oxygen-rich red hypergiant or red supergiant (O-rich RHG or RSG) and pulsating variable star 1.2 kiloparsecs (3,900 light-years) 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 current evidence has been found that it is part of a multiple-star system. Its great infrared (IR) excess makes it one of the brightest objects in the local part of the galaxy (Orion Arm) at wavelengths of 5 to 20 microns (μm) and indicates a dust shell or heated disk. It is about 17±8 times the mass of the Sun (M☉). It is surrounded by a complex asymmetric circumstellar envelope (CSE) 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 681 ly (209 pc). It lies near the young open cluster NGC 2362, and is thought to be a member. The radius of VY CMa is estimated at 1,420 times that of the Sun (R☉), 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. The star is usually compared to another similar star, NML Cygni, and has also been described as 'Betelgeuse on steroids'.

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 (extinction). 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 (1612 MHz), H2O (22235.08 MHz), and SiO (43122 MHz) masers, which has been proven to be typical of an OH/IR star. Molecules, such as HCN, NaCl, PN, CH, CO, CH3OH, TiO, and TiO2 have been detected. The variation in the star's brightness was first described in 1931, when it was listed (in German) 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 1.5±0.5 kiloparsecs (kpc) or 4,890±1,630 light-years (ly) 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 (about 3,900 light-years). 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 1.78±3.54 milliarcseconds (mas), and Gaia DR2 gives a purely notional parallax of −5.92±0.83 mas. In some cases, radio parallax (via long baseline interferometry) can be useful. For VY CMa this was achieved in 2008, with observations of H2O masers using VERA interferometry from the National Astronomical Observatory of Japan giving 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). 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). These imply the cloud (Sh 2-310) is less remote than thought or that VY CMa is a foreground object.

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