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.

Its great infrared (IR) excess makes it one of the brightest objects in the local part of the galaxy at wavelengths of 5 to 20 microns (μm) and indicates a dust shell or heated disk.

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).

[27] 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.

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.

The mass loss is due to strong convection in the tenuous outer layers of the star, associated with magnetic fields.

They assumed the distance of the cloud is approximately equal to that of the stars, which are members of the open cluster NGC 2362, that ionize the rim.

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), in which the "central" figure equates to 562 pc (1,832 ly).

In 2008, such observations of H2O 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).

[40] 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).

[16] The Gaia mission provides highly constrained parallaxes to some objects, but the data release 2 value of −5.92±0.83 mas for VY CMa is not meaningful.

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.

[48] 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.

[14][50] 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.

Adopting the latter classes with the temperature scale proposed by Emily Levesque gives a range of between 3,450 and 3,535 K.[54] The calculation of the radius of VY CMa is complicated by the extensive circumstellar envelope of the star.

[47] 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.

[29][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.[29][9] On 6 and 7 March 2011, VY CMa was observed at near-infrared wavelengths using interferometry at the Very Large Telescope.

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.

[47] However, this is probably larger than the actual size of the underlying star—this angular diameter estimate is heightened from interference by the envelope.

On this basis they considered VY CMa and another notable extreme cool hypergiant star, NML Cygni, as normal early-type red supergiants.

The size of the star was published at its Rosseland Radius, outside of which optical depth falls below 2⁄3,[55] given the mean of two most modern, similar but distinct distances.

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.

VY Canis Majoris is surrounded by an extensive and dense asymmetric red reflection nebula, with a total ejected mass of 0.2–0.4 M☉ and a temperature of 800 kelvin, based on a DUSTY model atmosphere that has been formed by material expelled from its central star.

[51] 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.

[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.

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