VY Canis MajorisVY Canis Majoris (VY CMa) is a red hypergiant star located in the constellation Canis Major. One of

the largest stars and also one of the most luminous of its type, it has a radius of approximately 1,420

± 120 solar radii[8] (equal to a diameter of 13.2 astronomical units, or about 1,976,640,000 kilometres

(1.22823×109 mi)), and is situated about 1.2 kiloparsecs (3,900 light-years) from Earth. VY CMa is a

single star categorized as a semiregular variable and has an estimated period of 2,000 days. It has

an average density of 5 to 10 mg/m3. If placed at the center of the Solar System, VY Canis Majoris's

surface would extend beyond the orbit of Jupiter, although there is still considerable variation in

estimates of the radius, with some making it larger than the orbit of Saturn.[10]

Nature[edit]

The first known recorded observation of VY Canis Majoris is in the star catalogue of Jérôme

Lalande, on 7 March 1801, which lists VY CMa as a 7th magnitude star. Further 19th-century studies

of its apparent magnitude demonstrate that the star has been fading since 1850.[11] Since 1847, VY

CMa has been known to be a crimson star.[11] During the 19th century, observers measured at least

six discrete components to VY CMa, suggesting the possibility that it was 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.[11][12]

VY CMa is a high-luminosity M star with an effective temperature of about 3,500 K, placing it at the

upper-right hand corner of theHertzsprung–Russell diagram and meaning it is a highly evolved star.

During its main sequence, it would have been an O star [13]  with a mass of 15 to 35 M☉.[8]

Distance[edit]

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.[14]

In 1976, Charles J. Lada and Mark J. Reid published the discovery of a bright-rimmedmolecular

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 12

COemission 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 kiloparsecsas determined from

its color-magnitude diagram.[13] VY CMa is projected onto the tip of the cloud 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.[15] A

more recent measurement of the distance to NGC 2362 gives 1.2 kpc.[16]

Size[edit]

Right to left: VY Canis Majoris compared to Betelgeuse, Rho Cassiopeiae, the Pistol Star, and theSun (too small to

be visible in this thumbnail). The orbits of Jupiter and Neptune are also shown.

Relative sizes of some planets in the Solar System and several well-known stars, including VY Canis Majoris

1. Mercury < Mars < Venus < Earth

2. Earth < Neptune < Uranus < Saturn < Jupiter

3. Jupiter < Proxima Centauri < Sun < Sirius

4. Sirius < Pollux < Arcturus < Aldebaran

5. Aldebaran < Rigel < Antares < Betelgeuse

6. Betelgeuse < VY Canis Majoris < NML Cygni < UY Scuti.

VY Canis Majoris is a Class M hypergiant. Its true size is debated, as it ejects much of its mass off its atmosphere in

wild flares that stretch well beyond the star itself. Artist's impression.

University of Minnesota Professor Roberta M. Humphreys originally approximated the radius of VY

CMa at 1,800-2,100 solar radii,[17] which would make it the largest known star by radius. However, a

more recent and more accurate VLTI measurement[8] gives the star a radius of 1420 ± 120 solar

radii.

Luminosity[edit]

In 2006, Humphreys used the spectral energy distribution distance of VY Canis Majoris 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 Canis

Majoris has a luminosity of 5.6×105 L☉.[17][18]

More recent estimates of the luminosity using a variety of methods give lower values of

around 3×105 L☉.

Circumstellar nebula[edit]

VY Canis Majoris is surrounded by an extensive nebula that shows condensations that were taken

as companion stars[12] and that has been extensively studied with the aid of the Hubble Space

Telescope, showing a complex structure with filaments and arcs caused by past eruptions, with a

structure similar of the one surrounding the yellow hypergiant IRC+10420, something that has led

some astronomers to suggest that VY Canis Majoris will become an object similar to the former,[19] and later a Wolf-Rayet star.[12]

Controversy[edit]

VY Canis Majoris ejects huge amounts of gas during its outbursts.[20]

There have been conflicting opinions of the properties of VY CMa. In one view,[17]the star is a very

large and very luminous red hypergiant. The various larger estimates of the size and luminosity fall

outside the bounds of current stellar theory, both beyond the maximum predicted size of any star

and far cooler than a star of its luminosity can become. In another opinion (such as Massey,

Levesque, and Plez's study),[10] the star is a normal red supergiant, with a radius around 600 solar

radii and falling comfortably inside models of stellar structure and evolution. More recent papers [7]

[8] produce intermediate values for radius and luminosity, falling at the very extreme for the expected

size and luminosity of red supergiants (or hypergiant based on its emission spectrum and high mass

loss rate).

VY Canis Majoris also illustrates the conceptual problem of defining the "surface" (and radius) of

very large stars. With an average density of 0.000005 to 0.000010 kg/m3, the star is a hundred

thousand times less dense than the atmosphere of the Earth (air) at sea level. It is also undergoing

strong mass loss with the outer layers of the star no longer gravitationally bound. The definition of

the boundary of such stars is based on the Rosseland Radius, the location at which the optical

depth is one (or sometimes a different value such as 2/3).[21] In cases such as VY CMa, the radius

may be defined on a different opacity value or on an opacity at a particular wavelength. [8]