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VY Canis Majoris (VY CMa) is the largest known star and also one of the most luminous. Located in the constellation Canis Major, it is a red hypergiant, between 1800 and 2100 solar radii, 8.4–9.8 astronomical units in radius, about 3.0 billion km or 1.9 billion miles in diameter, and about 1.5 kiloparsecs (4,900 light years, 4.6×1016 km or 2.9×1016 mi) distant from Earth. Unlike most hypergiant stars, which occur in either binary or multiple star systems, VY CMa is a single star. It is categorized as a semiregular variable and has an estimated period of 2,000 days. It has an average density of 0.000005 to 0.000010 kg/m3.

Placed at the center of our solar system, VY Canis Majoris’s surface would extend beyond the orbit of Saturn, although the astrophysicists Philip Massey, Emily Levesque and Bertrand Plez disagree about the star’s stated radius, suggesting it is smaller: merely 600 times the size of the Sun, extending past the orbit of Mars.¹²

Contents
[hide] 1 Nature of VY Canis Majoris
2 Measuring the distance
3 Size
4 Luminosity
5 Controversy
6 Life expectancy
7 See also
8 References
9 Further reading
10 External links

[edit] Nature of VY Canis Majoris

The first known recorded observation of VY Canis Majoris is in the star catalogue of Jérôme Lalande, on March 7, 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 1850.¹³

Since 1847, VY CMa has been known to be a crimson star.¹³ During the 19th century, observers measured at least six discrete components to VY CMa, suggesting the possibility that it is 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.⁷¹³

VY CMa is a high-luminosity M star with an effective temperature of about 3,000 K, placing it at the upper-right hand corner of the Hertzsprung–Russell diagram and suggesting that it is a complexly created star. During its main sequence, it would have been an O star¹¹ with a mass of about 30 to 40 M☉.⁷

[edit] Measuring the distance

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-rimmed molecular 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 12CO emission 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 kiloparsecs as determined from its color-magnitude diagram.¹¹

VY CMa is projected onto the tip of the 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.⁶

[edit] 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.

Relative sizes of the 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 < Wolf 359 < Sun < Sirius
4. Sirius < Pollux < Arcturus < Aldebaran
5. Aldebaran < Rigel < Antares < Betelgeuse
6. Betelgeuse < Mu Cephei < VV Cephei A < VY Canis Majoris.
University of Minnesota Professor Roberta M. Humphreys estimates the radius of VY CMa at 1,800 to 2,100 solar radii.⁹ To illustrate, if Earth’s Sun were replaced by VY Canis Majoris, its radius might extend beyond the orbit of Saturn (about 9 AU). Assuming the upper size limit of 2100 solar radii, light would take more than 8.5 hours to travel around the star’s circumference, compared to 14.5 seconds for the Sun. It would take 7×1015 Earths to fill the volume of VY Canis Majoris.¹⁵

If the Earth were to be represented by a sphere one centimeter (0.39") in diameter, the Sun would be represented as a sphere with a diameter of 109 centimeters (3½ ft.), at a distance of 117 meters. At these scales, VY Canis Majoris would have a diameter of approximately 2.3 kilometers (1.43 mi.), assuming the upper limit estimate of its radius.

[edit] Luminosity

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 4.3×105 L☉.⁹¹⁶

[edit] Controversy

VY Canis Majoris ejects huge amounts of gas during its outbursts.¹⁷
There are two conflicting opinions of the properties of VY CMa. In one view,⁹ the star is a very large and very luminous red hypergiant. In another opinion (such as Massey, Levesque, and Plez’s study),¹² the star is a normal red supergiant, with a radius around 600 solar radii. In this case, its surface would extend well past the orbit of Mars in comparison to our solar system.

Like its size, the luminosity of VY CMa is also the subject of debate. Humphreys contests that visual photometry is not sufficient for stars with enough circumstellar dust to reprocess the visual and red fluxes into the thermal infrared.⁹

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 thousand times less dense than the atmosphere of the Earth (air) at sea level. Since the star does not have a uniform density—as it must have a fusing core—the actual density of the outer “surface” at the radius and mass stated (mass ÷ volume, where the volume of a sphere is 4/3 * pi * r3) is far less dense than even the outer atmosphere of the Earth. This presents a dilemma for cataloging star radii since, technically, a supernova remnant like the Crab Nebula still has a fusing (neutron) “core” surrounded by an (11 light-year wide) “atmosphere”, while statements of our Sun’s radius never include its corona (atmosphere of hot gas). This is all the more problematic since our sun’s corona is hotter and thicker than VY Canis Majoris’s “surface”. A dividing line at which a star’s ejected layers are no longer included in the radius of a star has yet to be established.

[edit] Life expectancy

The star has been discovered to be very unstable, having thrown off much of its mass into its surrounding nebula. Astronomers, with the help of the Hubble Space Telescope, have predicted that VY Canis Majoris will be blown up, as a hypernova, in the next 100,000 years.¹⁸ Theoretically, a hypernova would cause gamma ray bursts that could damage the contents of the local galaxy, wiping out any cellular life within a number of light years; however, no hypergiant is located close enough to Earth to pose a threat
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