VY Canis Majoris (VY CMa) is a red hypergiant in the constellation Canis Major. It is one of the largest known stars by radius and also one of the most luminous of its type. It is approximately 1,420 ± 120 solar radii(6.6 astronomical units), about 1,975,000,000 kilometres (1.227×109 mi) in radius, and about 1.2 kiloparsecs (3,900 light-years) distant 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.
Nature of VY Canis Majoris
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.
Since 1847, VY CMa has been known to be a crimson star. 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.
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 meaning it is a highly evolved star. During its main sequence, it would have been an O star with a mass of 15 to 35 M
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.
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.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.[14] A more recent measurement of the distance to NGC 2362 gives 1.2 kpc.
Recent VLBI direct measurements of the parallax of VY CMa give a distance of 1.1–1.2 kpc.
Size
University of Minnesota Professor Roberta M. Humphreys estimates the radius of VY CMa at 1,800 to 2,100 solar radii.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 more than 1.2×1016 Earths to fill the volume of VY Canis Majoris,of which the average volume (VY Canis Majoris) is 1.414×1037 cubic meters.
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
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 5.6×105 L
More recent estimates of the luminosity using a variety of methods give lower values of around 3×105 L
Circumstellar nebula
As mentioned above, VY Canis Majoris is surrounded by an extensive nebula that shows condensations that were taken as companion stars[11] 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 have led some astronomers to suggest that VY Canis Majoris will become an object similar to the former,and later a Wolf-Rayet star.
Controversy
VY Canis Majoris ejects huge amounts of gas during its outbursts.
There have been conflicting opinions of the properties of VY CMa. In one view, the star is a very large and very luminous red hyper giant 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),the star is a normal red super giant with a radius around 600 solar radii and falling comfortably inside models of stellar structure and evolution. More recent papers produce intermediate values for radius and luminosity, falling at the very extreme for the expected size and luminosity of red super giants (or hyper giant 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 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 gravitational bound. The definition of the boundary of such stars is based on the Ross-eland Radius, the location at which the opacity is one. In cases such as VY CMa, the radius may be defined on a different opacity value or on an opacity at a particular wavelength.
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 Can-is Major-is will explode as a hypernova within the next 100,000 years[citation needed]. Theoretically, a hyper nova 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 hyper-giant is located close enough to Earth to pose a threat. The star is large enough to create an enormous black hole.