VY Canis Majoris 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), and about 1.2 kiloparsecs 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 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 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.
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 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☉.
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. 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 originally estimated the radius
of VY CMa at 1,800 to 2,100 solar radii, which would make it the largest known star by radius.
However, a more recent VLTI direct measurement gives a radius of 1420 ± 120 solar radii.
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 VY Canis Majoris is surrounded by an extensive
nebula that shows condensations that were taken as companion stars 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, and later a Wolf-Rayet star.
Controversy
There have been conflicting opinions of the properties of VY CMa. In one view, 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,
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 produce
intermediate values for radius and luminosity, falling at the very extreme for the expected
size and luminosity of red supergiants. VY Canis Majoris also illustrates the conceptual
problem of defining the "surface" 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 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. In cases
such as VY CMa, the radius may be defined on a different opacity value or on an opacity
at a particular wavelength. See also
Orders of magnitude List of largest known stars
List of most luminous stars WASP-17b
R136a1 NML Cygni
UY Scuti Westerlund 1-26
References
Further reading Kastner, Joel H.; Weintraub, David A.. "Hubble
Space Telescope Imaging of the Mass-losing Supergiant VY Canis Majoris". Astronomical
Journal 115: 1592–1598. Bibcode:1998AJ....115.1592K. doi:10.1086/300297.
External links Astronomers Map a Hypergiant Star's Massive
Outbursts, HubbleSite NewsCenter, 2007-01-08 "What is the Biggest Star in the Universe?",
Fraser Cain, Universe Today, published 2008-04-06, updated May 13, 2013
Amos, Jonathan. "Herschel telescope 'fingerprints' colossal star". BBC News. Retrieved 30 November
2009. "The death throes of one of the biggest stars known to science have been spied by
Europe's Herschel space telescope." American Association of Variable Star Observers
Remote Sensing Tutorial Page A-5 archive