VY Canis Majoris - Controversy

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 (such as Massey, Levesque, and Plez's study), 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 (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 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 opacity is one (or sometimes a different value such as 2/3). In cases such as VY CMa, the radius may be defined on a different opacity value or on an opacity at a particular wavelength.