When the French astronomer Jérôme Lalande first observed VY Canis Majoris in 1801, he described it as a seventh magnitude star like many others. Certainly, Lalande did not expect that this faint dot was actually a very bright red hypergiant and one of the largest stars in our Galaxy.
VY Canis Majoris is a star still shrouded in many mysteries, whose physical properties are little known.
One of the few well-known aspects is its variability: the star indeed varies its brightness over a cycle of 956 days, going from an apparent magnitude of 9.6 to one of 6.5, at the limit of visibility to the naked eye.
As for the distance, the value is very uncertain, but the best estimates suggest it is located 4000 light-years away from us.
From the spectrum of VY Canis Majoris, it has been found that it is an M-type star with a surface temperature below 4000 Kelvin, in addition to some information on the atmospheric composition.
From visible and infrared photometry, it has been derived that the star should be 350,000 times more luminous than the Sun, thus making it one of the most luminous stars in the entire Milky Way. The star, however, is surrounded by a shell of gas expelled in the past, which absorbs much of the radiation making it invisible to the naked eye from Earth.
According to the most recent estimates, the star is indeed expelling about 0.01 solar masses per year into space. Consequently, its current mass, estimated at 17 solar masses, is much lower compared to its original mass.
The estimate of the diameter of VY Canis Majoris is strongly influenced by the values assumed for the other stellar parameters and by the presence of the shell that surrounds it. Estimates of its size vary between 1400 and 2000 solar diameters, placing it in any case among the largest stars in the Milky Way.
In this artistic representation, the size of VY Canis Majoris is compared to that of the Sun and Earth's orbit.
Image Credit: Oona Räisänen.