The radius has been estimated to come in at about 8.2AU (possibly even up to 10.2AU!), but the term, ‘surface,’ has no real definition here. You see, in the outer layers of the star, its density is so low, that it may be more comparable to a vacuum than a star. Its gargantuan size and properties have even sparked debate as to whether or not we can consider it a definite star, or if its more akin to a spherical nebula burning at 3000k! It is generally agreed that it is a star – and it isn’t alone. VY CMa belongs to a very exclusive group of stars, dubbed hypergiants. Hypergiants are so massive that they devour themselves at exponential rates – in other words, the amount of energy our Sun emits in year is equal to what a hypergiant would release in just 6 seconds.
This enormous level of activity comes at a cost though, as stars belonging to this class typically live a very short and turbulent life. Their lives are only measurable in millions of years; for comparison our Sun is a 2nd generation main sequence star, and will live for about 10 billion years – so currently, its about halfway through its life cycle. As well as burning fuel faster than their smaller counterparts, their lives are further shorted by violent explosions and stellar outbursts. As a result, it is believed that VY CMa has already shed half of its original mass, so we can only speculate on how large the star was during its inception. Suffice to say, it could put our sun to shame.