Of all known stars, the VY Canis Majoris is the largest. This red Hypergiant star, found in the constellation Canis Major, is estimated to have a radius at least 1,800 that of the Sun’s. In astronomy-speak we use the term 1,800 solar radii to refer to this particular size. Although not the most luminous among all known stars, it still ranks among the top 50.
Hypergiants are the most massive and luminous of stars. As such, they emit energy at a very fast rate. Thus, hypergiants only last for a few million years. Compare that to the Sun and similar stars that can keep on burning up to 10 billion years.
VY Canis Majoris a.k.a. VY CMa is about 4,900 light years from the Earth. This value, however, is just a rough estimate because it is too far for parallax to be used. Parallax is the most common method for measuring star distances. It is actually a special kind of triangulation method, i.e., similar to the one employed by engineers that make use of angles and a fixed baseline.
As of January 2009, we now know about some other really big stars. One is called Eta Carinae. It has a size about 800 times that of our Sun, a mass about 100 times that of our Sun, and is about 4,000,000 times brighter than our Sun. And, yet, we do not think it is the biggest! Recent observations of a star called VY Canus Majoris show that it has a size between 600 and 2100 times the size of our Sun! However, this star is only about 500,000 times brighter than our Sun and 30 or so times more massive than our Sun. So how can it be so big? It is in a stage of its lifecycle called a red hypergiant, and while it is very big, it is rather cool in temperature compared to our Sun.