VY Canis Majoris (VY CMa) is a red hypergiant star located in the constellation Canis Major. It is the largest known star and also one of the most luminous stars known. It is located about 1.5 kiloparsecs (or 5,000 light-years) from Earth.
Roberta M. Humphreys (2006) estimates the radius of VY CMa at 1800 to 2100 solar radii. To illustrate, if our Sun was replaced with VY Canis Majoris, its surface would extend to the orbit of Saturn. Assuming the upper size limit of 2100 solar radii, light would take more than 8 hours to travel around the star's circumference.
Similarly, if a human could walk on the surface of Canis Majoris – assuming a speed of 5 km/h (3 mph) for 8 hours a day – the person would have to walk for 650,000 years to circle the star (compared with 2 years 11 months to complete the same task on the Earth, and 310 years 7 months on the Sun).
There are two controversial opinions of the properties of VY CMa. One view (such as Roberta M. Humphreys' study), the star is a very large and very luminous red hypergiant. In another opinion (such as Massey, Levesque, & Plez's study), the star is a normal red supergiant, with a radius around 600 solar radii. In this case, its surface would extend well past the orbit of Mars.