There are many categories, or classes, into which a star can form, based its temperature and luminosity. The size of the star is related to these factors. Supergiant stars are typically 10,000 times brighter than the Sun and 100 to 1000 time larger (that is that they have a radius 100 to 1000 times larger).
But how big is the biggest star? Well firstly lets explain that biggest is not necessarily brightest nor most massive. You might for example have a very dense, small star which is technically more massive than a low density giant star.
If we take biggest to mean physical dimension we can say that we looking for the star with the largest radius. The southern hemisphere star, Eta Carinae is almost 11 AUs in radius. AU stands for Astronomical Unit and is the distance between the Sun and Earth. 11 AU means that if you swapped Eta Carinae for the Sun, it would extend outward to engulf Jupiter’s orbit.
However it seems that the generally accepted winner in the largest star category is the Pistol Star, with an radius 340 times larger than the Sun’s. It is also 150 times more massive than our Sun and a million times more luminous.
Like Eta Carinae, it is part of the category of stars known as Luminous Blue Variables. It can found in the constellation of Sagittarius and only ranks in at a measly magnitude +4 when viewed from Earth compared to Sirius at -1.47 and Venus which sometimes reaches -4 or more (I should point out that the lower the magnitude, the brighter the star).
