Universe Today had a nice little column predicated on the question of how just how big is the biggest star in the universe is. Naturally, a difficult question given the size and scale of the universe, but something a little easier is how large is the largest star we puny humans have observed from our vantage point along the outer edge of the Milky Way galaxy?
Naturally, we use our parent star as the basis of comparison for other stars, using the terms solar mass (2 x 10 to the 30th power kilograms) and solar radius (432,000 miles). You could fit about 1 million Earth size planets inside the Sun, which accounts for 99.9% of all the mass in the solar system.
However, our sun is actually a pretty small to mid size star. The largest observed star in the Milky Way is called VY Canis Majoris, located in the Canis Majoris constellation around 5,000 light years from our solar system. Such a large star is very hard to measure because they generate so much solar wind that they lose a mass equal or greater than a gas giant size planet every year. Estimates of VY Canis Majoris size vary from as much as 2100 solar radii to 600 solar radii. The first estimate would place the surface of the star outside the orbit of Saturn, the latter outside the orbit of Mars - either way, really really big.
The author contacted the astronomer responsible for the larger of the VY Canis Majoris estimates, Roberta Humphreys, to ask how large a star could get in theory.
"I contacted Roberta Humphreys from the University of Minnesota, the researcher who calculated the size of VY Canis Majoris, and posed this question to her. She noted that the largest stars are the coolest. So even though Eta Carinae is the most luminous star we know of, it's extremely hot - 25,000 Kelvin - and so only a mere 400 solar radii. The largest stars will be the cool supergiants. For example, VY Canis Majoris is only 3,500 Kelvin. A really big star would be even cooler. At 3,000 Kelvin, a cool supergiant would be 2,600 times the size of the Sun.
That, she believes, is the largest possible star."
Interesting question, and an even more interesting answer.