VY Canis Majoris is the largest known star.
The largest known star is VY Canis Majoris, a red hypergiant measuring between 1,800 and 2,100 solar radii. Its volume is almost a billion times greater than that of the Sun, although its density is much lower. Canis Majoris means large dog in Latin. If it were located in the solar system, its surface would reach as far as the orbit of Saturn. Another way of saying this is that this star is about 9 astronomical units (AU) across, nine times the distance between Earth and the Sun. There must be larger stars located in other galaxies, but we currently don’t have telescopes powerful enough to to solve them. Hyperteloscopes can help in this regard.
Eventually, the star VY Canis Majoris will collapse in a supernova explosion and become a black hole.
VY Canis Majoris is a star in its final death throes. It is ejecting massive amounts of material into a surrounding nebula that causes the star to be blocked out in the visible spectrum. It should be observed in the infrared portion of the spectrum. The star’s death nebula is about 4,500 AU wide, about fifty times larger than the star itself, and much larger than our solar system. Within the gas nebula is a smaller circumstellar dust region, which has a temperature of 760 K and a width of about 260 AU. The star’s surface probably has a temperature of around 3650 K, extremely cold for a star.
Current telescope technology makes it impossible to find stars larger than VY Canis Majoris.
Unlike main sequence stars like our Sun, VY Canis Majoris does not have a distinct photosphere and thus simply disappears into space. While it is the largest known star, it is definitely not the most massive, in part because it has already expelled much of its mass into the surrounding nebula.
Like all red giants and hypergiants, VY Canis Majoris is so large because it has used up all the hydrogen fuel in its core and has begun to fuse hydrogen in a shell outside of a helium core. In fact, VY Canis Majoris is so big that it can fuse helium, lithium, etc., all the way down the periodic table to iron and beyond. Eventually, it will have a core made mostly of iron, just like the planets. The problem with iron post-fusion reactions is that they don’t produce energy and therefore can’t balance the gravitational pressure generated by the star. When all the fusion fuel is gone, the star will catastrophically collapse in a supernova explosion and become a black hole.