In 2005 it was reported by a team of astronomers from the Lowell Observatory in Arizona that the three largest stars ever known had been found. All three are red supergiants and the largest is named KW Sagitarii. It is almost 10 thousand light years away, in the constellation Sagittarius (hence the second half of its name). The other two supergiants are V354 Cephei and KY Cygni. KW Sagitarii is so large that, if it were placed in the position of the Sun, its outer edge would lie between the orbits of Jupiter and Saturn. It is a whopping 1500 times the size of the Sun.
Stars are born when clouds of gas and dust, nebulae, condense due to gravitational forces. When the mass and heat become great enough they burst into light and flame when nuclear reactions begin in the core. For the first part of a star's life, hydrogen is fused into helium. When the hydrogen is used up, after several billion years, the star expands into a red giant or supergiant and continues to shine by the energy produced from the nuclear fusion of helium into carbon and oxygen. Whether the star becomes a giant or supergiant depends on its original size. The Sun is medium-sized and will expand into a red giant. The star that expanded into KW Sagitarii must have been very much larger.
The color of a star is determined by its composition and its temperature. Very hot stars like Sirius appear to be bluish white. Stars of a medium temperature like the Sun appear yellow. The surface temperature of a red giant is relatively cool, which gives it a red color. A red star is a mere 2500 Kelvin, instead of 6000 K for a yellow star or 18000 K for a blue star. The brightness of a star is dependent on not only its absolute luminosity but how far away it is. KW Sagitarii is 38000 times as luminous as the Sun, but it is also very far away. There are other red giants that appear brighter than KW Sagitarii, such as Antares in the constellation Taurus, but this is because Antares is much closer than KW Sagitarii.
Although the surface of a red supergiant is relatively cool, the centre is subject to tremendous gravitational forces and has to burn much hotter to be stable. This has a big effect on the way that these stars die at the end of their stellar lives. Red supergiants like KW Sagitarii will come to a spectacular end. This will not occur for billions of years, but when the last of the helium is fused into heavier elements and the star has no more energy to burn, it will go out with a bang in a spectacular supernova. The Sun is too small for such a spectacular end. It will eventually expand into a red giant, which will consume the Earth, but when its nuclear fuel runs out, it will not have the mass to supernova. Instead, it will shrink into a white dwarf and eventually blink out.