The color of a star is defined by its temperature. The coolest stars are red, while the hottest stars are blue. And the temperature of a star is defined by its mass. The most massive stars in the Universe are the blue supergiant stars; then can have more than 20 times the mass of the Sun. Blue giant stars are very hot, with surface temperatures of 20,000-50,000 Kelvin. Just for comparison, our own Sun is only 6,000 Kelvin.
Blue supergiant star have extremely high masses, sometimes with dozens of times the mass of the Sun. They form in the largest, most active star forming regions where large amounts of mass can come together to form the biggest stars: star clusters, the arms of spiral galaxies and in irregular galaxies.
Perhaps the best known example of a blue supergiant star is Rigel, located in the constellation Orion. It has about 20 times the mass of the Sun, and puts out 60,000 times as much energy.
Blue supergiants can turn into red supergiants and vice versa. When the star is smaller and more compact, its luminosity is contained over a smaller surface area and so its temperature is much hotter; this is the blue supergiant phase. These stars can then puff up expanding to a much larger size, spreading their luminosity over a much larger area. Then they become red supergiant stars, and appear the cooler red color. Astronomers think supergiants can fluctuate back and forth between red and blue supergiant, puffing off an outer layer of material with each contraction.
Eventually a supergiant runs out of material to continue supporting fusion in its core, and will detonate as a supernova – one of the brightest explosions in the Universe.
We have written many articles about stars on Universe Today. Here’s an article that talks about the constellation Orion, including the star Rigel, and here’s a nice picture of Rigel passing behind Saturn.