Blue stars are typically hot, O-type stars that are commonly found in active star-forming regions, particularly in the arms of spiral galaxies, where their light illuminates surrounding dust and gas clouds making these areas typically appear blue.

Blue stars are also often found in complex multi-star systems, where their evolution is much more difficult to predict due to the phenomenon of mass transfer between stars, as well as the possibility of different stars in the system ending their lives as supernovas at different times.

Stars are mainly characterized by the strong Helium-II absorption lines in their spectra, and the hydrogen and neutral helium lines in their spectra that are markedly weaker than in B-type stars. Because blue stars are so hot and massive, they have relatively short lives that end in violent supernova events, ultimately resulting in the creation of either black holes or neutron stars. A star in the main sequence is in a state of hydrostatic equilibrium. Gravity is pulling the star inward, and the light pressure from all the fusion reactions in the star is pushing outward.

The inward and outward forces balance one another out, and the star maintains a spherical shape. Stars in the main sequence will have a size that depends on their mass, which defines the amount of gravity pulling them inward. Around 90 percent of the stars in the Universe are main-sequence stars, including our sun. The main sequence stars typically range from between one-tenth to 200 times the Sun’s mass.