O-type star

An O-type star is a hot, blue-white star of spectral type O in the Yerkes classification system employed by astronomers. They have temperatures in excess of 30,000 kelvin (K). Stars of this type have strong absorption lines of ionised helium, strong lines of other ionised elements, and hydrogen and neutral helium lines weaker than spectral type B. Stars of this type are very rare, but because they are very bright, they can be seen at great distances and four of the 90 brightest stars as seen from Earth are O type. Due to their high mass, O-type stars end their lives rather quickly in violent supernova explosions, resulting in black holes or neutron stars. Most of these stars are young massive main sequence, giant, or supergiant stars, but some central stars of planetary nebulae, old low-mass stars near the end of their lives, also usually have O spectra. O-type stars are typically located in regions of active star formation, such as the spiral arms of a spiral galaxy or a pair of galaxies undergoing collision and merger (such as the Antennae Galaxies). These stars illuminate any surrounding material and are largely responsible for the distinct coloration of a galaxy's arms. Furthermore, O-type stars often occur in multiple star systems, where their evolution is more difficult to predict due to mass transfer and the possibility of component stars exploding as supernovae at different times. O-type stars are classified by the relative strength of certain spectral lines. The key lines are the prominent He+ lines at 454.1 nm and 420.0 nm, which vary from very weak at O9.5 to very strong in O2–O7, and the He0 lines at 447.1 nm and 402.6 nm, which vary from absent in O2/3 to prominent in O9.5. The O7 class is defined where the 454.1-nanometer He+ and 447.1-nanometer He0 lines have equal strength. The very hottest O-type stars have such weak neutral He lines that they must be separated on the relative strength of the N2+ and N3+ lines. The luminosity classes of O-type stars are assigned on the relative strengths of the He+ emission lines and certain ionised nitrogen and silicon lines.