<P> 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 kelvins (K) and so appear to the left on the Hertzsprung--Russell diagram . Stars of this type are identified by their dominant Helium II absorption lines, strong lines of other ionised elements, and Hydrogen and neutral He lines weaker than spectral type B . </P> <P> Stars of this type are particularly rare; only 0.00003% of the main sequence are O - type stars . However, because they are usually very bright, they can be seen when further away than dimmer stars, and four of the 90 brightest stars as seen from Earth are O type . Due to high temperature and luminosity, 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 the central stars of planetary nebulae, old low - mass stars near the end of their lives, also usually have O spectra . </P> <P> 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 are also frequent in multiple star systems where their evolution is more difficult to predict due to mass transfer and the possibility of component stars going supernova at different times . </P> <P> 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 He 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 He lines have equal strength . The very hottest O - type stars have such weak neutral He lines that they are best separated on the relative strength of the N and N lines . </P>

Where do you expect to find an o−type star