O-type main-sequence star
Main-sequence stars of spectral type O (or O-stars of luminosity class V ) are the largest, hottest and most massive ( 15 to 90 times the solar mass ) stars of the main sequence and are located on the top left in the Hertzsprung -Russell diagram. The most massive stars are in their initial phase of this class of stars.
Stars of this type usually have a mass of about 50 solar masses. Their luminosity 30000-1 million times larger than that of the sun. Due to the higher energy radiation has a higher frequency and appear as a blue light. These stars shine in the ultraviolet range. You have a physical resemblance to Blue Giant, but are smaller than them. Their surface temperatures are between 30,000 K and 52,000 K.
Since the relative proportion of massive stars is small and their life short (due to the faster inner fusion reaction with increasing mass ), there are very few stars of this type ( only about one in 10 million stars, totally in the Milky Way about 20,000 ).
O stars are very common in binary systems. the proportion has been estimated to be more than 75 %, with the discovery of a pet by the high brightness of the O- star, and the rapid rotation of the star early difficult. The fast rotation propagated by the Doppler effect, the spectral lines and in combination with the high luminosity, the spectral lines of the companion no longer detectable. Because of the high rate of double stars, the O -star probably arise mainly from the merger in interacting multiple systems. These massive stars are the precursors of core-collapse supernovae, gamma ray bursts, a part of, Millisekundenpulsare and the double neutron star.
The blue gnomes on and near the main sequence are classified in subclass O Vz based on the relation of the equivalent width of the helium lines HeII4686/HeI4541. These stars with a relatively weak HeI4541 absorption line are on average younger and have both a lower luminosity and stellar winds. The emergence of Vz subclass could also be strongly dependent on the metallicity. If the HeII4686 line in emission on as the stars are classified as Of stars. These stars have strong stellar winds and evolve rather been away from the main sequence.
A case of suitable observation conditions visible to the naked eye representatives of these stars is the smaller companion binary star almaak ( γ And) in the constellation Andromeda like with an apparent magnitude of about 5. Other examples of O- stars of the main sequence are ζ Ophiuchi, σ Orionis A and 10 Lacertae.