Short answers, pulled from the story.
O-type main-sequence stars have surface temperatures between 30,000 and 50,000 Kelvin, making them the hottest class of hydrogen-burning stars. Their extreme temperatures give them a blue-white color and cause them to radiate heavily in ultraviolet light.
There are estimated to be no more than 20,000 O-type stars in the entire Milky Way, roughly one in every ten million stars. Their scarcity results from their extremely short lifespans of only a few million years.
O-type main-sequence stars range from about 15 to 90 times the mass of the Sun. Their luminosities range from 40,000 to one million times that of the Sun.
The foundational standards were set in the 1943 Morgan-Keenan-Kellerman atlas, with revisions in 1953 by Johnson and Morgan and again in 1973 by Morgan and Keenan. The earliest subtypes received formal luminosity-class standards in the 1978 atlas by Morgan, Abt, and Tapscott, and spectral class O2 was defined by Walborn and colleagues in 2002.
Zeta Ophiuchi is the brightest O-type main-sequence star in the sky, shining at third magnitude. It is classified as an O9.5 main-sequence star and was also used as a classification standard for that spectral subtype.
As of 2026, no planets have been discovered around any O-type main-sequence star, primarily because these stars live for only a few million years. One brown dwarf companion has been found orbiting the O-type star CEN 16, making it the only known substellar companion around such a star.