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The O5 spectral type is within the O-class, specifically, stars with a surface temperature of around 45000 K. Characteristics of main-sequence O5 stars:
| 12 | radius(solar) |
| 60 | mass(solar) |
| -5.7 | absolute magnitude(visual) |
| 790,000 | bolometric luminosity(solar) |
| 3,000,000 years | lifetime |
| 0.00001% | abundance of all O-type stars |
Until the 1970s, O5 was the earliest (hottest) defined spectral type. (O stars are rare, and I presume as time has passed, a larger number of O stars have been found, including hotter stars, thus larger spectral-type numbers came to be used.)