| Term | Definition | Contributor | Modified |
|---|---|---|---|
| O stars | A luminous, hot, blue star whose spectrum is dominated by the lines of hydrogen, atomic helium, and ionized helium; also known as O-type star. This is the earliest spectral type and the only main sequ ence star in which ionized helium is present. The effective temperatures of these stars range from about 30,000 K to 50,000 K, their luminosities from 50,000 to 1,000,000 times that of solar luminosity, and their masses from about 20 to 100 solar masses. The hottest O-type stars display high ionization emission features such as N III and He II, Of star. They are divided into subtypes O2, the hottest, to O9.7, the coldest. O-type stars are relatively rare, for each star of 100 solar masses there are 10^6 stars of solar mass. They are relatively short-lived since they spend only a few million years on the main sequence. The brightest O-type star in the sky visible with naked eye is Alnitak. | Ryan McGranaghan | 2023.04.16 |
| Ostend--Siege, 1601-1604 | No definition provided | Christopher Rauch | 2023.12.01 |
| Osteoarthritis | No definition provided | Christopher Rauch | 2023.12.01 |
| Osteoclasis | No definition provided | Christopher Rauch | 2023.12.01 |
| Osteology | No definition provided | Christopher Rauch | 2023.12.01 |
| Osteomalacia | No definition provided | Christopher Rauch | 2023.12.01 |
| Osteomyelitis | No definition provided | Christopher Rauch | 2023.12.01 |
| Osteopathy | No definition provided | Christopher Rauch | 2023.12.01 |
| Osteoplasty | No definition provided | Christopher Rauch | 2023.12.01 |
| Osteotomy | No definition provided | Christopher Rauch | 2023.12.01 |
| Ostiaks | No definition provided | Christopher Rauch | 2023.12.01 |
| Ostracoda | No definition provided | Christopher Rauch | 2023.12.01 |
| Ostracopoda | No definition provided | Christopher Rauch | 2023.12.01 |
| Ostraka | No definition provided | Christopher Rauch | 2023.12.01 |
| Ostrea | No definition provided | Christopher Rauch | 2023.12.01 |
| Ostrich | No definition provided | Christopher Rauch | 2023.12.01 |
| Ostrich farms and farming | No definition provided | Christopher Rauch | 2023.12.01 |
| Ostrogoths | No definition provided | Christopher Rauch | 2023.12.01 |
| Ostyaks | No definition provided | Christopher Rauch | 2023.12.01 |
| O subdwarf stars | No definition provided | Ryan McGranaghan | 2023.04.16 |