Class O — O-type (Blue)

O-type stars are the hottest and most luminous main-sequence stars. With surface temperatures exceeding 30,000 K, they appear blue-white and emit strongly in the ultraviolet. They are extremely rare, comprising less than 0.00003% of main-sequence stars, but their enormous luminosity makes them visible across vast distances. O-type stars have short lifetimes of only a few million years and end their lives as supernovae, often leaving behind neutron stars or black holes. Examples include the Trapezium stars in Orion.

O-type stars are the hottest, most luminous, and rarest stars in the universe. With surface temperatures exceeding 30,000 K and luminosities up to millions of times that of the Sun, they are cosmic powerhouses that dominate the appearance of young star-forming regions despite comprising less than 0.00003% of main-sequence stars. Their intense ultraviolet radiation ionizes surrounding hydrogen gas, creating the glowing H II regions and emission nebulae that mark spiral arms in galaxies.

Characteristics

O-type stars appear blue-white and have masses ranging from 15 to over 100 solar masses. Their spectra show ionized helium (He II) absorption lines — a defining feature, as helium requires extreme temperatures to become doubly ionized. These stars burn through their hydrogen fuel at prodigious rates: while the Sun will shine for 10 billion years, an O-type star exhausts its fuel in just 1-10 million years before exploding as a core-collapse supernova, often leaving behind a neutron star or black hole. Their powerful stellar winds eject material at speeds exceeding 2,000 km/s.

Notable Examples

Famous O-type stars include Alnitak (Zeta Orionis), the leftmost star of Orion's Belt at type O9.5Ib and 250,000 times the Sun's luminosity. Naos (Zeta Puppis, O4If) is one of the hottest stars visible to the naked eye at approximately 40,000 K. Theta-1 Orionis C, the brightest member of the Trapezium Cluster illuminating the Orion Nebula, is a young O6 star responsible for sculpting one of the sky's most famous nebulae. Many O stars are found in OB associations — loose groupings of young, hot stars born from the same molecular cloud.

HIP 52558 HIP 52628 HIP 52697 HIP 52849 HIP 53686 HIP 54175 HIP 54358 HIP 54475 HIP 54902 HIP 55078 HIP 55833 HIP 55995 HIP 56021 HIP 56134 HIP 56196 HIP 56331 HIP 56726 HIP 56757 HIP 56769 HIP 56833 HIP 56897 HIP 56961 HIP 59288 HIP 61482 HIP 61772 HIP 61958 HIP 63117 HIP 63449 HIP 64018 HIP 64624 HIP 64737 HIP 65307 HIP 65637 HIP 65890 HIP 67642 HIP 6844 HIP 68995 HIP 69261 HIP 69628 HIP 69892 HIP 70052 HIP 70074 HIP 72510 HIP 74941 HIP 77391 HIP 77994 HIP 7845 HIP 79172

Frequently Asked Questions

What is a class O star?

Class O (O-type (Blue)) stars are blue stars. O-type stars are the hottest and most luminous main-sequence stars. With surface temperatures exceeding 30,000 K, they appear blue-white and emit strongly in the ultraviolet. They are extremely rare,

How hot are class O stars?

Class O stars have surface temperatures between 30,000 K and 50,000 K.

What color are class O stars?

Class O stars appear blue.

How many class O stars are in the StarFYI database?

StarFYI currently catalogs 264 class O stars.

How luminous are class O stars?

Extremely luminous, 30,000-1,000,000 solar luminosities