OB stars are hot, massive stars of spectral types O or early-type B that form in loosely organized groups called OB associations. They are short lived, and thus do not move very far from where they formed within their life. During their lifetime, they will emit much ultraviolet radiation. This radiation rapidly ionizes the surrounding interstellar gas of the giant molecular cloud, forming an H II region or Strömgren sphere.

In lists of spectra the "spectrum of OB" refers to "unknown, but belonging to an OB association so thus of early type".^{[dubious – discuss]}

• O-type main-sequence star
• B-type main-sequence star
• Stellar kinematics

• M. Pantaleoni González (2021). "The Alma catalogue of OB stars - II. A cross-match with Gaia DR2 and an updated map of the solar neighbourhood". Monthly Notices of the Royal Astronomical Society. 661 (2): 2968–2982. arXiv:2103.02748. Bibcode:2021MNRAS.504.2968P. doi:10.1093/mnras/stab688.

• B. Cameron Reed (2003). "Catalog Of Galactic OB Stars". The Astronomical Journal. 125 (5): 2531–2533. Bibcode:2003AJ....125.2531R. doi:10.1086/374771.

• Bouy, Hervé and Alves, João: Cosmography of OB Stars in the Solar Neighborhood Astronomy & Astrophysics (December 2015). A three-dimensional map of OB star density within 500 pc of the Sun.
• [1] — Scientia Astrophysical Organization's star classification page
• Philippe Stee's homepage: Hot and Active Stars Research