Strömgren

Strömgren is a Swedish surname. A notable bearer is Bengt Strömgren (1908–1991), an influential astronomer who developed the Strömgren uvbyβ photometric system, a four-filter approach used to classify and analyze stars.

The Strömgren photometric system uses four intermediate-band filters, labeled u, v, b, and y, together with an Hβ filter. The resulting indices (u−v), (v−b), (b−y), and the Hβ measurement enable estimates of a star’s effective temperature, surface gravity, metallicity, and interstellar reddening. It is especially used for F to B-type stars and in studies of star clusters and stellar populations.

Strömgren sphere: In astrophysics, the Strömgren sphere is the region of ionized hydrogen around a hot, young star. Ionizing ultraviolet photons balance recombinations within a roughly spherical volume, defining a Strömgren radius R_s that depends on the ionizing photon rate Q_H, the ambient electron density n_e, and the recombination coefficient α_B. The concept is central to the study of H II regions and the early evolution of massive stars.