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Strömgren sphere(strohm -grĕn) An approximately spherical region of ionized gas, mainly ionized hydrogen (H II), that surrounds a hot O or B star. If a hot star were embedded in very low-density gas of uniform distribution, the ultraviolet radiation from the star would almost completely ionize the gas out to a particular radius – the Strömgren radius. Equilibrium is established within this volume when the rate of ionization is balanced by the rate of recombination of ions with electrons. Outside this volume lies an ionization front – a thin transition zone between ionized gas and much cooler neutral gas. When the star ‘turns on’, the ionization front expands outward at about 10 km s–1 until the Strömgren radius is reached. If the gas cloud is relatively small and of low density, the entire cloud can be ionized. The Strömgren radius depends on the gas density and on the flux of ionizing radiation, i.e. of ultraviolet photons. The flux is dependent mainly on the star's temperature. See also emission nebula.
Strömgren sphere[′strəm·grən ‚sfir]
An approximately spherical region of ionized hydrogen that surrounds a hot star.