Ionized hydrogen regions and the Strömgren photon budget
An ionization-bounded pure-hydrogen region in steady balance equates absorbed external ionizing photons to net recombinations: Q_H=∫α_B nₑnₚ dV.
Key knowledge
An ionization-bounded pure-hydrogen region in steady balance equates absorbed external ionizing photons to net recombinations: Q_H=∫α_B nₑnₚ dV. Case B excludes direct ground-state recombinations whose ionizing photons are locally reabsorbed. A uniform spherical fully ionized model has R_S=[3Q_H/(4πα_B nₑnₚ)]^(1/3). This is a photon budget, not an assertion that every nebula is spherical or in thermodynamic equilibrium.
Values and equations
Q_H=∫α_B nₑnₚ dV; R_S=[3Q_H/(4πα_B nₑnₚ)]^(1/3)
- Q_H
- absorbed external hydrogen-ionizing photon rate s⁻¹
- α_B
- case-B recombination coefficient at the specified temperature m³ s⁻¹
- nₑ,nₚ
- proper electron and proton number densities m⁻³
- V
- ionized gas volume m³
- R_S
- uniform spherical equilibrium radius m
Conditions and scope
- Fixed source, uniform temperature/coefficient in the spherical estimate, negligible dust absorption or escape, negligible collisional ionization and the case-B local reabsorption approximation. For pure fully ionized hydrogen nₑ=nₚ; other species require adjusted electron and photon accounting.
- Density-bounded gas, clumping, expansion, variable sources, leaking radiation and dust alter the result. Photoionized low-density gas need not obey the LTE Saha relation; no universal region radius, temperature or star-formation conversion is inferred.
Sources · 3
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
University of Maryland — J. P. Harrington teaching notes2 sources
- Physics of Ionized Gasses in the Interstellar Medium, slides3–6: case A/B and rate balanceUniversity of Maryland — J. P. Harrington teaching notesReference guide · PDF
- Strömgren Spheres and Recombination Lines (2019), slides1–3: photon integral, uniform radius and limitsUniversity of Maryland — J. P. Harrington teaching notesReference guide · PDF
- Chapter4 §4.2, equations4.3 and4.6–4.11 and density-bounded distinctionNRAO — Essential Radio AstronomyReference guide
Evidence and review
Read actual UMD case-B ground-state recycling versus case-A escape, source photon integral and uniform balance/radius body, plus NRAO ionization/density-bounded definitions and volumetric budget. The volume integral is the disclosed extension of the uniform local rate. Excluded source malformed c² prefactor in a blackbody substitution, numerical table examples and universal dust-free observational claims.
2026-10-10 · Independent AI editorial review: All44 exact root-authored compact records, equations/variable units/conditions and140 origin edges accepted after independently reading final6/18 with bounded primary bodies and matching prior38/122 scientific fingerprints unchanged. Exact claim/edge review is distinct from combined inventory scope and separate CLE26 approval; no entire cited books or old-corpus experimental revalidation is asserted. Receipt: cosmos-completion-independent-content-and-inventory-review.1.
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