H II Regions

Why H II regions matter

• Star-formation tracers. H-alpha luminosity is a workhorse gauge of a galaxy's current star-formation rate — it lights up only where massive stars have formed in the last few million years.
• Cosmic chemistry. The relative strengths of emission lines yield abundances of oxygen, nitrogen, sulfur and helium, mapping how galaxies enrich with metals over time.
• Stellar nurseries. They mark the surviving skin of the molecular cloud that just gave birth to a massive star cluster, complete with protoplanetary disks (proplyds).
• Feedback laboratories. Ionization, radiation pressure and stellar winds carve, compress and eventually disperse the parent cloud — the "feedback" that regulates galactic star formation.
• Distance and dynamics. Giant extragalactic H II regions serve as bright, resolvable beacons for tracing spiral arms and measuring rotation curves.
• Physics on display. They are natural low-density plasma labs where "forbidden" atomic transitions, invisible on Earth, shine brightly.

How an H II region works, step by step

1. A massive star ignites. Inside a cold molecular cloud, gravity assembles a star of at least ~15–20 solar masses. Its surface reaches 25,000–50,000 K, so a large fraction of its light emerges as ultraviolet radiation.
2. Ionization. Photons carrying more than 13.6 eV (wavelengths shorter than 912 Å, the Lyman limit) knock the electron clean off a hydrogen atom, converting neutral H I into ionized H II — a soup of bare protons and free electrons.
3. An ionization front sweeps outward. The star ionizes gas around it faster than the gas can recombine, and the boundary between ionized and neutral gas — a sharp ionization front — races outward until balance is reached.
4. Recombination and the red glow. Electrons and protons continually recombine. Because a captured electron usually lands in an excited level and cascades down, it emits a ladder of photons. The n=3→2 step is H-alpha at 656.3 nm — the crimson that defines these nebulae.
5. Forbidden lines add color. Free electrons collisionally excite trace ions of oxygen, nitrogen and sulfur. In the near-vacuum, these ions radiate "forbidden" lines like the green [O III] 500.7 nm doublet, impossible to see in a dense lab.
6. A thermostat locks in ~10,000 K. Photoelectrons heat the gas; the forbidden lines cool it. This balance keeps almost every H II region near 8,000–10,000 K regardless of the star.
7. Equilibrium: the Stromgren sphere. The ionized zone grows until every ionizing photon is exactly consumed by a recombination somewhere inside it. The result is a roughly spherical bubble of ionized gas — the Stromgren sphere — bounded by the neutral cloud.

The Stromgren sphere: the key equation

In 1939 Bengt Stromgren showed that a single hot star surrounds itself with a sharply bounded ball of ionized gas. Setting the star's total ionizing-photon output equal to the total number of recombinations inside a uniform-density sphere gives the Stromgren radius:

R_S = ( 3 Q / 4π n² α_B )^1/3

• R_S — Stromgren radius (cm, often converted to light-years or parsecs)
• Q — rate of hydrogen-ionizing photons the star emits (photons s⁻¹); ~10⁴⁹ s⁻¹ for an O6 star, ~10⁴⁸ for a B0
• n — number density of hydrogen (cm⁻³); typically 10² to 10⁴ in these clouds
• α_B — Case B recombination coefficient, ≈ 2.6 × 10⁻¹³ cm³ s⁻¹ at 10,000 K (excludes recombinations straight to the ground state, which would just re-ionize a neighbor)

The n^−2/3 scaling is the crucial insight: denser clouds have far smaller ionized bubbles, because recombinations (which go as n²) devour photons fast. For an O6 star (Q ≈ 10⁴⁹ s⁻¹) in a cloud of n = 100 cm⁻³, R_S works out to roughly 3 parsecs — about 10 light-years in radius (some 20 light-years across). Double the density and the radius shrinks by a factor of 1.6. Real H II regions are messier than a perfect sphere — clumpy, blister-shaped where they burst out of their cloud — but Stromgren's balance sets the scale.

Key numbers and famous examples

Property	Typical value	Note
Ionization energy of H	13.6 eV (λ < 912 Å)	The Lyman limit; sets who can power one
Electron temperature	~8,000–10,000 K	Thermostat set by forbidden-line cooling
Density	~10²–10⁴ cm⁻³	Diffuse, but denser than most of the ISM
H-alpha wavelength	656.3 nm (red)	Balmer n=3→2; dominant visible line
[O III] doublet	495.9 & 500.7 nm (green)	Forbidden line; once called "nebulium"
Exciting stars	O and B spectral types	T_surface ≈ 25,000–50,000 K
Lifetime of exciting star	~1–10 Myr	Why they trace recent star formation
Orion Nebula (M42)	1,344 ly, ~24 ly wide	Nearest and best-studied
Rosette Nebula	~5,000 ly, ~130 ly wide	Central cavity cleared by stellar winds
30 Doradus (Tarantula)	~161,000 ly (LMC)	Most luminous H II region in the Local Group

A history: from "nebulium" to plasma physics

When 19th-century observers first put nebulae through spectroscopes, they saw not a smooth rainbow but a handful of bright emission lines. One brilliant green pair at 500.7 and 495.9 nm matched no known element, and astronomers proposed a new one — nebulium — much as helium had first been found in the Sun. The mystery held until 1927, when Ira Sprague Bowen realized the lines came from ordinary doubly-ionized oxygen making a transition so improbable it is called "forbidden." It shines in nebulae only because the gas is a near-perfect vacuum where an excited ion is left undisturbed long enough to finally emit. Twelve years later Bengt Stromgren supplied the geometry, proving a hot star maintains a sharply bounded ionized sphere. Together these results turned glowing nebulae from curiosities into precise plasma diagnostics — line ratios now yield temperatures, densities, and the abundances of a dozen elements across the universe.

Worked example: sizing Orion's ionized bubble

The Orion Nebula is ionized mainly by Theta-1 Orionis C, an O6–O7 star with an ionizing output of roughly Q ≈ 1 × 10⁴⁹ photons s⁻¹. Take a representative density of n ≈ 3,000 cm⁻³ for the bright inner nebula and α_B ≈ 2.6 × 10⁻¹³ cm³ s⁻¹. The Stromgren radius is

R_S = ( 3 × 10⁴⁹ / (4π × (3000)² × 2.6×10⁻¹³) )^1/3 ≈ 1.0 × 10¹⁸ cm ≈ 0.33 pc ≈ 1.1 light-years.

That small value reflects Orion's high density near the Trapezium; in the more diffuse outer gas the ionized volume extends much farther, and the true nebula spans some 24 light-years. The example shows the key lever: because R_S ∝ n^−2/3, the same star produces a compact bubble in dense gas and a sprawling one in thin gas. It also shows why H II regions are so bright — the recombination rate per unit volume climbs as n², so dense, compact regions blaze even though they are small.

Common misconceptions

• "H II" is not "H-eleven." The II is a Roman numeral for ionization stage; it reads "H two" and means singly ionized (electron removed) hydrogen.
• The red glow is not reflected starlight. H II regions are emission nebulae — the gas itself emits H-alpha after recombination. Blue reflection nebulae, by contrast, just scatter starlight off dust.
• The gas is not "on fire." There is no combustion; the light is recombination and collisionally excited line emission from a ~10,000 K plasma.
• Forbidden lines are not truly forbidden. They are highly improbable transitions that proceed readily in the near-vacuum of space, where collisions are too rare to de-excite the ion first.
• They are not the same as planetary nebulae. Both glow by the same physics, but a planetary nebula is gas shed by a dying low-mass star, while an H II region surrounds hot, newly born massive stars.
• They don't last forever. Within a few million years the exciting stars die or blow away the gas; the nebula fades and disperses.