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Hydrogen — one element, three faces

August 30, 2026 · 8 min read

A good nine out of ten atoms in the universe are hydrogen — the simplest atom, one proton and one electron. Practically everything that glows red in the images of this gallery is this one element, and almost all of that red is in fact a single spectral line. Anyone who understands which states hydrogen occurs in, and why it glows, holds the key to nearly every image.

One element, three faces

Hydrogen occurs in space in three states, and the images show two of them directly: as the molecule H₂ it forms the cold, pitch-dark clouds in which stars are born — visible in a photograph only as a silhouette against a brighter background. As a neutral atom it fills vast spaces between the clouds, but is practically invisible in visible light. And as ionized gas — proton and electron separated — it forms the brilliantly red emission nebulae around hot stars.

The three states of hydrogen: molecular in cold clouds, atomic and invisible, ionized in H II regions. Starlight drives the chain to the right, recombination leads back and produces the red glow along the way.H₂ · molecular~20 K, pitch dark —stars are born herestarlight breaks apartH · atomicneutral, invisible in a photograph —broadcasts at 21 cm in the radioUV above 13.6 eV ionizesH⁺ · ionized~10,000 K, the “H II region” —glows in HαRecombination: a proton captures an electron — and the red light is born

Starlight drives the chain from left to right: it breaks molecules apart and ionizes atoms. The way back — recombination — runs without interruption, and it is precisely that which produces the glow. Where the chain meets in space, an ionization front forms.

Two details here are remarkable. First: H₂ hardly ever forms in open space — two hydrogen atoms barely find one another there. The molecules form on the surfaces of dust grains, which act as catalyst and parasol at once: they bring the atoms together and shield the finished molecule from the destructive starlight. No dust, no molecular clouds; no molecular clouds, no stars. Second: the invisible neutral atom gives itself away in the radio range through the famous 21 cm line — with it, the spiral structure of the Milky Way was mapped for the first time in the 1950s, straight through all the dust.

Why it glows red of all colors

An H II region does not glow because it is red hot like a stovetop — its light is the bookkeeping of recombination. When a proton captures an electron again, that electron usually lands in a high orbit and then falls down the energy ladder, rung by rung. Each jump releases a photon of precisely defined energy:

Energy level diagram of the hydrogen atom: a free electron is captured and falls downward step by step. The jump from n=3 to n=2 produces Hα at 656 nanometers — the deep red of the emission nebulae; n=4 to n=2 produces the blue-green Hβ.freen = 5n = 4n = 3n = 2n = 1capture of a free electronHα · 656 nmn 3 → 2 — the red of the nebulaeHβ · 486 nmn 4 → 2 — blue-green, fainter→ n 1: Lyman series, invisible UV
The jumps that end at n = 2 make up the Balmer series — the only part of the ladder that lands in visible light. The most common of them, n 3 → 2, produces Hα at 656 nm: the one deep red that all the emission nebulae in this gallery have in common.

H₂, H II, Hα — a small untangling

The notations look confusingly alike and mean three different things. H₂ (subscript number) is the molecule made of two atoms — cold, dark, the raw material. H II (Roman numeral, pronounced “H two”!) is the spectroscopists' notation for the ionization state: I means neutral, II singly ionized — so an “H II region” is a cloud of H⁺. And Hα is not a kind of gas but a spectral line: the light that ionized hydrogen emits as it recombines. The same convention also explains the “[O III]” from the other articles: doubly ionized oxygen, whose blue-green line is the second channel of dual-band exposures.

A few things worth knowing about this:

  • The eye does not see this red. The dark-adapted eye is nearly blind to deep red — visual observers see even bright emission nebulae as gray to greenish, because the [O III] line then dominates. The rich red of the photographs is no exaggeration; it is what the eye withholds.
  • This is why the dual-band filter works. It lets exactly two narrow windows through: Hα and [O III]. Everything else — street lamps, moonlight, airglow — is shut out. Emission nebulae send out their light almost entirely in a few lines; a continuum object such as a galaxy loses nearly everything to the same filter.
  • Hβ is the little brother. The same process produces a blue-green line at 486 nm, roughly three times fainter than Hα. In very bright regions it lends the red a magenta cast — which is why emission nebulae often look pinkish-red rather than pure red in broadband images.
  • H II regions are signposts. Because they exist only around short-lived hot stars, in other galaxies they mark exactly the sites of fresh star formation — the Hα knots in M 33 trace out its spiral arms the way lit buoys trace a shipping channel.
  • The scale is absurd. NGC 604 in M 33, a small bright patch in the images, is about 1,500 light-years across — the Orion Nebula, the showpiece of our own sky, measures about 25. One and the same phenomenon, across two orders of magnitude.
AI-generated illustration: on the left a completely dark molecular cloud as a silhouette, on the right a deep-red glowing H II region with embedded blue stars, and a bright rim between them.

Two of the three faces in one scene: on the left the cold molecular hydrogen as a black silhouette, on the right the ionized hydrogen as a red glowing region around young stars — and at the boundary the bright rim of the front. The third state, the neutral atom, is consistently invisible. This image is AI-generated — not a photograph, but an illustration.

In our own sky

All three roles of hydrogen appear in this gallery — you only have to know that the dark part belongs to it too:

Dark does not mean empty: the Great Rift in Cygnus5 objects — show
The Great Rift in Cygnus: what is missing here is the image. The dark lanes are molecular hydrogen with dust — so dense that an entire Milky Way disappears behind them. “Dark does not mean empty” is the short version of this article.My photograph · C27 Crescent · 0.74° × 1.13° · view gallery entry
LDN 935 — the cloud that draws the continent2 objects — show
LDN 935, the cloud between the North America and Pelican Nebulae: a molecular cloud as a paper cutout in front of glowing H II. The “coastline” of the continent is not a property of the glowing gas but of the dark cloud in front of it — the picture is drawn by the H₂ and lit by the H⁺.My photograph · NGC 7000 · 1.11° × 1.26° · view gallery entry
IC 1318 and LDN 889 — the Butterfly is a silhouette3 objects — show
The Butterfly Nebula IC 1318: the two “wings” are a single H II region, cut in half by the dark dust lane LDN 889. Glowing and dark gas create the shape together — neither one alone would make a butterfly.My photograph · IC 1318 Panorama · 1.90° × 1.60° · view gallery entry
NGC 604 and NGC 59510 objects — show
NGC 604 and NGC 595 in M 33: H II regions two and a half million light-years away, and yet effortlessly visible — NGC 604 is among the largest known anywhere. That individual red knots in another galaxy can be photographed at all is precisely because Hα packs its light into one single narrow line.My photograph · M 33 · 22.3′ × 23.8′ · view gallery entry

Further reading

  • Balmer series — the family of lines whose red member colors the nebulae (Wikipedia).
  • 21 cm line — how neutral hydrogen becomes visible in the radio range (Wikipedia).
  • Molecular cloud — the cold, dark raw material of star formation (Wikipedia).
  • Emission nebula — the umbrella term for everything that glows red here (Wikipedia).

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