Apollo — perceptual mechanics

The mechanism

A star is hotter inside than out. The light leaving it starts as a continuous band — every wavelength at once, brightest in the yellow-green where the eye happens to be most sensitive — and then it has to cross the star's own outer atmosphere on the way out. Every element up there absorbs at its own exact set of wavelengths and at no others. So the light that arrives has holes in it, and the holes say what the star is made of.

Joseph von Fraunhofer found them in sunlight in 1814 and labelled the strongest with letters. Some of those letters are still the names: the sodium D lines, calcium H and K, the magnesium b triplet. He did not know what any of them were. Nobody did for another forty-five years.

Apollo is that mechanism with the controls exposed. Ten elements sit on faders; moving one puts that element in the light's path, and its lines appear in the band because it is now absorbing. Two or three together is how a star is actually read. Clicking a dark line sounds its wavelength as a pitch, which means the thing you play is the absence.

Hydrogen, computed

Hydrogen has one electron, and one electron is the case that has a closed-form answer. The Rydberg formula gives every line in the series exactly:

1/λ = R (1/2² − 1/n²), for n = 3, 4, 5, …

with R = 1.096776e+7 per metre — the Rydberg constant with the correction for a proton that is heavy but not infinitely heavy, which is the difference between landing on the published wavelengths and missing them in the third figure. The formula gives a vacuum wavelength; every table of visible lines gives an air wavelength, because that is what a spectrograph on the ground measures, so the result is passed through the standard refraction formula. These are computed on page load, not typed in:

656.288 nm n = 3 to 2 · H-alpha · 456.8 Hz
486.138 nm n = 4 to 2 · H-beta · 616.7 Hz
434.051 nm n = 5 to 2 · H-gamma · 690.7 Hz
410.178 nm n = 6 to 2 · H-delta · 730.9 Hz
397.011 nm n = 7 to 2 · H-epsilon · 755.1 Hz
388.909 nm n = 8 to 2 · H-zeta · 770.9 Hz
383.542 nm n = 9 to 2 · H-eta · 781.6 Hz
379.794 nm n = 10 to 2 · H-theta · 789.4 Hz
377.067 nm n = 11 to 2 · 795.1 Hz
375.019 nm n = 12 to 2 · 799.4 Hz

The lines crowd tighter as n rises and converge on the series limit at 364.601 nm, just past the violet edge of the band. They do not stop there because anything runs out — infinitely many of them pile up in the last fraction of a nanometre, and past the limit the spectrum goes continuous. That convergence is the best thing the scene has to show, which is why it is calculated rather than listed.

Everything else is looked up, and has to be. Sodium's doublet, iron's forest and helium's scatter come out of many-body quantum mechanics that nobody solves in closed form, in a browser or anywhere else. Deriving them would be work that does not show and would produce wrong numbers.

Wavelength as pitch

A wavelength has a real frequency: the speed of light divided by it. For the sodium D line that is about 509 trillion cycles a second, which is not a sound. Dividing by 1e+12 is the entire mapping — no scale, no rounding to the nearest note, no tuning per element — so the intervals you hear are the intervals you see.

That puts the whole visible band between 400 Hz at the deep red end and 789 Hz at the violet. Which is a fact about light rather than a choice: 750 divided by 380 is 1.97, so the visible spectrum is almost exactly one octave wide. The instrument has one octave and cannot have more.

Shorter wavelength is higher frequency, so violet is treble and red is bass. Sodium is the case worth listening to: its two lines are 0.597 nm apart, which after the division is 0.52 Hz apart — and two tones half a hertz apart are not two notes, they are one note that swells and fades about every two seconds. The visible spacing is the harmonic relationship. Iron, at the other end, has fifty lines here and sounds like a wall.

The sun, playing itself

Apollo has an idle state, which nothing else on this site has: every other scene sits still until it is touched. Turn on Sunlight and the instrument puts the sun's own composition in the light and lets its lines sound on their own — irregular, unsynchronised, each one weighted by how deep it actually is.

The composition is not an even mix and not a taste. Once the three atmospheric oxygen bands are set aside — those are absorbed by Earth's air on the way in, not by the sun — five elements own every remaining labelled line in the standard Fraunhofer table:

Magnesium 100% in the light · Fraunhofer b4 516.7nm, b2 517.3nm, b1 518.4nm Three close lines in the green — the b triplet. The middle case between sodium’s two and iron’s hundreds: still countable, already a chord rather than a beat.
Calcium 95% in the light · Fraunhofer K 393.4nm, H 396.8nm, g 422.7nm Two enormous lines crammed against the violet edge, and a scatter of ordinary ones across the rest. H and K are among the deepest features in real sunlight, so this fader is the one that makes the band look most like a photograph of the sun.
Hydrogen 85% in the light · Fraunhofer C 656.3nm, F 486.1nm, f 434.1nm, h 410.2nm Four lines wide apart, then a crowd. The tutorial element: sparse enough to count, and the only one here whose wavelengths are calculated rather than looked up.
Sodium 80% in the light · Fraunhofer D2 589.0nm, D1 589.6nm Two lines so close together they read as one until you look. Sonically this is the point of the whole instrument: the pair is half a hertz apart after transposition, and half a hertz apart is not two notes — it is one note that pulses.
Iron 70% in the light · Fraunhofer G 430.8nm, e 438.4nm, c 495.8nm, E 527.0nm A wall. Fifty lines here and thousands in reality, most of them jammed into the blue and violet, and struck together they are not a chord but a cluster — noise with a shape. Iron is in the instrument precisely because it breaks it.

The other five elements in the instrument sit at zero, which is a fact about the sun rather than an omission. Helium is the one worth naming. It was found in the sun in 1868, twenty-seven years before anyone found it on Earth, and it is still not part of the sun's visible fingerprint: its D3 line at 587.6nm belongs to the chromosphere and to prominences, not to the photospheric absorption spectrum this band draws. The element named after the sun is not in the sun's visible signature.

A note on how much of each, because a number in a table reads as measured. The ordering above is sourced — it is the Fraunhofer table. The fader positions are not: what a fader controls is column density in this particular model, with Gaussian line profiles and NIST intensities standing in for strength, and no published quantity maps onto that. Solar equivalent widths would be the right physical input and no machine-readable table of them was within reach. Photospheric abundances are available and would be actively wrong here, since they would put helium second and calcium near nothing, when calcium's H and K are the deepest features in the visible solar spectrum. Abundance is not line strength. So the values were set to reproduce the sourced ordering and each was checked by computing the optical depth it produces, rather than by looking at the result.

And one place where it does not come out right, said plainly because it is measurable. Magnesium's b triplet is comparable in depth to sodium's D lines in the real solar spectrum. Here it cannot be: NIST's emission intensity for b1 is 70 against sodium D2's 1000, so at the maximum fader position magnesium's strongest line still transmits about a third of the light passing through it. That is the emission-versus-absorption caveat further down this page turning into a specific number. Inventing a per-element correction to make one line look right would be worse — it would be taste wearing the costume of data.

The ten elements

Curated for what they do to the band and to the sound, not for coverage. Most of the periodic table is inert here — the transition metals are indistinguishable forests and most of everything else has nothing in the visible range at all — so an element earns a fader by producing a distinct look or a distinct sound, and preferably both. 218 lines in total.

Where the numbers come from

NIST Handbook of Basic Atomic Spectroscopic Data (Sansonetti & Martin), strong-lines tables, physics.nist.gov/PhysRefData/Handbook/ — a US Government work, public domain. Retrieved 2026-09-02.
CODATA recommended value for the Rydberg constant, with the reduced-mass correction for hydrogen applied here.
Edlén (1966), the vacuum-to-air refraction formula adopted as the IAU standard.
Wyman, Sloan & Shirley (2013), “Simple Analytic Approximations to the CIE XYZ Color Matching Functions”, Journal of Computer Graphics Techniques 2(2).

One honest note about the relative intensities. NIST publishes emission intensities — how bright a line is when the element is made to glow — and this scene draws absorption, how deep a line cuts when the element sits in front of something hotter. The two track each other closely, because both follow the same transition probabilities, but they are not the same quantity: a real absorption depth also depends on temperature, on ionization state, and on how much of the element is in the path. The intensities here are used as a line-strength proxy for an instrument, not as a photometric claim.