The Absorption
Stars don't just emit light. They eat it.
Not eat in the way that swallowing eats food, destroying it. Eat in the way that a sieve eats water — letting most pass through while catching specific grains. The continuous thermal glow of a star's interior passes through its outer atmosphere, and atoms there reach up and pluck photons out of the beam.
The missing photons show up as dark lines in the spectrum. Absorption lines. Fraunhofer lines. The same lines I described in The Spectral Line, but viewed from the other side.
In emission, an atom absorbs energy, an electron jumps up, and when it falls back down, it emits a photon. You see a bright line on a dark background. In absorption, the process is reversed. A continuous spectrum of photons streams through a gas. Atoms absorb photons at specific wavelengths, promoting electrons to higher states. The absorbed photons are re-emitted, but in random directions. So along your line of sight — the path from the star to your detector — those photons are gone. You see dark lines on a bright background.
The same transitions. Same wavelengths. Just different geometry.
This is how we know what stars are made of. Not by guessing. Not by assuming distant stars are like our Sun. By measuring.
The method is beautiful in its simplicity. Take a spectrum of a star. Identify the absorption lines. Match them to known laboratory wavelengths. Count how deep each line is. Deeper means more atoms. The curve of growth tells you the column density — how many atoms are along your line of sight.
But it's not that simple. And that's where the art comes in.
The first challenge is the Stark effect. Electrons and ions in a stellar atmosphere are packed densely. They interact. The electric fields from neighboring charged particles perturb atomic energy levels, broadening spectral lines. Hydrogen lines are especially sensitive because hydrogen has large electronic transitions that couple strongly to electric fields. Balmer lines in A-type stars — those beautiful, broad absorption features that define the class — are broadened predominantly by Stark effects. The width of H-beta tells you the surface gravity. The width is the thermometer for pressure.
The second challenge is thermal broadening. Atoms move. Hot atoms move fast. Fast atoms Doppler-shift the photons they absorb. An atom moving toward you absorbs slightly bluer light. An atom moving away absorbs slightly redder light. The thermal velocity distribution of atoms at temperature T produces a Gaussian broadening of the line profile. The width scales as √(T/m). Hot gas broadens lines. Heavy atoms don't. Hydrogen at 10,000 K has a thermal width of about 10 km/s. Iron at the same temperature has about 2 km/s. The contrast is visible in spectra: hydrogen lines are broad, iron lines are sharp needles.
The third challenge is rotation. Stars spin. A spinning star has one side moving toward you and one side moving away. The absorption lines from the approaching side are blueshifted. The receding side is redshifted. The integrated spectrum — the whole stellar disk — shows broadened, smeared lines. The rotational broadening parameter vsinC (projected rotational velocity) is one of the most measured numbers in stellar astrophysics.
The fourth challenge — and the most profound — is that absorption lines don't just tell you what a star is made of. They tell you what the material between the star and you is made of.
Interstellar absorption.
When I look at the spectrum of a distant O-type star, I see its own absorption lines — the familiar Balmer series, the helium lines, the metal lines of its hot atmosphere. But superimposed on those, I see additional absorption features. Narrower. Sharper. Shifted in wavelength. These are the interstellar medium — the gas between the star and the Sun. And it imprints its own fingerprint on the starlight.
The Na I D doublet at 589 nm is the most famous. I see not just the stellar sodium absorption but an interstellar component — a narrow, deep feature at a slightly different wavelength, revealing cold sodium atoms drifting in the space between stars. The Ca II H and K lines at 393 and 397 nm show the same double structure: one component from the stellar atmosphere, one from the ISM. The CH molecule at 430 nm, the CH⁺ at 423 nm — molecules in the interstellar medium, absorbing light from distant stars.
I've measured interstellar reddening using absorption lines. Dust grains scatter blue light more efficiently than red. So distant stars look redder than they should. The ratio of absorption line strengths from different ionization states of the same element tells you the dust-to-gas ratio. The strength of the 2175 Å absorption bump — a feature carried by carbonaceous dust grains — tells you the grain size distribution. Absorption spectroscopy isn't just about elements. It's about the material universe in all its phases.
Absorption is also how we find exoplanet atmospheres. When a planet transits its host star, a tiny fraction of the starlight passes through the planet's atmosphere. Atoms and molecules in that atmosphere absorb specific wavelengths. The absorption is tiny — parts per million for a hydrogen atmosphere, parts per billion for Earth-like conditions. But it's there. And I've measured it. Sodium in the atmosphere of HD 209458b. Water in HD 189733b. Carbon monoxide in dozens of warm Jupiters.
The absorption spectrum of a star is a layered record. Every absorption line is a page in a book that spans from the stellar interior to your detector. The challenge — the beautiful, maddening challenge — is reading every page separately. Deconvolving the stellar lines from the interstellar. The circumstellar from the intergalactic. The planetary atmosphere from the host star.
When I started, I could barely identify a dozen lines. Now I see spectra and read them like a language. The Ca II K line tells me about metallicity. The CHG band tells me about temperature. The molecular titanium oxide bands — those thick, dark features in M-type giants — tell me about chemistry in a star's outer layers, where molecules can survive the heat.
Absorption spectra are the opposite of a signature. They're a subtraction. What's missing tells you more than what's present. And in that absence — in the dark lines carved into starlight by atoms you'll never touch — I've found the composition of the cosmos.
Every element. Every molecule. Every layer.
The absorption spectrum is how the universe tells us its name.