In 1814, the German optician Joseph von Fraunhofer — refining a spectroscope of his own design — looked at sunlight passed through a prism and saw something nobody had catalogued before: hundreds of dark lines crossing the otherwise-continuous solar spectrum. He labelled the strongest with letters A through K, the Fraunhofer lines, and observed that the same lines appeared in light from Sirius. Forty-five years later, Robert Bunsen and Gustav Kirchhoff in Heidelberg established that the dark lines were absorption lines produced by specific elements in the solar atmosphere, by matching them against laboratory emission spectra of known elements. Auguste Comte, in 1835, had argued that humans would never know the chemical composition of stars; by 1860 the laboratory had been brought to the stars instead.
A spectrum — intensity as a function of wavelength — encodes, against a single one-dimensional axis, an enormous amount of physics. Emission lines appear when excited atoms de-excite and release photons at characteristic wavelengths set by quantum-mechanical transitions; the Balmer series of hydrogen, He II 4686, the forbidden lines of nebular oxygen each carry an unmistakable signature. Absorption lines appear when cool gas in front of a hot continuum source absorbs at the same characteristic wavelengths; what Fraunhofer saw on the Sun was the cool outer atmosphere absorbing the continuum produced by the hotter photosphere below. Doppler shifts turn the lines into a velocimeter — lines redshift when the source recedes, blueshift when it approaches. Line broadening discloses temperature, density, and rotation; line ratios constrain ionization state. From a single well-resolved spectrum, an astronomer can read off chemistry, temperature, density, motion, and magnetic field strength. The instrumental story since 1859 is largely about expanding the wavelength range and overcoming Earth's atmosphere. Most of the electromagnetic spectrum is opaque or severely distorting from the ground; the rest requires space platforms — Hubble in optical and ultraviolet, Chandra in X-ray, Fermi in gamma-ray, JWST in the infrared. The two most consequential ground-based developments since the 1990s are adaptive optics — deformable mirrors correcting atmospheric turbulence in real time — and interferometry, combining signals from multiple separated telescopes to synthesize a larger virtual aperture. The Event Horizon Telescope, a planet-sized synthetic radio aperture, imaged the shadow of the supermassive black hole at the centre of M87 (2019) and Sgr A* (2022). Gravitational-wave observatories (LIGO, Virgo, KAGRA) extend the methodology to a non-electromagnetic channel.
JWST — 6.5 m primary, parked at L2 1.5 million km from Earth, passively cooled to ~50 K, in science operations since July 2022 — is the most capable infrared telescope ever flown, and is doing the first systematic atmospheric spectroscopy of transiting exoplanets. The next decade brings the Extremely Large Telescope (ESO, Chile, first light ~2028) at 39 m, the Vera Rubin Observatory surveying the entire visible southern sky every few nights, the Square Kilometre Array radio interferometer beginning science around 2030, and a third generation of gravitational-wave detectors (Einstein Telescope, Cosmic Explorer) planned for the 2030s. Every spectacular astronomical image flows through the methodology of photons collected, separated by wavelength, and inferred from spectra.