Light & Spectra

Deformable mirrors that flex hundreds of times a second to cancel the blurring of Earth's turbulent atmosphere, giving ground telescopes near space-quality sharpness.

Combining the light of separate telescopes so they act as one much larger instrument, achieving resolution impossible for a single dish. Radio interferometry links antennas across continents; it produced the first images of black-hole shadows.

The precise measurement of the brightness of a star or galaxy, in one or more filters. Photometry over time reveals variable stars, transiting planets, and supernovae; photometry in different colours reveals temperature.

Measuring the polarisation of light, which carries information no brightness or spectrum can — magnetic-field geometry, scattering by dust and grains, and the physics of everything from stellar surfaces to the cosmic microwave background.

Sorting stars by the lines in their spectra into the sequence O, B, A, F, G, K, M — a classification that turned out to be an ordering by temperature, and the basis of the Hertzsprung–Russell diagram.

Spreading light into its spectrum to read the fingerprints of the elements. Spectral lines reveal what an object is made of, how hot it is, how it moves (through the Doppler shift), and the strength of its magnetic fields — the single most powerful tool in astronomy.

Astronomy's logarithmic, backwards brightness scale, in which smaller numbers are brighter. Apparent magnitude is how bright an object looks; absolute magnitude is how bright it truly is — and the difference gives its distance.