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Photometry measures stellar brightness in different wavelength bands; color indices compare magnitudes at different wavelengths, revealing temperature. Spectral classification (O, B, A, F, G, K, M types) orders stars by temperature and composition. Together, photometry and spectroscopy enable measurement of distance, luminosity, temperature, and mass for stars.
Stars emit light across a broad range of wavelengths, and the shape of that emission — how much energy comes out at each wavelength — is determined primarily by the star's surface temperature. A hot star (say, 30,000 K) peaks in the ultraviolet and appears blue-white; a cool star (3,000 K) peaks in the infrared and appears red. Photometry exploits this by measuring a star's brightness through standardized filters that each transmit only a specific wavelength band. The most common system uses U (ultraviolet), B (blue), and V (visual/green) filters. By comparing the brightness measured through different filters, you construct a color index — for instance, B−V, the difference in magnitude between blue and visual bands. A small or negative B−V means the star is brighter in blue light, indicating high temperature; a large positive B−V means the star is brighter in the visual band relative to blue, indicating low temperature.
Spectral classification goes further by spreading starlight into its full spectrum and examining the pattern of absorption lines — dark features at specific wavelengths where atoms in the star's atmosphere absorb photons. The sequence O, B, A, F, G, K, M (from hottest to coolest) was established by organizing stars according to the strength of these absorption features, which turned out to correlate tightly with surface temperature. O-type stars are so hot that hydrogen is mostly ionized, so hydrogen absorption lines are weak; A-type stars have the strongest hydrogen lines because the temperature is just right for hydrogen atoms to populate the energy level that absorbs visible light; M-type stars are cool enough for molecules like titanium oxide to survive, producing broad absorption bands. The Sun is a G2 star — middle of the sequence, with prominent lines of ionized calcium and neutral metals.
The power of combining photometry and spectroscopy is that together they let you determine a star's fundamental physical properties from its light alone. Color index gives surface temperature quickly and cheaply (you only need two filter measurements). The spectral type refines the temperature and adds information about chemical composition and surface gravity. Once you know the temperature and luminosity — the latter requiring a distance measurement, which is where your knowledge of parallax and the distance ladder comes in — you can place the star on the Hertzsprung-Russell diagram, the central organizing tool of stellar astronomy. A star's position on the HR diagram reveals its evolutionary stage, mass, and remaining lifetime, all derived from measuring how bright it is and what color its light is.