Astronomers measure cosmic distances using a hierarchy of techniques known as the cosmic distance ladder, in which each method calibrates the next, extending reach from nearby stars to the edge of the observable universe. At the smallest scales, parallax exploits the apparent shift in a star's position as Earth orbits the Sun and is used to measure distances to nearby stars. The technique defines the parsec: a star with a parallax of one arcsecond lies at a distance of one parsec, or about 3.26 light-years. Farther out, Cepheid variable stars serve as standard candles because their pulsation period correlates predictably with their true luminosity, while Type Ia supernovae extend the ladder to the deepest extragalactic distances because their peak brightness can be standardized.
Astronomers also analyze light directly through spectroscopy, spreading it into its component wavelengths to determine a star's composition, temperature, motion, and other physical conditions. This is how the spectral classes O, B, A, F, G, K, and M were established. Even to the naked eye, a few stars stand out: Sirius is the brightest star in Earth's night sky, while Polaris, the North Star, sits near the north celestial pole and has long served as a navigational reference. Beyond the visible, space-based observatories avoid the distorting effects of Earth's atmosphere. The Hubble Space Telescope, launched in 1990, transformed optical astronomy, and its infrared successor, the James Webb Space Telescope (JWST), launched in 2021, peers even deeper into the early universe from its station at the Earth-Sun Lagrange point L2, about 1.5 million kilometers away. A Lagrange point is a gravitationally stable position in a two-body system where a small object can remain parked relative to both bodies.