A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
More than half of all stars are members of binary or multiple star systems where two or more stars orbit their common center of mass. Binary stars are the primary tool for determining stellar masses, derived from Kepler's third law modified for two massive bodies. Visual binaries are resolved telescopically; spectroscopic binaries are detected through periodic Doppler shifts in their absorption lines; eclipsing binaries reveal sizes and masses when one star transits the other. In close binaries, mass transfer from a giant companion onto a white dwarf can trigger recurrent novae or, if the white dwarf reaches the Chandrasekhar limit, a Type Ia supernova.
Work through the stellar mass calculation for a visual binary with known orbital period and separation. Study a real eclipsing binary light curve and interpret its shape in terms of the sizes, temperatures, and orbital inclination of the two stars.
Most stars in the Milky Way are not solitary like the Sun — they are members of binary or multiple systems, two or more stars gravitationally bound and orbiting a common center of mass. This is not a curiosity; binary stars are the cornerstone of stellar astrophysics because they provide the only direct way to measure stellar masses. From your prerequisite work with Kepler's laws, you know that the orbital period and semi-major axis of any orbiting system encode the mass of the central body. For a binary, both stars orbit the system's center of mass, and the modified form of Kepler's third law — P² = 4π²a³ / G(M₁ + M₂) — yields the total system mass from the observed period and separation.
Astronomers detect binary stars three different ways, each suited to different orbital geometries. Visual binaries are close enough (in angular terms) that a telescope resolves both stars as distinct points; their orbital motion is tracked directly over years or decades. Spectroscopic binaries cannot be resolved, but as the stars orbit, their radial velocities change periodically — you see alternating blueshifts and redshifts in the absorption lines of the combined spectrum (or, if both stars are bright, a periodic splitting of each line). Eclipsing binaries happen when the orbital plane is nearly edge-on to us: one star periodically crosses in front of the other, dimming the total brightness in a characteristic pattern. The shape of that light curve encodes the relative sizes and temperatures of both stars.
In close binary systems, interesting physics occurs when one star evolves into a giant and its outer envelope overflows into the gravitational domain of the companion — a process called mass transfer. If the companion is a white dwarf, the transferred hydrogen accumulates on its surface. When the layer becomes dense and hot enough, hydrogen burning ignites in a sudden thermonuclear runaway visible across the galaxy as a nova. Critically, neither star is destroyed: the white dwarf survives, and mass transfer can resume, producing recurrent novae. If the white dwarf accretes enough mass to approach the Chandrasekhar limit (~1.4 solar masses), electron degeneracy pressure fails and the entire star detonates as a Type Ia supernova — a catastrophic endpoint rather than a surface flash.
Type Ia supernovae are important far beyond the binary systems that produce them. Because they all detonate at approximately the same mass and therefore the same intrinsic luminosity, they serve as "standard candles" for measuring cosmological distances. The observation in 1998 that distant Type Ia supernovae appeared fainter than expected — meaning they were farther away than standard cosmology predicted — was the key evidence for the accelerating expansion of the universe and the existence of dark energy.