Castor revealed as a sextuple system (1719–1920)
In 1719, James Pound resolved Castor—one of the brightest stars in the northern sky—into two components. By 1905, spectroscopy showed each component was itself a tight binary. In 1920, a third faint pair was found orbiting the inner four, making Castor a sextuple system. Each revelation took decades and a new instrument.
Castor became the prototype for understanding hierarchical multi-star architectures, proving that gravitationally bound systems can nest multiple orbits within orbits.
The two-century unfolding of Castor's complexity established that the apparent simplicity of a star is often an observational limitation, not a physical reality—a lesson that TESS is now confirming at industrial scale.
TIC 120362137's discovery mirrors the Castor story compressed into months instead of centuries: TESS photometry first showed a binary, then a triple, then spectroscopy revealed the fourth star. Modern instruments are collapsing discovery timelines from centuries to single observing campaigns.