Castor, also known as Alpha Geminorum, is the second-brightest star in the constellation Gemini. Although it appears as a single white point of light beside Pollux, Castor is actually an extraordinary system of six stars arranged as three binary pairs. Its name comes from the mortal twin Castor in Greek mythology, whose immortal brother was Pollux.
Easy to Find Beside Pollux in Gemini
Castor and Pollux mark the heads of Gemini’s celestial twins. From mid-northern latitudes, they are especially prominent on winter and early-spring evenings. One way to find them is to begin with Orion and look northeast of the constellation’s bright reddish star Betelgeuse. Pollux is the slightly brighter and more orange of the pair, while Castor appears whiter and lies about 4.5° to its north. The two stars look like close neighbors, but this is only a line-of-sight effect. Pollux is approximately 34 light-years away, while Castor lies about 51 light-years from Earth.
The 23rd-Brightest Star in the Night Sky
With a combined apparent magnitude of approximately 1.6, Castor is the 23rd-brightest star in the night sky. It is noticeably fainter than Pollux, which ranks 17th. This makes their Bayer designations slightly surprising: Castor is Alpha Geminorum, while brighter Pollux is Beta Geminorum. However, the Greek letters assigned by Johann Bayer in the early 17th century do not always follow a strict brightness order. Position, tradition and the limited precision of naked-eye brightness estimates also influenced how stars were labeled.
One Point of Light Is Actually Six Stars
Castor looks like one star without optical aid, but it is actually a hierarchical system containing six stars. They are organized into three binary pairs known as Castor A, Castor B and Castor C.
Castor A contains stars Aa and Ab, Castor B contains Ba and Bb, and Castor C contains Ca and Cb. Each pair consists of two stars bound together by gravity, while the three pairs also belong to the same wider system. All six stars are thought to have formed together approximately 290 million years ago. That makes Castor far younger than the roughly 4.6-billion-year-old Sun.
A Small Telescope Can Split Castor Into Two Stars
A small telescope under steady atmospheric conditions can separate Castor’s two brightest visible components, Castor A and Castor B. They currently appear several arcseconds apart and take roughly 450 years to orbit their shared center of mass. The remaining four stars orbit too closely to their companions to be separated individually through an ordinary backyard telescope.
Castor was recorded as a double star by English astronomers James Pound and James Bradley in 1718 and 1719, although Giovanni Domenico Cassini may have resolved it several decades earlier. William Herschel later recognized that the changing positions of the two stars showed they were physically orbiting each other rather than merely appearing close together. The discovery was historically important because it helped establish that Newton’s law of gravity operates between distant stars—not only among objects in our Solar System.
Its White Light Comes Mostly From Two Hot Stars
Most of the light we see from Castor comes from Castor Aa and Ba, two hot A-type main-sequence stars. Castor Aa has roughly 2.4 times the Sun’s mass and is about twice as wide, while Castor Ba has around 1.8 solar masses and a radius approximately 1.6 times that of the Sun. Modern interferometric measurements give their radii as about 2.09 and 1.65 solar radii, respectively.
Castor Aa has a surface temperature of approximately 10,000 K and shines around 30 times more brightly than the Sun. Castor Ba is slightly cooler at roughly 8,800 K and produces around 14 times the Sun’s luminosity. Their combined light gives Castor its white or subtly blue-white naked-eye appearance.
Neither bright star is alone. Castor Aa is orbited by the faint red dwarf Castor Ab every 9.2 days, while Castor Ba and another red dwarf, Castor Bb, complete an orbit in approximately 2.9 days. The companions are much cooler and less massive than the two A-type stars and contribute very little visible light. Because these red dwarfs are too close and faint to resolve conventionally, astronomers originally inferred their presence from recurring shifts in the systems’ spectra. Long-baseline interferometry has since resolved the companions directly and allowed all four stars’ masses to be measured with remarkable precision.
The Outer Pair Lies Around 1,000 AU Away
Castor C is the third binary pair and lies much farther from Castor A and B than those two systems lie from each other. Its projected separation is roughly 1,000 astronomical units—around 25 times Pluto’s average distance from the Sun.
Despite that enormous separation, Castor C shares Castor A and B’s distance and motion through space and is gravitationally connected to the wider system. Its orbit around the central four stars probably takes many thousands of years, making the system’s slow outer motion extremely difficult to measure directly. Castor C is also called YY Geminorum and consists of two similar red dwarfs, bringing the system’s total population to six stars.
Its Two Outer Stars Eclipse Each Other Every 19.5 Hours
The two red dwarfs in YY Gem orbit each other in only about 19.5 hours. Their orbit is aligned so that, from Earth, the stars repeatedly pass in front of one another and partially block each other’s light. This makes YY Gem an eclipsing binary.
Both stars are also magnetically active flare stars. Dark starspots and sudden flares can produce additional changes in brightness, complicating the otherwise regular pattern created by the eclipses. Because the two stars are so similar and their masses and sizes can be measured accurately, YY Gem has become an important laboratory for testing models of low-mass red dwarfs.
Its Three Binary Orbits Point in Different Directions
Castor’s six stars did not settle into one tidy, flat arrangement. Modern measurements show that the orbital planes of Castor A and Castor B are strongly tilted relative to the much wider orbit carrying those two pairs around each other. Castor A’s inner orbit is oriented almost at right angles to the wide A–B orbit, while Castor B’s orbit is also steeply inclined. Castor C adds another extremely distant pair to this already complicated architecture.
The result is a remarkable nested system: one naked-eye point of light containing three binary pairs, six individual stars and orbits ranging from less than one day to many thousands of years.