1 Geminorum

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1 Geminorum (1 Gem) is a star in the constellation Gemini. Its apparent magnitude is 4.15.

In the 19th century, John Flamsteed numbered the brighter stars in each constellation from west to east, and 1 Geminorum was the first star listed in Gemini. It is also listed in the Bright Star Catalogue as star 2134, usually designated HR 2134 with the HR standing for the Harvard Revised catalog which was the precursor to the Bright Star Catalogue.

1 Geminorum is a triple star system. The primary component of the system is a K-type red giant star. It is almost twice the mass of the Sun. It is orbited by a spectroscopic binary pair of stars at a separation of about 9.4 astronomical units every 4877.6 days. It has not been resolved, but periodic Doppler shifts in the spectrum indicate the orbital motion of two separate stars. The pair, an F-type subgiant and a solar-mass star that may be G-type, are separated by 0.1234 astronomical units.

1 Geminorum is listed as a suspected variable star with an amplitude of 0.05 magnitudes.