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Propus (η Geminorum) Worksheets

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Key Facts & Information

  • Propus (η Geminorum) is a triple star system found in the constellation of the celestial twins, Gemini. It is located 380 light years away from our Sun. Eta Geminorum is one of the brightest stars in its constellation and is also visible to the naked eye. Propus is a semiregular variable star and also an Algol-type variable system.
  • Propus is an evolved cool luminous star which is currently in a period of stellar evolution undertaken by all of the low to intermediate stars in their life.
  • Red dwarfs are usually associated with red giants and stars of spectral class M.
  • It is very likely that Propus will become a red giant star after completing its AGB stage.
  • USS Propus (AK-132) was a cargo ship converted from a liberty ship to a United States Navy Crater in service during World War II that was named after the star.


  • The traditional names Tejat Prior and Propus originate from the Greek meaning “forward foot”.
  • This name refers to the position of the star in the constellation of Gemini, marking the right foot of Castor, one of the celestial twins.
  • The word Propus in Latin means “close”.
  • The other traditional names Praepes and Pish Pai are of Persian origin, meaning “foreleg”.
  • The Working Group on Star Names by the International Astronomical Union approved the name Propus for the star in July 2016.
  • Eta Geminorum, Alhena (Gamma Geminorum), Tejat Posterior (Mu Geminorum), Nu Geminorum, and Xi Geminorum were Al Han’ah or “The Brand”.
  • These stars were also associated in Al Nuḥātai, the dual form of Al Nuḥāt, “a Camel’s Hump”.
  • In the Chinese lunar mansion, Propus is the only member of the lunar mansion “Battle Axe”.
  • The star also has other designated names: 7 Geminorum, HD 42995, HR 2216, BD+22°1241, HIP 29655, SAO 78135, CCDM J06149+2230, and ADS 4841.

Observational History

  • In 1865, German astronomer and geophysicist Julius Schmidt first reported that Propus was a variable star.
  • In 1881, American astronomer Sherburne Burnham observed that Eta Geminorum had a close companion, which is Eta Geminorum B, measured at a distance of 1.08”.
  • In 1902, William Campbell reported that the star exhibits radial velocity variations.
  • In 1944, the orbit was calculated with a period of 2,983 days (8.17 years) and an eccentricity of 0.53.
  • On July 11, 1837, Propus was occulted by the Planet Mercury.
  • On July 27, 1910, the star was last occulted by a planet, the Venus.
  • The assumption was that the star was a spectroscopic binary, but no period or orbital parameters were known.

Stellar System

  • The primary star, Eta Geminorum Aa, is an asymptotic giant branch star with spectral type M2 IIIa.
  • It is also classified both as a semiregular variable star and an eclipsing variable.
  • Its apparent magnitude varies from 3.15 to 3.90 in a period of around 234 days and its absolute magnitude is around -1.87.
  • The primary star is much bigger than our Sun with a mass of 2.5 solar masses and a radius of 153 solar radii.
  • Eta Geminorum Aa is a highly evolved luminous star and is much cooler than our Sun.
  • It has an estimated radial velocity of around 22.39 km/s
  • Its average surface temperatures is around 3,548 K.
  • The primary star is also an energetic one, being 3.162 times more luminous than our Sun.
  • The companion star, Eta Geminorum B is of spectral type G0 III.
  • It has an apparent magnitude of 6.04.
  • Eta Geminorum B is also suspected to be a much fainter M-class star.
  • Further information about the companion star is still unknown.
  • The third star, Eta Geminorum Ab, is yet to be observed.
  • It is suggested that the third component is very likely to be much smaller than the primary star, Eta Geminorum Aa.
  • This star is also blamed for the eclipsing periods.
  • The semi-regular variations have been classified as type SRa, which indicates that a relatively predictable periodicity with variations in amplitude and light curve shape.
  • These variables are very similar to the Mira variables but with smaller amplitudes.
  • The eclipsing period of Eta Geminorum Aa and Ab is known to last for around 8.17 years, corresponding to the orbit of a companion which is not visible.
  • The orbit of these stars has a semi-major axis of 0.77’ and an eccentricity of around 0.53.
  • Eta Geminorum A and Eta Geminorum B are separated by a distance of around 1.65 arcseconds.
  • Their orbit is known to last for around 473.7 years with a semi-major axis of 1.08” and an eccentricity of 0.54.
  • These eclipses are questioned long ago and observations are still being conducted to gather more information and to establish their predictability.


  • Tejat Prior (Eta Geminorum) and Tejat Posterior (Mu Geminorum) are both Mercury-Venus stars according to Ptolemy.
  • They both exhibit the ability to express ideas in reasonable and acceptable ways.
  • Propus is also known to rule the left breast in the human body.
  • The constellation of Gemini is said to cause trouble and disgrace, sickness, loss of fortune, affliction, and danger to the knees.


  • Propus is located in the constellation of Gemini and marks the right foot of one of the celestial twins, Castor.
  • In its constellation, Eta Geminorum is located at around 2 degrees west of Mu Geminorum and 2 degrees southeast of the open star cluster Messier 35.
  • It is also located 0.9 degrees south of the ecliptic, thus, it can be occulted by the moon, and rarely, by planets.
  • The constellation of the celestial twins is the northernmost and one of the brightest constellations.
  • Gemini is the 30th largest constellation in the night sky with an area of 514 square degrees and is one of the 12 constellations of the Zodiac.
  • It is also one of the 48 constellations listed in the Almagest by Greco-Roman astronomer Ptolemy in the 2nd century.
  • The deep-sky objects of the Gemini constellation are best observed during the month of February.
  • The 10 brightest stars of Gemini are: Pollux (Beta Gem, 1.14 mag.), Alhena (Gamma Gem, 1.93 mag.), Castor (Alpha Gem, 1.98), Tejat (Mu Gem, 2.88 mag.), Mebsuta (Epsilon Gem, 2.98), Propus (Eta Gem, 3.28 mag.), Alzirr (Xi Gem, 3.36 mag.), Wasat (Delta Gem, 3.53 mag.), Kappa Gem (3.57 mag), and Lambda Gem (3.58 mag).
  • In Greek mythology, Castor and Pollux are the children of Leda.
  • Pollux was the son of Zeus, who seduced Leda, and Castor was the son of Tyndareus, the husband of Leda and the King of Sparta.
  • Pollux and Castor became close to each other as they grew up.
  • When Castor died, since he was mortal, Pollux begged his father Zeus to give Castor immortality.
  • Zeus agreed and united the twins in the heavens.
  • The constellation of Gemini is also home to interesting celestial objects such as its stars Pollux and Castor which mark the heads of the twins, a neutron star Geminga, planetary nebulae Medusa Nebula and Caldwell 39, supernova remnant Jellyfish Nebula, open stars clusters M35 or NGC 2168 and NGC 2158.
  • Geminids are a bright meteor shower with approximately 100 meteors per hour every year around mid-December.