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3 Geminorum, HD42087, HIP29225, HR2173

3 Geminorum is a blue supergiant star that can be located in the constellation of Gemini. The description is based on the spectral class. The star can be seen with the naked eye, that is, you don't need a telescope/binoculars to see it.

3 Geminorum's Alternative Names

The Id of the star in the Yale Bright Star Catalogue is HR2173. HIP29225 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD42087.

3 Geminorum has alternative name(s) :- , PU Gem.

Flamsteed designations are named after the creator, Sir John Flamsteed. Sir John numbered the stars in the constellation with a number and the latin name, this star's Flamsteed designation is 3 Geminorum with it shortened to 3 Gem.

BD number is the number that the star was filed under in the Durchmusterung or Bonner Durchmusterung, a star catalogue that was put together by the Bonn Observatory between 1859 to 1903. The star's BD Number is BD+23 1226.

More details on star alternative names can be found at Star Names .

Location of 3 Geminorum

The location of the star in the night sky is determined by the Right Ascension (R.A.) and Declination (Dec.), these are equivalent to the Longitude and Latitude on the Earth. The Right Ascension is how far expressed in time (hh:mm:ss) the star is along the celestial equator. If the R.A. is positive then its eastwards. The Declination is how far north or south the star is compared to the celestial equator and is expressed in degrees. For 3 Geminorum, the location is 06h 09m 43.99 and +23° 06` 48.5 .

Proper Motion of 3 Geminorum

All stars like planets orbit round a central spot, in the case of planets, its the central star such as the Sun. In the case of a star, its the galactic centre. The constellations that we see today will be different than they were 50,000 years ago or 50,000 years from now. Proper Motion details the movements of these stars and are measured in milliarcseconds. The star is moving -2.65 ± 0.45 miliarcseconds/year towards the north and -0.92 ± 0.88 miliarcseconds/year east if we saw them in the horizon. The Radial Velocity, that is the speed at which the star is moving away/towards us is 16.00000 km/s with an error of about 4.30 km/s .

Physical Properties (Colour, Temperature) of 3 Geminorum

3 Geminorum has a spectral type of B2.5Ib. This means the star is a blue supergiant star. The star has a B-V Colour Index of 0.19 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 7,480 Kelvin.

3 Geminorum Apparent and Absolute Magnitudes

3 Geminorum has an apparent magnitude of 5.75 which is how bright we see the star from Earth. Apparent Magnitude is also known as Visual Magnitude. If you used the 1997 Parallax value, you would get an absolute magnitude of If you used the 2007 Parallax value, you would get an absolute magnitude of -7.44. Magnitude, whether it be apparent/visual or absolute magnitude is measured by a number, the smaller the number, the brighter the Star is. Our own Sun is the brightest star and therefore has the lowest of all magnitudes, -26.74. A faint star will have a high number.

Distance to 3 Geminorum

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as -0.51 which gave the calculated distance to 3 Geminorum as -6395.36 light years away from Earth or -1960.78 parsecs. It would take a spaceship travelling at the speed of light, -6395.36 years to get there. We don't have the technology or spaceship that can carry people over that distance yet.

In 2007, Hipparcos data was revised with a new parallax of 0.23 which put 3 Geminorum at a distance of 14181.01 light years or 4347.83 parsecs. It should not be taken as though the star is moving closer or further away from us. It is purely that the distance was recalculated. 3 Geminorum brightness ranges from a magnitude of 5.821 to a magnitude of 5.778 over its variable period. The smaller the magnitude, the brighter the star. Its variable/pulsating period lasts for 6.8 days (variability).

Source of Information

The source of the information if it has a Hip I.D. is from Simbad, the Hipparcos data library based at the University at Strasbourg, France. Hipparcos was a E.S.A. satellite operation launched in 1989 for four years. The items in red are values that I've calculated so they could well be wrong. Information regarding Metallicity and/or Mass is from the E.U. Exoplanets. The information was obtained as of 12th Feb 2017.

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3 Geminorum Facts

Visual Facts


 3 Geminorum
Alternative NamesHD 42087, HIP 29225, HR 2173, 3 Gem, BD+23 1226, PU Gem
Spectral TypeB2.5Ib
Multiple Star SystemYes
Star Type very luminous Supergiant Star
Colour blue
GalaxyMilky Way
ConstellationGemini
Visual / Apparent Magnitude5.75
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)06h 09m 43.99
Declination (Dec.)+23° 06` 48.5
Galactic Latitude1.77 degrees
Galactic Longitude187.75 degrees
1997 Distance from Earth-0.51 Parallax (milliarcseconds)
 -6395.36 Light Years
 -1960.78 Parsecs
2007 Distance from Earth0.23 Parallax (milliarcseconds)
 14181.01 Light Years
 4347.83 Parsecs
Proper Motion Dec.-2.65 ± 0.45 milliarcseconds/year
Proper Motion RA.-0.92 ± 0.88 milliarcseconds/year
B-V Index0.19
Radial Velocity16.00 ± 4.30 km/s

Companions (Multi-Star and Exoplanets) Facts


Exoplanet CountNone/Unaware

Variable Star Details


Mean Variability Period in Days6.807
Variable Magnitude Range (Brighter - Dimmer)5.778 - 5.821

Estimated Facts


Calculated Effective Temperature7,480 Kelvin

Sources and Links


SIMBAD SourceLink

Multi-Star System

The star has been identified as being a multi-star system, one in which there is at least one star in close orbit to another star or two or more stars orbiting a central point. The stars may be of equal mass, unequal mass where one star is stronger than the other or be in groups orbiting a central point which doesn't necessarily have to be a star. More information can be found on my dedicated multiple star systems page. The source of the info is Simbad. The file is dated 2000 so any differences between this and any other source will be down to the actual source from where the information came from.


Proper Motion mas/yr
H.D. IdB.D. IdStar CodeMagnitudeR.A.Dec.SpectrumColourYear
42087+23 1226.0A5.80000-2.000009.00000B1Blue/White
B9.900001891
C14.400001891

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