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81 Ceti, HD16400, HIP12247, HR771

81 Ceti is a white to yellow giant star that can be located in the constellation of Cetus. 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. 81 Ceti has at least 1 Extrasolar Planets believed to be in orbit around the star.

81 Ceti's Alternative Names

The Id of the star in the Yale Bright Star Catalogue is HR771. HIP12247 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD16400.

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 81 Ceti with it shortened to 81 Cet.

The Gould star designation is one that was designed by American astronomer, Benjamin Apthorp Gould. Gould stars are predominantly in the Southern and Equatorial constellations but do appear in northern constellations such as Bootes and Orion. The star has the designation 277 G. Ceti. There are no stars with a Gould designation in Ursa Major for example.

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-04 436.

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

Location of 81 Ceti

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 81 Ceti, the location is 02h 37m 41.78 and -03° 23` 45.8 .

Proper Motion of 81 Ceti

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 -42.91 ± 0.37 miliarcseconds/year towards the north and 40.18 ± 0.45 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 8.76000 km/s with an error of about 0.28 km/s .

81 Ceti Luminosity

Luminosity is the amount of energy that a star pumps out and its relative to the amount that our star, the Sun gives out. The figure of 58.04 that I have given is based on the value in the Simbad Hipparcos Extended Catalogue at the University of Strasbourg from 2012.

Physical Properties (Colour, Temperature, Mass) of 81 Ceti

81 Ceti has a spectral type of G5III:. This means the star is a white to yellow giant star. The star is 7,453.00 Parsecs from the Galactic Centre or terms of Light Years is 24,308.95 s. The star has a B-V Colour Index of 1.02 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 4,811 Kelvin.

81 Ceti Radius has been calculated as being 9.69 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 6,743,884.85.km. If you need the diameter of the star, you just need to multiple the radius by 2. However with the 2007 release of updated Hipparcos files, the radius is now calculated at being round 9.21. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures. The star's solar mass is 2.40 times that of the Sun's. The Sun's Mass is 1,989,100,000,000,000,000,000 billion kg. which to calculate using this website is too large. To give idea of size, the Sun is 99.86% the mass of the solar system.

The star's metallicity is -0.060000, this value is the fractional amount of the star that is not Hydrogen (X) or Helium (Y). An older star would have a high metallicity whereas a new star would have a lower one.

81 Ceti Apparent and Absolute Magnitudes

81 Ceti has an apparent magnitude of 5.65 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 0.71 If you used the 2007 Parallax value, you would get an absolute magnitude of 0.82. 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 81 Ceti

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 10.29 which gave the calculated distance to 81 Ceti as 316.97 light years away from Earth or 97.18 parsecs. It would take a spaceship travelling at the speed of light, 316.97 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 10.81 which put 81 Ceti at a distance of 301.72 light years or 92.51 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.

The star's Galacto-Centric Distance is 7,453.00 Parsecs or 24,308.95 Light Years. The Galacto-Centric Distance is the distance from the star to the Centre of the Galaxy which is Sagittarius A*.

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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81 Ceti Facts

Visual Facts


Primary / Proper / Traditional Name81 Ceti
Alternative NamesHD 16400, HIP 12247, HR 771, 277 G. Ceti, 81 Cet, BD-04 436
Spectral TypeG5III:
Multiple Star SystemNo / Unknown
Star TypeGiant Star
Colour white to yellow
GalaxyMilky Way
ConstellationCetus
Absolute Magnitude0.71 / 0.82
Visual / Apparent Magnitude5.65
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)02h 37m 41.78
Declination (Dec.)-03° 23` 45.8
Galactic Latitude-55.25 degrees
Galactic Longitude174.49 degrees
1997 Distance from Earth10.29 Parallax (milliarcseconds)
 316.97 Light Years
 97.18 Parsecs
2007 Distance from Earth10.81 Parallax (milliarcseconds)
 301.72 Light Years
 92.51 Parsecs
Galacto-Centric Distance24,308.95 Light Years / 7,453.00 Parsecs
Proper Motion Dec.-42.91 ± 0.37 milliarcseconds/year
Proper Motion RA.40.18 ± 0.45 milliarcseconds/year
B-V Index1.02
Radial Velocity8.76 ± 0.28 km/s
Iron Abundance0.01 ± 0.02 Fe/H
Eccentricity0.10
Semi-Major Axis6795.00
Stellar Luminosity (Lsun)58.04

Companions (Multi-Star and Exoplanets) Facts


Exoplanet Count1

Estimated Facts


Calculated Effective Temperature4,811 Kelvin
Mass Compared to the Sun2.40

Sources and Links


SIMBAD SourceLink

List of Extrasolar Planets orbiting 81 Ceti


NameStatusMass (Jupiters)Orbital Period (Days)EccentricityDiscoveredSemi-Major AxisPeriastronInclination
81 Cet bConfirmed952.7000.20620082.5175.0002008.000

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