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EM Cephei, HD208392, HIP108073

EM Cephei is a blue eclipsing binary system subgiant star that can be located in the constellation of Cepheus. The description is based on the spectral class. The star can not be seen by the naked eye, you need a telescope to see it.

EM Cephei's Alternative Names

HIP108073 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD208392.

EM Cephei has alternative name(s) :- , EM Cep.

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+61 2216.

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

Location of EM Cephei

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 EM Cephei, the location is 21h 53m 48.11 and +62° 36` 51.9 .

Proper Motion of EM Cephei

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 -0.88 ± 0.45 miliarcseconds/year towards the north and -2.52 ± 0.54 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 -23.10000 km/s with an error of about 1.10 km/s .

EM Cephei 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 568.61 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) of EM Cephei

EM Cephei has a spectral type of B1IV:. This means the star is a blue subgiant star. The star is 7,587.00 Parsecs from the Galactic Centre or terms of Light Years is 24,746.01 s. The star has a B-V Colour Index of 0.25 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 7,483 Kelvin.

EM Cephei Radius has been calculated as being 15.80 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 10,996,605.00.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 10.83. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures.

EM Cephei Apparent and Absolute Magnitudes

EM Cephei has an apparent magnitude of 7.01 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 -2.27 If you used the 2007 Parallax value, you would get an absolute magnitude of -1.45. 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 EM Cephei

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 1.39 which gave the calculated distance to EM Cephei as 2346.50 light years away from Earth or 719.42 parsecs. It would take a spaceship travelling at the speed of light, 2346.50 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 2.03 which put EM Cephei at a distance of 1606.72 light years or 492.61 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 7,587.00 Parsecs or 24,746.01 Light Years. The Galacto-Centric Distance is the distance from the star to the Centre of the Galaxy which is Sagittarius A*.

Variable Type of EM Cephei

The star is a eclipsing binary system W Ursae Majoris variable type which means that its size changes over time. The Variable Type is usually named after the first star of that type to be spotted. EM Cephei brightness ranges from a magnitude of 7.158 to a magnitude of 7.040 over its variable period. The smaller the magnitude, the brighter the star. Its variable/pulsating period lasts for 0.8 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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EM Cephei Facts

Visual Facts


Primary / Proper / Traditional NameEM Cephei
Alternative NamesHD 208392, HIP 108073, BD+61 2216, EM Cep
Spectral TypeB1IV:
Multiple Star SystemYes
Star TypeSubgiant Star
Colour blue
GalaxyMilky Way
ConstellationCepheus
Absolute Magnitude-2.27 / -1.45
Visual / Apparent Magnitude7.01
Naked Eye VisibleRequires a 7x50 Binoculars - Magnitudes
Right Ascension (R.A.)21h 53m 48.11
Declination (Dec.)+62° 36` 51.9
Galactic Latitude6.46 degrees
Galactic Longitude104.03 degrees
1997 Distance from Earth1.39 Parallax (milliarcseconds)
 2346.50 Light Years
 719.42 Parsecs
2007 Distance from Earth2.03 Parallax (milliarcseconds)
 1606.72 Light Years
 492.61 Parsecs
Galacto-Centric Distance24,746.01 Light Years / 7,587.00 Parsecs
Proper Motion Dec.-0.88 ± 0.45 milliarcseconds/year
Proper Motion RA.-2.52 ± 0.54 milliarcseconds/year
B-V Index0.25
Radial Velocity-23.10 ± 1.10 km/s
Eccentricity0.08
Semi-Major Axis7669.00
Stellar Luminosity (Lsun)568.61

Companions (Multi-Star and Exoplanets) Facts


Exoplanet CountNone/Unaware

Variable Star Details


Variable Star ClassEclipsing binary system
Variable Star TypeW Ursae Majoris
Mean Variability Period in Days0.806
Variable Magnitude Range (Brighter - Dimmer)7.040 - 7.158

Estimated Facts


Calculated Effective Temperature7,483 Kelvin

Sources and Links


SIMBAD SourceLink

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