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BC Cygni, HIP100404

BC Cygni Location in Cygnus

BC Cygni is a red pulsating supergiant star that can be located in the constellation of Cygnus. The description is based on the spectral class. BC Cygni is not part of the constellation but is within the borders of the constellation.

The star can not be seen by the naked eye, you need a telescope to see it.

BC Cygni is one of the biggest stars that has currently been discovered in the milky way, it is not the largest, that title goes to UY Scuti. We shouldn`t underestimate the size of the star, it is about 856 - 1553 times the size of the radius of the Sun. The radius fluctuates because it is a variable star.

BC Cygni's Alternative Names

HIP100404 is the reference name for the star in the Hipparcos Star Catalogue.

BC Cygni has alternative name(s) :- , BC Cyg.

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+37 3903.

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

Location of BC Cygni

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 BC Cygni, the location is 20h 21m 38.55 and +37° 31` 59.0 .

Radial Velocity and Proper Motion of BC Cygni

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 -5.85 ± 0.62 miliarcseconds/year towards the north and -3.64 ± 0.75 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 the Sun is -3.00 km/s with an error of about 4.50 km/s . When the value is negative then the star and the Sun are getting closer to one another, likewise, a positive number means that two stars are moving away. Its nothing to fear as the stars are so far apart, they won't collide in our life-time, if ever.

Physical Properties (Colour, Temperature) of BC Cygni

BC Cygni has a spectral type of M3.5Ia. This means the star is a red supergiant star. The star has a B-V Colour Index of 3.03 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 0 Kelvin.

BC Cygni Apparent and Absolute Magnitudes

BC Cygni has an apparent magnitude of 8.42 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 0.69 If you used the 2007 Parallax value, you would get an absolute magnitude of -1.18. 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 BC Cygni

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 2.84 which gave the calculated distance to BC Cygni as 1148.46 light years away from Earth or 352.11 parsecs. It would take a spaceship travelling at the speed of light, 1148.46 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 1.20 which put BC Cygni at a distance of 2718.03 light years or 833.33 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.

Using the 2007 distance, the star is roughly 171,885,453.28 Astronomical Units from the Earth/Sun give or take a few. An Astronomical Unit is the distance between Earth and the Sun. The number of A.U. is the number of times that the star is from the Earth compared to the Sun.

Variable Type of BC Cygni

The star is a pulsating Slow Irregular 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. BC Cygni brightness ranges from a magnitude of 8.820 to a magnitude of 8.208 over its variable period. The smaller the magnitude, the brighter the star. Its variable/pulsating period lasts for 0.6 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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BC Cygni Facts

Visual Facts


Primary / Proper / Traditional NameBC Cygni
Alternative NamesHIP 100404, BD+37 3903, BC Cyg
Spectral TypeM3.5Ia
Constellation's Main StarNo
Multiple Star SystemNo / Unknown
Star Type very luminous Supergiant Star
Colour red
GalaxyMilky Way
ConstellationCygnus
Absolute Magnitude 0.69 / -1.18
Visual / Apparent Magnitude8.42
Naked Eye VisibleRequires a 7x50 Binoculars - Magnitudes
Right Ascension (R.A.)20h 21m 38.55
Declination (Dec.)+37° 31` 59.0
Galactic Latitude0.41 degrees
Galactic Longitude75.85 degrees
1997 Distance from Earth2.84 Parallax (milliarcseconds)
 1148.46 Light Years
 352.11 Parsecs
2007 Distance from Earth1.20 Parallax (milliarcseconds)
 2718.03 Light Years
 833.33 Parsecs
 171,885,453.28 Astronomical Units
Proper Motion Dec.-5.85 ± 0.62 milliarcseconds/year
Proper Motion RA.-3.64 ± 0.75 milliarcseconds/year
B-V Index3.03
Radial Velocity-3.00 ± 4.50 km/s

Companions (Multi-Star and Exoplanets) Facts


Exoplanet CountNone/Unaware

Variable Star Details


Variable Star ClassPulsating
Variable Star TypeSlow Irregular
Mean Variability Period in Days0.597
Variable Magnitude Range (Brighter - Dimmer)8.208 - 8.820

Estimated Facts


Sources and Links


SIMBAD SourceLink

Related Stars


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