Universe Guide

BT Capricorni

BT Capricorni Facts

  • BT Capricorni is a pulsating giant star that can be located in the constellation of Capricornus. The description is based on the spectral class.
  • BT Capricorni is not part of the constellation outline but is within the borders of the constellation.
  • Based on the spectral type (M3III) of the star, the star's colour is red .
  • The star can not be seen by the naked eye, you need a telescope to see it.
  • Using the most recent figures given by the 2007 Hipparcos data, the star is 994.40 light years away from us. Distance

BT Capricorni's Alternative Names

HIP108236 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD208271.

BT Capricorni has alternative name(s) :- BT Cap, BT Cap.

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-21 6120.

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

Location of BT Capricorni

The location of the giant 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 object is compared to the celestial equator and is expressed in degrees. For BT Capricorni, the location is 21h 55m 38.39 and -21° 08` 23.6 .

Proper Motion of BT Capricorni

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 -29.46 ± 0.33 milliarcseconds/year towards the north and -36.69 ± 0.57 milliarcseconds/year east if we saw them in the horizon. . 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 of BT Capricorni

BT Capricorni Colour and Temperature

Based on the star's spectral type of M3III , BT Capricorni's colour and type is red giant star. The star has a B-V Colour Index of 1.48 which means the star's temperature is about 4,025 Kelvin. The temperature was calculated using information from Morgans @ Uni.edu.

BT Capricorni 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 296.30 that I have given is based on the value in the Simbad Hipparcos Extended Catalogue at the University of Strasbourg from 2012.

BT Capricorni Radius

BT Capricorni estimated radius has been calculated as being 18.59 times bigger than the Sun. The Sun's radius is 695,800km, therefore the star's radius is an estimated 12,937,649.27.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 22.355010396890543782544733794. The figure is derived at by using the formula from SDSS rather than peer reviewed papers. It has been known to produce widely incorrect figures.

BT Capricorni Apparent and Absolute Magnitudes

BT Capricorni has an apparent magnitude of 7.09 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 0.07 If you used the 2007 Parallax value, you would get an absolute magnitude of -0.33. 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 BT Capricorni

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 3.95000 which gave the calculated distance to BT Capricorni as 825.73 light years away from Earth or 253.16 parsecs. If you want that in miles, it is about 4,854,157,329,398,902.29, based on 1 Ly = 5,878,625,373,183.61 miles.

In 2007, Hipparcos data was revised with a new parallax of 3.28000 which put BT Capricorni at a distance of 994.40 light years or 304.88 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 62,885,575.94 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. The star's Galacto-Centric Distance is 7,242.00 Parsecs or 23,620.75 Light Years. The Galacto-Centric Distance is the distance from the star to the Centre of the Galaxy which is Sagittarius A*.

Travel Time to BT Capricorni

The time it will take to travel to this star is dependent on how fast you are going. U.G. has done some calculations as to how long it will take going at differing speeds. A note about the calculations, when I'm talking about years, I'm talking non-leap years only (365 days).

The New Horizons space probe is the fastest probe that we've sent into space at the time of writing. Its primary mission was to visit Pluto which at the time of launch (2006), Pluto was still a planet.

DescriptionSpeed (m.p.h.)Time (years)
Airbus A380736906,061,380.27
Speed of Sound (Mach 1)767.269869,136,086.40
Concorde (Mach 2)1,534.54434,567,476.82
New Horizons Probe33,00020,207,914.42
Speed of Light670,616,629.00994.40

Variable Type of BT Capricorni

The star is a pulsating Semi-Regular Star w 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. BT Capricorni brightness ranges from a magnitude of 7.139 to a magnitude of 7.030 over its variable period. The smaller the magnitude, the brighter the star. Its variable/pulsating period lasts for 23.3 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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Additional BT Capricorni Facts and Figures

Visual Facts

Primary / Proper / Traditional NameBT Capricorni
Alternative NamesBT Cap, HD 208271, HIP 108236, BD-21 6120, BT Cap
Spectral TypeM3III
Constellation's Main StarNo
Multiple Star SystemYes
Star TypeGiant Star
GalaxyMilky Way
Absolute Magnitude 0.07 / -0.33
Visual / Apparent Magnitude7.09
Naked Eye VisibleRequires a 7x50 Binoculars - Magnitudes
Right Ascension (R.A.)21h 55m 38.39
Declination (Dec.)-21° 08` 23.6
Galactic Latitude-49.64760054 degrees
Galactic Longitude31.62414795 degrees
1997 Distance from Earth3.95000 Parallax (milliarcseconds)
 825.73 Light Years
 253.16 Parsecs
2007 Distance from Earth3.28000 Parallax (milliarcseconds)
 994.40 Light Years
 304.88 Parsecs
 62,885,575.94 Astronomical Units
Galacto-Centric Distance23,620.75 Light Years / 7,242.00 Parsecs
Proper Motion Dec.-29.46000 ± 0.33000 milliarcseconds/year
Proper Motion RA.-36.69000 ± 0.57000 milliarcseconds/year
B-V Index1.48
Stellar Luminosity (Lsun)296.3000000

Companions (Multi-Star and Exoplanets) Facts

Exoplanet CountNone/Unaware

Variable Star Details

Variable Star ClassPulsating
Variable Star TypeSemi-Regular Star w
Mean Variability Period in Days23.300
Variable Magnitude Range (Brighter - Dimmer)7.030 - 7.139

Estimated Calculated Facts

Radius (x the Sun)22.36
Effective Temperature4,025 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
208271-21 6120.0A7.20000-38.00000-28.00000M3Red

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