21 Persei is a blue rotating star that can be located in the constellation of Perseus. 21 Persei is the brightest star in Perseus based on the Hipparcos 2007 apparent magnitude. The star can be seen with the naked eye, that is, you don't need a telescope/binoculars to see it.
HIP13775 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD18296.
21 Persei has alternative name(s), LT Per.
The location of the star in the galaxy 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 21 Persei, the location is 02h 57m 17.28 and +31d56`03.5 .
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 -31.15 ± 0.18 miliarcseconds/year towards the north and 2.55 ± 0.27 miliarcseconds/year east if we saw them in the horizon.
21 Persei has a spectral type of B9p.... This means the star is a blue star. The star is 7479.00000000 Parsecs from the Galactic Centre or terms of Light Years is 24393.7564977600000000s. The star has a B-V Colour Index of 0 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 10,293 Kelvin.
21 Persei Radius has been calculated as being 3.33 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 2,313,697.28.km. However with the 2007 release of updated Hipparcos files, the radius is now calculated at being round 2.75. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures. The star's Iron Abundance is 0.90 with an error value of 9.99 Fe/H with the Sun has a value of 1 to put it into context.
21 Persei has an apparent magnitude of 5.10 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 -0.27 If you used the 2007 Parallax value, you would get an absolute magnitude of 0.14. 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.
Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 8.43 which gave the calculated distance to 21 Persei as 386.91 light years away from Earth or 118.62 parsecs. It would take a spaceship travelling at the speed of light, 386.91 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 10.20 which put 21 Persei at a distance of 319.77 light years or 98.04 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 7,479.00 Parsecs or 24,393.76 Light Years. The Galacto-Centric Distance is the distance from the star to the Centre of the Galaxy which is Sagittarius A*.
The star is a rotating Alpha2 Canum Venaticorum 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. 21 Persei brightness ranges from a magnitude of 5.113 to a magnitude of 5.089 over its variable period. The smaller the magnitude, the brighter the star. Its variable/pulsating period lasts for 2.9 days (variability).
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.
|Traditional/Proper Name||21 Persei|
|Flamsteed Name||21 Persei|
|Flamsteed Short Name||21 Per|
|Short Name||LT Per|
|Hipparcos Library I.D.||13775|
|Bonner Durchmusterung||BD+31 509|
|Henry Draper Designation||18296|
|Absolute Magnitude||-0.27 / 0.14|
|Visual / Apparent Magnitude||5.10|
|Naked Eye Visible||Yes - Magnitudes|
|Right Ascension (R.A.)||02h 57m 17.28|
|Galactic Latitude||-23.77 degrees|
|Galactic Longitude||151.88 degrees|
|1997 Distance from Earth||8.43 Parallax (milliarcseconds)|
|386.91 Light Years|
|2007 Revised Distance from Earth||10.20 Parallax (milliarcseconds)|
|319.77 Light Years|
|Galacto-Centric Distance||24,393.76 Light Years / 7,479.00 Parsecs|
|Proper Motion Dec.||-31.15 ± 0.18 milliarcseconds/year|
|Proper Motion RA.||2.55 ± 0.27 milliarcseconds/year|
|Radial Velocity||8.50 ± 1.60 km/s|
|Iron Abundance||0.90 ± 9.99 Fe/H|
|Variable Star Class||Rotating|
|Variable Star Type||Alpha2 Canum Venaticorum|
|Mean Variability Period in Days||2.885|
|Variable Magnitude Range (Brighter - Dimmer)||5.089 - 5.113|
|Calculated Effective Temperature||10,293 Kelvin|