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 object is compared to the celestial equator and is expressed in degrees. For HAT-P-21, the location is 11h 25m 06.00 and 41° 01` 41.00 .
HAT-P-21 has a spectral type of G3. This means the star is a yellow star.
HAT-P-21 has been calculated as 1.11 times bigger than the Sun. The Sun's radius is 695,800km, therefore the star's radius is an estimated 768,859.00.km. If you need the diameter of the star, you just need to multiple the radius by 2.
The star's metallicity is 0.080000, 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.
The star is believed to be about 10.20 Billion years old. To put in context, the Sun is believed to be about five billion years old and the Universe is about 13.8 billion years old.
HAT-P-21 has an apparent magnitude of 11.46 which is how bright we see the star from Earth. Apparent Magnitude is also known as Visual Magnitude. Using the supplied Parallax value, you would get an absolute magnitude of 4.44 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.
The Parallax of the star is given as 3.94 which gives a calculated distance to HAT-P-21 of 828.46 light years from the Earth or 254.00 parsecs. It would take a spaceship travelling at the speed of light, 828.46 years to get there. We don't have the technology or spaceship that can carry people over that distance yet.
The star is roughly 52,390,895.72 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.
|Primary / Proper / Traditional Name||HAT-P-21|
|Constellation's Main Star||No|
|Multiple Star System||No / Unknown|
|Age||10.20 Billion Years Old|
|Visual / Apparent Magnitude||11.46|
|Naked Eye Visible||Requires a 4.5 - 6 Inch Telescope - Magnitudes|
|Right Ascension (R.A.)||11h 25m 06.00|
|Declination (Dec.)||41° 01` 41.00|
|Distance from Earth||3.94 Parallax (milliarcseconds)|
|828.46 Light Years|
|52,390,895.72 Astronomical Units|
|Radius (x the Sun)||1.11|
|Name||Status||Mass (Jupiters)||Orbital Period (Days)||Eccentricity||Discovered||Semi-Major Axis||Periastron||Inclination|
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