Alpheratz (Alpha Andromedae, 21 Andromedae) Star Facts

Alpheratz Facts
Alpheratz's star type is star that can be located in the constellation of Andromeda. The description is based on the spectral class.
Alpheratz is a main star of the constellation outline.
Based on the spectral type (B9p) of the star, the star's colour is blue .
Alpha Andromedae is the Bayer name for the star. It was assigned this name by Johann Bayer in 1603. The closer to the start of the Greek Alphabet the name, the brighter the star is. Alpha stars tend to be the brightest in the constellation. A notable exception is Pollux (Beta Geminorum) which is the brighest star in the Gemini constellation.
Alpheratz is the 55th brightest star in the night sky and is the brightest star in Andromeda 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.
Using the most recent figures given by the 2007 Hipparcos data, the star is 97.01 light years away from us. Distance
Alpheratz's Alternative Names
Alpha Andromedae (Alf And) is the Bayer Classification for the star. The Bayer Classification was created by Johann Bayer in the early nineteenth century. The brightest star in the constellation is normally given the Alpha designation although there are exceptions such as Pollux which is Beta Geminorum.

The Id of the star in the Yale Bright Star Catalogue is HR15. HIP677 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD358.

Alpheratz has alternative name(s) :- Sirrah, Sirah. In Arabic, it is known as Surrat al-Faras.

Flamsteed designations are named after the creator, Sir John Flamsteed. Sir John numbered the stars in the constellation with a number and the latin name, this star's Flamsteed designation is 21 Andromedae with it shortened to 21 And.

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+28 4.

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

Location of Alpheratz
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 Alpheratz, the location is 00h 08m 23.17 and +29° 05` 27.0 .

Radial Velocity and Proper Motion of Alpheratz
Proper Motion
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 -163.44 ± 0.17 milliarcseconds/year towards the north and 137.46 ± 0.35 milliarcseconds/year east if we saw them in the horizon.

Radial Velocity
The Radial Velocity, that is the speed at which the star is moving away/towards the Sun is -10.10 km/s with an error of about 0.20 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.

Alpheratz 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 146.79 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 Alpheratz
Alpheratz Colour and Temperature
Alpheratz has a spectral type of B9p. This means the star is a blue star. 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.

Alpheratz Radius
Radius has been calculated as being 3.37 times bigger than the Sun. The Sun's radius is 695,800km, therefore the star's radius is an estimated 2,345,885.60.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 3.37. 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 0.38 with an error value of 9.99 Fe/H with the Sun has a value of 1 to put it into context.

Alpheratz Apparent and Absolute Magnitudes
Alpheratz has an apparent magnitude of 2.07 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 -0.30 If you used the 2007 Parallax value, you would get an absolute magnitude of -0.30. 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 Alpheratz
Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 33.60 which gave the calculated distance to Alpheratz as 97.07 light years away from Earth or 29.76 parsecs. It would take a spaceship travelling at the speed of light, 97.07 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 33.62 which put Alpheratz at a distance of 97.01 light years or 29.74 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 6,134,272.59 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 7,409.00 Parsecs or 24,165.44 Light Years. The Galacto-Centric Distance is the distance from the star to the Centre of the Galaxy which is Sagittarius A*

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.