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Alpheratz (Alpha Andromedae, 21 Andromedae) Star Facts

Alpheratz Facts

  • Alpheratz is a 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 .
  • 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.
  • Alpheratz has a radius that is 2.70 times bigger than the Suns. Radius
  • 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 1603. The brightest star in the constellation is normally given the Alpha designation, 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 named the stars in the constellation with a number and its latin name, this star's Flamsteed designation is 21 Andromedae. The Flamsteed name can be 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 -10.10000 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.

Physical Properties of Alpheratz

Alpheratz Temperature and Colour

Based on the star's spectral type of B9p , Alpheratz's colour and type is blue star. The star's effective temperature is 13,800 Kelvin which is hotter than our own Sun's effective Temperature which is 5,777 Kelvin.

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.

Alpheratz Radius

Alpheratz Radius has been calculated as being 2.70 times bigger than the Sun. The Sun's radius is 695,800km, therefore the star's radius is an estimated 1,878,660.00.km. If you need the diameter of the star, you just need to multiple the radius by 2.

Alpheratz Mass

The Alpheratz's solar mass is 3.80 times that of our star, the Sun. The Sun's Mass is 1,989,100,000,000,000,000,000 billion kg. which to calculate using this website is too large. To give idea of size, the Sun is 99.86% the mass of the solar system.

Alpheratz Iron Abundance

Alpheratz 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. The value comes from the Hipparcos Extended Catalog.

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 33.60000 which gave the calculated distance to Alpheratz as 97.07 light years away from Earth or 29.76 parsecs. If you want that in miles, it is about 570,638,164,974,933.02, based on 1 Ly = 5,878,625,373,183.61 miles.

In 2007, Hipparcos data was revised with a new parallax of 33.62000 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*.

Travel Time to Alpheratz

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)
Walking416,264,129,794.82
Car120542,137,659.83
Airbus A38073688,392,009.75
Speed of Sound (Mach 1)767.26984,789,714.14
Concorde (Mach 2)1,534.5442,394,801.82
New Horizons Probe33,0001,971,409.67
Speed of Light670,616,629.0097.01

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 Alpheratz Facts and Figures

Visual Facts


Primary / Proper / Traditional NameAlpheratz
Alternative NamesAlpha Andromedae, Alf And, Sirrah, Sirah, Surrat al-Faras, HD 358, HIP 677, HR 15, 21 Andromedae, 21 And, BD+28 4
Spectral TypeB9p
Constellation's Main StarYes
Multiple Star SystemYes
Star TypeStar
ColourBlue
GalaxyMilky Way
ConstellationAndromeda
Absolute Magnitude -0.30 / -0.30
Visual / Apparent Magnitude2.07
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)00h 08m 23.17
Declination (Dec.)+29° 05` 27.0
Galactic Latitude-32.84290326 degrees
Galactic Longitude111.73192384 degrees
1997 Distance from Earth33.60000 Parallax (milliarcseconds)
 97.07 Light Years
 29.76 Parsecs
2007 Distance from Earth33.62000 Parallax (milliarcseconds)
 97.01 Light Years
 29.74 Parsecs
 6,134,272.59 Astronomical Units
Galacto-Centric Distance24,165.44 Light Years / 7,409.00 Parsecs
Proper Motion Dec.-163.44000 ± 0.17000 milliarcseconds/year
Proper Motion RA.137.46000 ± 0.35000 milliarcseconds/year
B-V Index0
Radial Velocity-10.10000 ± 0.20 km/s
Iron Abundance0.3800 ± 9.99 Fe/H
Eccentricity0.11300
Inclination105.66000
Semi-Major Axis6662.0000000
Stellar Luminosity (Lsun)146.7900000
Orbital Period (Days)96.69600
Argument Of Periastron77.31000
Brightest in Night Sky55th

Companions (Multi-Star and Exoplanets) Facts


Exoplanet CountNone/Unaware

Estimated Calculated Facts


Radius (x the Sun)2.70
Effective Temperature10,293 Kelvin
Mass Compared to the Sun3.80

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
358+28 4.0A2.20000133.00000-168.00000A0White
B11.400001.00000-16.000001954

Location of Alpheratz in Andromeda


Alpheratz Location in Andromeda

The map was generated using Night Vision, an awesome free application by Brian Simpson.

Andromeda Main Stars


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