AF Andromedae (AF And) is a

luminous blue variable Luminous blue variables (LBVs) are massive evolved stars that show unpredictable and sometimes dramatic variations in their spectra and brightness. They are also known as S Doradus variables after S Doradus, one of the brightest stars of the Larg ...

(LBV), a type of

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as ...

. The star is one of the most luminous variables in M31, the

Andromeda Galaxy The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with the diameter of about approximately from Earth and the nearest large galaxy to the Milky Way. The gala ...

Discovery

The star was discovered to be variable in 1927, with a photographic magnitude range of 15.3 to 16.5, at the

Harvard College Observatory The Harvard College Observatory (HCO) is an institution managing a complex of buildings and multiple instruments used for astronomical research by the Harvard University Department of Astronomy. It is located in Cambridge, Massachusetts, United St ...

and designated HV 4013. It was considered to be the brightest variable star in M31 Two years later it was given the variable star designation AF Andromedae. Between 1917 and 1953, five or six major eruptions were detected and two or three minor ones. More eruptions were observed in 1970-74, 1987-92, 1998-2001, and 2017. AF And was often referred to as var 19, after its number in a Hubble list of variable stars in M31 and M33. It was identified as one of the five

Hubble–Sandage variable Luminous blue variables (LBVs) are massive evolved stars that show unpredictable and sometimes dramatic variations in their spectra and brightness. They are also known as S Doradus variables after S Doradus, one of the brightest stars of the Lar ...

s: Var A, Var B, Var C, and Var 2 in M33, and Var 19 in M31. On the basis of color–color comparisons, it was assigned as spectral type B and described as related to the

P Cygni P Cygni (34 Cygni) is a variable star in the constellation Cygnus. The designation "P" was originally assigned by Johann Bayer in ''Uranometria'' as a nova. Located about 5,300 light-years (1,560 parsecs) from Earth, it is a hypergiant ...

variables. Observations from 1960 to 1970 showed irregular variations in the B (blue) magnitude between 15.5 and 17.6, with visual magnitudes somewhat brighter. The first detailed spectrum was published in 1975.

Spectrum

AF And in outbursts has a peculiar emission line spectrum described as very much like

Eta Carinae Eta Carinae (η Carinae, abbreviated to η Car), formerly known as Eta Argus, is a stellar system containing at least two stars with a combined luminosity greater than five million times that of the Sun, located around distant in t ...

, likely due to a dense stellar wind. When quiescent, the spectrum is similar to late Of or WN stars. AF And has prominent allowed and forbidden Fe_II and

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

lines in its

emission spectrum The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an electron making a atomic electron transition, transition from a high energy state to a lower energy st ...

, as well as weaker He_I lines. The variability and lack of absorption lines defy a normal spectral classification, but it was suggested that it may be close to class A. The 250.7 nm Fe_II line is unusually strong in emission. The same feature in Eta Carinae's spectrum has been attributed to a UV laser.

Properties

AF And was the brightest star in M31 when it was first noticed during an outburst, at an apparent magnitude around 15, over a million times more luminous than the Sun. Newer calculations give a luminosity slightly less than a million times that of the Sun. The star's mass has not been calculated explicitly, but this type of star is massive, typically .

References

{{DEFAULTSORT:AF Andromedae Andromeda (constellation) Andromeda Galaxy Extragalactic stars Luminous blue variables Andromedae, AF ? Stars in the Andromeda Galaxy

Discovery

Spectrum

Properties

See also

References