AF Andromedae

Lore & Background

AF Andromedae was discovered to be variable in 1927 at the Harvard College Observatory, with a photographic magnitude range of 15.3 to 16.5, and was designated HV 4013. Two years later it received the variable star designation AF Andromedae. Between 1917 and 1953, five or six major eruptions and two or three minor ones were detected, with further eruptions observed in 1970–74, 1987–92, 1998–2001, and 2017. It was often referred to as var 19, after its number in a Hubble list of variable stars in M31 and M33, and identified as one of the five Hubble–Sandage variables. On the basis of color–color comparisons, it was assigned spectral type B and described as related to the P Cygni variables. Observations from 1960 to 1970 showed irregular variations in the B magnitude between 15.5 and 17.6, with visual magnitudes somewhat brighter. The first detailed spectrum was published in 1975.

Reader's Guide

AF Andromedae holds significance as one of the five Hubble–Sandage variables, a group of luminous blue variables in M31 and M33 that have been studied for their extreme eruptions and spectral peculiarities. Its spectrum during outbursts closely resembles that of Eta Carinae, likely due to a dense stellar wind, while in quiescence it is similar to late Of or WN stars. The star exhibits prominent allowed and forbidden FeII and hydrogen emission lines, with weaker HeI lines, and the 250.7 nm FeII line is unusually strong in emission—a feature also seen in Eta Carinae and attributed to a UV laser. The variability and lack of absorption lines defy normal spectral classification, though it was suggested it may be close to class A. AF And was the brightest star in M31 when first noticed during an outburst, at an apparent magnitude around 15, with newer calculations giving a luminosity around half a million times that of the Sun. Its mass has not been calculated explicitly, but such stars are typically 50–120 M☉. The star's repeated eruptions over decades make it a key object for understanding the behavior of LBVs in external galaxies.

Did You Know?

• AF And was originally designated HV 4013 upon discovery in 1927.
• It was considered the brightest variable star in M31.
• The 250.7 nm FeII line in its spectrum is unusually strong and has been attributed to a UV laser in Eta Carinae.
• Its mass is not explicitly calculated, but LBVs typically range from 50 to 120 M☉.

From Var 19 to a New Stellar Class

AF Andromedae entered astronomical records not under its familiar Bayer designation but as Var 19, one of the variable stars Edwin Hubble catalogued in the Andromeda Galaxy back in 1929. For decades it remained a curiosity—an irregular variable, remarkable chiefly for ranking among the brightest stars in its host galaxy. The first detailed look at Var 19 came in 1953, when Hubble and Allan Sandage examined it alongside four other extragalactic variables, though they noted only in a passing footnote that S Doradus might share their nature. It was not until the 1970s that AF Andromedae was formally added to the growing roster of what researchers were beginning to call Hubble–Sandage variables. Spectral analysis revealed P Cygni profile emission lines, drawing comparisons to η Carinae and hinting at a deeper physical connection. By the late 1970s, Roberta Humphreys' work on eight variables in M31 and M33 helped cement the link between these extragalactic stars and the S Doradus type, and AF Andromedae stood as one of the key members anchoring the emerging class.

A Titan Among Stars

As a member of the luminous blue variable class, AF Andromedae belongs to a population of massive, unstable supergiant or hypergiant stars that occupy a narrow band on the Hertzsprung–Russell diagram known as the S Doradus instability strip. In their quiescent state these stars typically present as B-type objects, occasionally slightly hotter, and display unusual emission lines in their spectra. The luminosity range across the class is staggering: the faintest members glow at roughly 10,000 kelvin with output around 250,000 times the Sun's, while the brightest reach temperatures near 25,000 kelvin and luminosities exceeding one million solar units. Masses span an equally wide interval, from about 10 solar masses at the low end—stars that were considerably heavier on the main sequence before rapid mass loss trimmed them down—to objects approaching or even surpassing 100 solar masses. AF Andromedae, as one of the brightest stars in the Andromeda Galaxy, sits comfortably within this extraordinary regime, making it one of the most luminous stars known to astronomers.

Cycles of Outburst and Eruption

The defining behavior of AF Andromedae and its luminous blue variable kin is a restless pattern of brightness and spectral change. During a typical outburst the star's surface temperature drops to roughly 8,500 kelvin, while its bolometric luminosity generally holds steady, producing a visual brightening of one or two magnitudes. These events follow what researchers call strong-active cycles, with two distinct periodicity regimes: variations stretching beyond twenty years or those completing in under ten. A gentler form, the weak-active cycle, produces shifts of less than half a magnitude and always operates on sub-decadal timescales. Beyond these regular pulses, some LBVs are capable of giant eruptions—catastrophic episodes of mass loss and luminosity so extreme that several were originally misidentified as supernovae. Three-dimensional simulations suggest helium opacity fluctuations may drive these violent events. AF Andromedae, like other members of the class, also exhibits small-amplitude variability on sub-annual periods reminiscent of Alpha Cygni stars, alongside entirely stochastic fluctuations that resist simple periodic explanation.

Forging a Stellar Category

AF Andromedae played a quiet but essential role in the slow crystallization of an entire stellar classification. For much of the twentieth century, stars like it were simply filed under irregular variables in extragalactic surveys. The term luminous blue variable first appeared in 1970s literature as a descriptive label rather than a formal name, applied to a handful of stars in M31 and M33 whose spectra bore P Cygni profile lines. The breakthrough came in 1978 when Roberta Humphreys explicitly connected these extragalactic objects to the S Doradus class, and in 1984 when Peter Conti, presenting at an IAU symposium, formally unified S Doradus variables, Hubble–Sandage variables, η Carinae, P Cygni, and other similar stars under the single umbrella of luminous blue variables, abbreviating it to LBV. Conti also drew a clear boundary separating these objects from Wolf–Rayet stars. The General Catalogue of Variable Stars later adopted the acronym SDOR to sidestep confusion with P Cygni spectral profiles, and AF Andromedae remained a cornerstone example of the class it helped define.