AE Andromedae

Lore & Background

AE Andromedae was discovered to be variable in 1928 at the Harvard College Observatory, with a photographic magnitude range of 14.7-15.6, and was designated HV 4476. A year later it received the variable star designation AE Andromedae. At that time it was the brightest stellar object in M31 and maintained a similar brightness for about 20 years. Early in the investigations leading to the definition of LBVs, AE And was identified as similar to the five Hubble–Sandage variables. On the basis of color–color comparisons, it was assigned as 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 16.2 and 17.6, with very similar V magnitudes, and U magnitudes around 0.4 brighter. The first detailed spectrum was published in 1975.

The star's spectrum is peculiar, with emission lines described as very much like Eta Carinae, likely due to a dense stellar wind. The spectrum in 2010 showed weaker emission lines and some weak and variable absorption lines. AE And has prominent allowed and forbidden FeII and hydrogen lines, as well as weaker HeI and NII lines. Some features suggest a B2-B3 spectral type but the emission and variability defy normal classification. The 250.7 nm FeII line is unusually strong in emission, a feature also seen in Eta Carinae and attributed to a UV laser.

AE And was the brightest star in M31 when first noticed during an outburst, at an apparent magnitude around 15, equivalent to an absolute magnitude of −10.2, or around a million times brighter than the Sun. This implies the outburst may have increased the luminosity, which would be unusual for an LBV. Since then, AE And has mostly been in the quiescent, hot LBV phase, with small irregular brightness fluctuations. The temperature is generally considered to be around 20,000 K, consistent with a position on the S Doradus instability strip. The star's stellar winds are strong at 3×10⁻⁵ M☉/yr but slow, around 100 km/s. There is a shell of 6×10⁻³ M☉ thought to have been ejected during the early 20th century outburst at a rate of at least 3×10⁻⁴ M☉/yr. The effective radius when quiescent has been modelled at 55 R☉, based on an effective temperature of 21,000 K.

Reader's Guide

AE Andromedae holds significance as one of the earliest identified luminous blue variables and as a key object in the definition of the LBV class. Its discovery in 1928 and subsequent identification as similar to the Hubble–Sandage variables helped establish the category of extremely luminous, eruptive stars in external galaxies. The star's outburst, during which it became the brightest object in M31 at an absolute magnitude of −10.2, is notable because such an increase in luminosity is unusual for an LBV. Its spectrum, closely resembling that of Eta Carinae with strong FeII and hydrogen emission lines, including a UV laser feature at 250.7 nm, provides a rare laboratory for studying dense stellar winds and mass loss in massive stars. The star's long-term behavior—mostly quiescent with irregular fluctuations—and the presence of an ejected shell from its early 20th century outburst offer insights into the eruptive history and wind dynamics of LBVs. Its mass, though not explicitly calculated, is typical for such stars at 50–120 M☉, underscoring its role as a massive stellar laboratory in the Andromeda Galaxy.

Did You Know?

• AE And was the brightest stellar object in M31 when first discovered and maintained that brightness for about 20 years.
• The 250.7 nm FeII line in its spectrum is unusually strong in emission and has been attributed to a UV laser in Eta Carinae.