SU Andromedae

Lore & Background

Thomas Espin noted the possible variability of this star in 1895. Williamina Fleming, in 1906, was examining photographic plates taken for the purpose of creating the Henry Draper Catalogue when she independently discovered and confirmed it as a variable star. The star varies from an apparent visual magnitude of 8.5 at minimum brightness to a magnitude of 8.0 at maximum brightness with no clear period. The spectrum of SU Andromedae is dominated by Swan bands from the molecule C2. These stars were classified as type N under the Harvard scheme, stars with the blue continuum completely obscured by molecular absorption bands. Carbon star spectral types were later refined in the Morgan-Keenan system and SU Andromedae was typically classified as C64, indicating a fairly cool carbon star and the subscript 4 showing modest Swan band intensity. Under the modern revised Morgan-Keenan system, SU Andromedae is classified as C-N5 C26-. The C-N spectral type is to distinguish those stars from the C-R type where the blue continuum is not entirely hidden by absorption bands. A classification based on the infrared spectrum is C5 II, again a moderately cool carbon star with a luminosity class of II for a bright giant. SU Andromedae is 22" from a magnitude 12.77 star, probably an F0 main sequence star. This star has a Gaia Data Release 2 parallax of 0.7479±0.0905 and an absolute magnitude of about +2.4. It has an almost identical space motion as SU Andromedae and is assumed to be a distant co-moving companion. Based on that assumption, the absolute magnitude of SU Andromedae is calculated to be about −2.2.

Reader's Guide

SU Andromedae is significant as a carbon star whose variability was independently discovered by two astronomers—Thomas Espin in 1895 and Williamina Fleming in 1906—using photographic plates from the Henry Draper Catalogue. Its classification history illustrates the evolution of stellar spectral typing: originally classed as type N under the Harvard scheme, it was later refined to C64 in the Morgan-Keenan system, and then to C-N5 C26- under the modern revised Morgan-Keenan system, distinguishing it from C-R type stars. An infrared classification of C5 II further places it as a moderately cool carbon bright giant. The star's possible distant co-moving companion, an F0 main sequence star 22 arcseconds away with nearly identical space motion, allows an estimated absolute magnitude of −2.2 for SU Andromedae, though this remains an assumption. Its slow irregular pulsations with no clear period and a small magnitude range of 0.5 make it a typical example of a slow irregular variable supergiant carbon star.

Did You Know?

• Thomas Espin noted the possible variability of SU Andromedae in 1895.
• Williamina Fleming independently discovered and confirmed its variability in 1906 while examining Henry Draper Catalogue plates.