R Andromedae

Lore & Background

R Andromedae shows periodic brightness variations every 409 days, with maximum brightness reaching mv = 5.8 and minima nearly 10 magnitudes fainter. The rise to maximum is about twice as fast as the decline. It is classified as a Mira variable, contracting and expanding regularly, changing size and temperature, which causes the brightness variations.

Its spectral type varies with brightness. At maximum it is S5/4.5e, an S-type red giant with ZrO bands about twenty times stronger than TiO bands. At minimum it has been classified as late as S8,8e. The star is an asymptotic giant branch star, having exhausted core helium and burning helium in a shell and hydrogen in a shell closer to the surface. It undergoes dredge-up events that bring fusion products to the surface, enhancing carbon and zirconium.

R Andromedae varies in temperature and luminosity but is typically about 2,500 K and 6,300 L☉. Angular diameter measurements give radii of 493±129 R☉ and 476±120 R☉ on different dates, assuming a distance of 532 pc. Other measurements based on spectral energy distribution give a luminosity of 6,000 L☉ and a temperature of 1,900 K, implying a radius of 730 R☉. It is one of the largest known stars.

Reader's Guide

R Andromedae holds significant scientific importance as one of the stars in which Paul Merrill discovered absorption lines of technetium, an unstable element, providing direct evidence that nucleosynthesis occurs in stars. This finding was foundational for understanding stellar evolution and element production. Additionally, the SH molecule was found for the first time outside Earth in the star's atmosphere, marking a milestone in astrochemistry. As a Mira variable on the asymptotic giant branch, it exemplifies the late stages of stellar evolution, undergoing regular pulsations, mass loss at a rate of 1.09×10−6 M☉/yr, and dredge-up events that alter its surface composition. Its S-type spectrum, with strong ZrO bands and a carbon-to-oxygen ratio near unity, places it in an intermediate class between carbon stars and oxygen-rich giants. The star's large size, with radii estimates ranging from about 476 to 730 R☉, makes it one of the largest known stars. Its variability and spectral changes provide a natural laboratory for studying stellar atmospheres and nucleosynthesis.

Did You Know?

• The SH molecule was detected for the first time outside Earth in the atmosphere of R Andromedae.
• R Andromedae loses mass at a rate of 1.09×10−6 M☉/yr due to stellar winds.
• Its angular diameter has been measured at 8.63±1.42 mas and 8.32±1.27 mas on different dates.