OU Andromedae
OU Andromedae is a bright, variable star in the constellation Andromeda. Its visible brightness changes from magnitude 5.87 to 5.94. It is classified as G1IIIe, a giant star with emission lines, currently on the subgiant branch as it evolves toward the red giant stage. The star was identified as variable in 1985 and named OU Andromedae in 1986.
OU Andromedae is also a strong X-ray source, indicating an active corona. A notable X-ray flare was observed by the Chandra X-ray Observatory in 2001 and was likely caused by a coronal mass ejection; additional X-ray activity was reported in 2007.
The star rotates unusually fast for an evolved star: its projected rotation speed is about 21.5 km/s and its rotation period is around 24.3 days. This rapid spin could result from engulfing a nearby giant planet or from retaining a strong magnetic field from an Ap-star phase.
The corona shows high magnetic activity, with solar-like active regions covering about 30% of the surface and hot plasma at roughly 7.5 million kelvin, fueling frequent flares.
OU Andromedae is about 449 light-years away. It has a mass of roughly 2.85 solar masses, a radius near 9.5 solar radii, and a luminosity about 71 times that of the Sun. Its surface temperature is about 5360 K, with near-solar metallicity. Studying OU Andromedae helps scientists understand magnetic activity in evolved stars.
This page was last edited on 2 February 2026, at 13:22 (CET).