STAR OF THE MONTH
Epsilon Pegasi is full of intriguing surprises
The Great Square of Pegasus is one of the iconic asterisms of autumn, forming a large, and – to be honest – rather blank pattern which is supposed to depict the torso of an upside-down flying horse. Markab (Alpha (_) Pegasi) marks the southwest corner of the square but is not the brightest star in Pegasus. That honour goes to mag. +2.4 Enif (Epsilon (¡) Pegasi) which forms the horse’s nose. In truth the star Sirrah (Delta (b) Pegasi) should be brightest but, marking the northeast corner of the Great Square, this star was re-assigned to become Alpheratz (Alpha (_) Andromedae) back in 1928 and so is not technically in Pegasus.
Enif itself is a cool orange supergiant of spectral class K2. Its distance as measured by the Hipparcos satellite is 690 lightyears and it has a diameter 185 times larger than our Sun’s. Through the eyepiece it is an optical double with a mag. +8.7 line-of-sight companion, 144 arcseconds away at position angle 318°. Position angle is the angle of the
companion as measured eastward from north. The view is worthy of note because it creates an unusual effect.
Centre the pair in the eyepiece using a magnification around 60-100x. While looking at the pair, tap the side of the eyepiece to make the view wobble at right angles to the imaginary line connecting both stars. When you do this, Enif appears to move normally back and forth while the dimmer companion appears to swing like the pendulum of a clock but out of
sync. This optical illusion is believed to be due to the longer time it takes the light from the dimmer star to register on your retina, and is the reason why Enif and its companion are collectively known as the ‘Pendulum Star’.
Enif has other surprises too. In 1972 it became as bright as Altair (Alpha (_) Aquilae), a five-fold increase in brightness. This was very short lived, lasting for around 10 minutes and is believed to have been caused by a rare superflare eruption on the star.