Enif, the nose of Pegasus, and its companion star create a pendulum-like optical illusion

By Pete Lawrence

Published: Friday, 20 September 2024 at 09:42 AM

Enif, also known as Epsilon (ε) Pegasi, is a star in the constellation Pegasus that’s full of intriguing surprises 

The Great Square of Pegasus is one of the iconic asterisms of autumn, forming a large pattern that’s supposed to depict the torso of an upside-down flying horse.

Discover our favourite autumn stargazing targets, autumn constellations and autumn meteor showers.

Star Enif, Epsilon Pegasi, as seen by the Digitized Sky Survey DSS2

Markab (Alpha (α) 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 (ε) Pegasi) which forms the horse’s nose.

In truth the star Sirrah (Delta (δ) Pegasi) should be brightest but, marking the northeast corner of the Great Square, this star was re-assigned to become Alpheratz (Alpha (α) 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 the Sun’s.

A view of the sky using data captured by ESA's Hipparchus satellite. Labelled is Alpha Persei, or Mirfak, in the constellation Perseus. Credit: ESA
A view of the sky using data captured by ESA’s Hipparchus satellite. Labelled is Alpha Persei, or Mirfak, in the constellation Perseus. Click to expand. Credit: ESA

Observing star Enif and its pendulum effect

Through the eyepiece, Enif 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.

Chart showing the constellation Pegasus, the Great Square and star Enif, Epsilon Pegasi. Credit: BBC Sky at Night Magazine
Chart showing the constellation Pegasus, the Great Square and star Enif, Epsilon Pegasi. Click to expand. Credit: BBC Sky at Night Magazine

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 (α) Aquilae), a five-fold increase in brightness.

This was very short-lived, lasting for around 10 minutes and believed to have been caused by a rare superflare eruption on the star.

Have you managed to recreate the pendulum effect while observing Enif? If so, let us know by emailing contactus@skyatnightmagazine.com

This guide appeared in the September 2018 issue of BBC Sky at Night Magazine