Categories: Stars

Star Facts: Epsilon Pegasi

Star Enif in Pegasus shown closest to M15 globular cluster

Enif (Epsilon Pegasi) is a massive, highly evolved orange giant, and the brightest star in the constellation Pegasus with an apparent visual magnitude of +2.4. It is also the 81st brightest star in the entire night sky, and according to measurements made by the Hipparcos astrometry satellite is situated approximately 690 light years from Earth.

Quick Facts

• Constellation: Pegasus
• Coordinates: RA 21h 44m 11.15614s|Dec. +09° 52′ 30.0311″
• Distance to Earth: 690 ± 20 light years
• Star Type: K2 Ib
• Mass: 11.7 sol
• Radius: 185 sol
• Apparent Magnitude: 2.399
• Luminosity: 12,250 sol
• Surface Temperature: 4,379K
• Rotational Velocity: 8 km/sec
• Age: 20.0 ± 4.5 million years
• Other Designations: Enif, Enf, Enir, Al Anf, Os Pegasi, 8 Peg, BD+09° 4891, FK5815, HD 206778, HIP 107315, HR 8308, SAO 127029


The constellation Pegasus can be seen from July to January from the northern hemisphere, although best seen at about 9 PM Local Time in October, when it is highest in the sky. The image below indicates the position of the star Epsilon Pegasi, where it marks out the “muzzle” of the horse. Although the star is readily visible without optical aid, the easiest way to find it is to look for it a few degrees to the southwest of the globular cluster M15.

Physical Properties

The star’s stellar classification as a K-type star indicates that it is a highly evolved orange giant that has swollen to a diameter of about 185 times that of the Sun. Although Epsilon Pegasi’s temperature of 4,337K is much lower than that of the Sun, its enormous bulk means that all of its light is radiated from a much larger surface area, hence its luminosity of 12,250 sol.

Due to its mass, Enif has exhausted its supply of hydrogen in only about 20 million years, and it is expected to die within the next few million years or so. However, since its mass straddles the dividing line between stars that explode as supernova and those that do not, the mechanism of its demise is not certain. Nonetheless, Enif has been observed to brighten in dramatic fashion on several occasions, which suggests that super giant stars like Enif tend to erupt in massive and violent flares that dwarf similar events on the Sun.

Despite the irregular flares or mass ejections, Enif is a known slow irregular variable star whose brightness varies between +0.7 to +3.5 magnitudes. The stars’ spectrum also displays an overabundance of the elements strontium and barium, likely as the result of the S-process of nucleosynthesis that is now occurring in the outer layers of the star.

One other notable aspect of this star is its high peculiar velocity of 21.6 km/sec. Unlike proper motions and radial velocities of objects that are calculated relative to the Sun, an object’s peculiar motion refers to that objects’ motion relative to a galactic rest frame, which is assumed to contain some objects that are stationary relative to the moving object. Thus, Enif’s high peculiar motion refers to its movement relative to the average motion of local objects in Milky Way galaxy, and not its motion across the sky relative to the motion of the Sun.


The star’s traditional name “Enif” derives from the Arabic word for “nose”, as a reference to the stars’ position at the horses’ muzzle. One other traditional name for this star is “Fom al Feras”, meaning “The horses’ mouth”.

In China, the star Enif forms an asterism known as Wei Sù, which means “Rooftop”, along with the stars Alpha Aquarii and Theta Pegasi. Due to its position in this asterism, Epsilon Pegasi is known as Wei Sù san, or “[the] Third Star of Rooftop”.

Peter Christoforou :