Pegasus
What is Pegasus?
Constellations· Autumn evenings (N) · the Great Square. The Great Square of autumn, the season's main signpost, is the body of the winged horse. From one corner, a line of stars through Andromeda leads to the Andromeda Galaxy.
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Stars of the figure
| Star | Magnitude |
|---|---|
| Enif | 2.4 |
| Markab | 2.5 |
| Matar | 2.9 |
The 3 brightest of the 8 stars used in the figure (8 lines). Lower magnitude means brighter.
Constellation basics
| Season | autumn |
|---|---|
| Shape | the Great Square |
| Next door | gateway to Andromeda |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Alpheratz, the corner of the Square that belongs next door
The Great Square of autumn is drawn with four stars, but the one at the north-east corner, Alpheratz, is not officially part of Pegasus. It belongs to Andromeda. Old star charts shared it between the two figures, and it once also carried the name Delta Pegasi. So one of the four stars of the horse's body now belongs to another constellation.
This came about when the International Astronomical Union fixed the constellation boundaries around 1930. Before that, charts disagreed about which figure a star between two constellations belonged to. The IAU divided the whole sky into 88 regions with no gaps, and since then every star lies in exactly one constellation. Alpheratz was assigned to Andromeda as the princess's head, and the Great Square keeps a trace of that rule.
The constellation in depth
The body of Pegasus is the Great Square of autumn. Bright stars are scarce in the autumn sky, so this large square of four stars stands out. In star charts it is a winged horse flying upside down, with a line of stars reaching west from the Square as its neck and head.
Counting how many stars the naked eye can see inside the Square shows how dark the night is. From a city the answer is none, or one or two; under a dark country sky it is a dozen or more. Because the Square holds only faint stars, it makes a handy yardstick for sky brightness.
In 1995 a planet was found around the star 51 Pegasi in this constellation, the first planet confirmed around a Sun-like star. It is a gas giant about half as heavy as Jupiter, yet it orbits so close to its star that a year there lasts just over four days. The two scientists who found it shared the 2019 Nobel Prize in Physics.
From Alpheratz, the north-east corner of the Square, a line of stars runs east into Andromeda, and just above that line lies the Andromeda Galaxy, M31. Viewing the Square and Andromeda together in the Sky view on an autumn evening shows the whole route from the Square to the galaxy.
Frequently asked
- When can I see Pegasus?
- Pegasus is best seen in autumn.
- What is the brightest star in Pegasus?
- The brightest star in Pegasus is Enif, at magnitude 2.4.
- In one line, what is Pegasus?
- Constellations — The Great Square of autumn, the season's main signpost, is the body of the winged horse.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.