The Guide to Autumn Constellations

Map of autumn constellations with Pegasus — The autumn sky reveals its treasures: Pegasus, Andromeda, Cassiopeia... Dive into a stellar journey through the cool, starry nights.
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Scientific Summary

This article presents the autumn constellations of the northern hemisphere, a sky rich in deep-sky objects. The prominent figures are Pegasus (identifiable by its square), Andromeda (home to the M31 galaxy, visible to the naked eye), Perseus (associated with the Perseids), and Cassiopeia (W-shaped, circumpolar). The article details notable objects: the Andromeda Galaxy (M31, 2.5 million light-years away), the Double Cluster in Perseus (NGC 869/884, ~13 million years old), and the variable star Algol (period of 2.87 days). Observation tips are provided (moonless nights, 10x50 binoculars, distance from cities).

Which constellations can be observed in the autumn sky?

The autumn sky in the northern hemisphere offers a rich spectacle of mythological constellations and deep-sky objects. The prominent figures are Pegasus, the winged horse, recognizable by its famous "Pegasus Square" formed by four bright stars; Andromeda, the chained princess, home to the M31 galaxy, the only extragalactic object visible to the naked eye (2.5 million light-years away, apparent diameter of 6 full moons); Perseus, the hero, associated with the Perseid meteor shower (peak around October 8) and containing the Double Cluster NGC 869/884; and Cassiopeia, the vain queen, whose "W" shape is circumpolar for observers above 35° north latitude, accompanied by its neighbor Cepheus. Autumn is also the ideal season to observe remarkable objects: the Andromeda Galaxy (M31), visible to the naked eye under a dark sky; the Double Cluster in Perseus (NGC 869 and NGC 884), two open clusters of young stars (12 to 13 million years old); and the star Algol (β Persei), the prototype of eclipsing variable stars (period of 2.87 days). To observe them, it is recommended to favor moonless nights, use 10x50 binoculars, and stay away from urban areas.

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Flagship Constellations of the Northern Autumn

Autumn offers a sky rich in deep-sky objects and mythological figures. Among the most notable constellations are:

Pegasus (the winged horse), identifiable by its characteristic "square" formed by 4 bright stars (magnitudes 2.1 to 2.8), one of which, Alpheratz, actually belongs to the constellation Andromeda since the IAU delimitation of 1930.
Andromeda, home to the M31 (messier 31) galaxy, visible to the naked eye under a dark sky (distance: 2.5 million light-years, according to HST (hubble Space Telescope) data).
Perseus, associated with the Perseid (autumn Perseids) meteor shower (peak around October 8).
Cassiopeia, shaped like a "W", and Cepheus, its celestial neighbor, both circumpolar for observers above 35° north latitude.

Location and Observation

To locate these constellations, use the Polaris (alpha Ursae Minoris) star as a starting point:

Find the Big Dipper (low on the northwest horizon in autumn).
Draw an imaginary line through the "guards" of the Big Dipper (Dubhe and Merak) to reach Polaris.
From Polaris, move downward toward the east to find Cassiopeia, then Pegasus.

Astronomer Johannes Hevelius (1611-1687) depicted these three constellations in his celestial atlas Firmamentum Sobiescianum (1690), one of the most detailed star catalogs of the 17th century, where Pegasus, Andromeda, and Perseus appear grouped according to their mythological and celestial proximity.

Notable Celestial Objects

This season is ideal for observing:

The Andromeda Galaxy (M31): the only extragalactic object visible to the naked eye from the northern hemisphere. Its apparent diameter is equivalent to 6 full moons (source: ESA (european Space Agency)/Gaia, 2023).
The Double Cluster in Perseus (NGC 869 and NGC 884), two open clusters of young stars (estimated age: 12-13 million years), known since antiquity (cataloged by Hipparchus around 130 BCE) and resolved into two distinct clusters by William Herschel. Notably, despite their brightness, they are absent from Charles Messier's catalog (1730-1817).
The star Algol (β Persei), the prototype of eclipsing variable stars (period of 2.87 days, discovered by John Goodricke (1764-1786)).

Comparative Table of Autumn Constellations

Main Autumn Constellations and Their Characteristics
Constellation	Brightest Star	Mag (apparent Magnitude) max	Notable Deep-Sky Object	Visibility (Northern Hemisphere)
Pegasus	Enif (ε Peg)	2.38	M15 (Globular Cluster)	September to December
Andromeda	Alpheratz (α And)	2.06	M31 (Galaxy)	August to January
Perseus	Mirfak (α Per)	1.79	NGC 869/884 (Open Clusters)	October to March
Cassiopeia	Schedar (α Cas)	2.24	NGC 7635 (Bubble Nebula)	Circumpolar >35°N

References

FAQ: Everything You Need to Know About Autumn Constellations

What are the main constellations visible in autumn in the northern hemisphere?

The main autumn constellations are:
• Pegasus: recognizable by its "square" of four bright stars.
• Andromeda: located next to Pegasus, it contains the famous M31 galaxy.
• Cassiopeia: W-shaped, circumpolar for observers above 35° north latitude.
• Cepheus: neighbor of Cassiopeia, less bright but identifiable.
• Perseus: visible from October, associated with the Perseid meteor showers.
• The Triangle: small constellation located between Andromeda and Aries.

How to find autumn constellations in the sky?

To locate autumn constellations, use the Pole Star as a starting point:
• Start by finding the Big Dipper (low on the northwest horizon in autumn).
• Draw an imaginary line through the two "guard" stars (Dubhe and Merak) to reach Polaris (the North Star).
• From Polaris, move downward toward the east to find Cassiopeia (W-shaped).
• From Cassiopeia, locate the Pegasus Square nearby.
• Andromeda is located right next to the Pegasus Square, and Perseus is to the northeast of Andromeda.

What deep-sky objects are visible in the autumn constellations?

Autumn offers several remarkable deep-sky objects:
• The Andromeda Galaxy (M31): the only extragalactic object visible to the naked eye from the northern hemisphere. It is located in the constellation Andromeda. Its apparent diameter is equivalent to 6 full moons (source: ESA/Gaia, 2023).
• The Double Cluster in Perseus (NGC 869 and NGC 884): two open clusters of young stars, aged about 12 to 13 million years, located in the constellation Perseus.
• The Globular Cluster M15: located in the constellation Pegasus, a very dense and ancient cluster of stars.
• The Bubble Nebula (NGC 7635): located in the constellation Cassiopeia.

What is the Double Cluster in Perseus and how to observe it?

The Double Cluster in Perseus (NGC 869 and NGC 884) is a set of two open clusters of young stars, located in the constellation Perseus. They are aged about 12 to 13 million years and are located approximately 7,500 light-years away. It is one of the most beautiful objects in the autumn sky. To observe it, use 10x50 binoculars or a small telescope; the two clusters appear as two groups of bright and close stars, visible even in semi-urban environments.

What is the star Algol and why is it famous?

Algol (β Persei) is a star in the constellation Perseus, famous for being the prototype of eclipsing variable stars. Its brightness varies regularly over a period of 2.87 days (about 2 days 20 hours and 49 minutes). This variation is due to the presence of a dark companion that passes in front of the main star, significantly reducing its brightness (from magnitude 2.1 to 3.4). This variation was discovered in 1782 by astronomer John Goodricke.

What tips for observing autumn constellations?

To optimize your autumn observations:
• Favor moonless nights (check a lunar calendar).
• Use 10x50 binoculars to distinguish M31, the Perseus clusters, or the Bubble Nebula.
• Stay away from urban areas (light pollution) to benefit from a darker sky.
• Dress warmly (autumn nights are cool) and bring a red-light lamp to preserve your night vision.
• Start your observations after sunset, around 8-9 PM, to enjoy the constellations at their highest.

What is the mythological significance of autumn constellations?

Autumn constellations are rich in Greek mythology:
• Pegasus: the winged horse born from Medusa's blood, ridden by Bellerophon.
• Andromeda: the Ethiopian princess chained to a rock to be devoured by the sea monster Cetus, saved by Perseus.
• Perseus: the hero who killed Medusa and saved Andromeda.
• Cassiopeia: the vain queen of Ethiopia, mother of Andromeda, who was punished for her pride.
• Cepheus: the king of Ethiopia, husband of Cassiopeia and father of Andromeda.