As summer turns to autumn in the northern hemisphere, the changing season brings many changes to the night sky. The Summer Triangle, which has dominated overhead for several months, now begins to sink lower in the west, while Andromeda and Pegasus become prominent in the east and northeast during the evening. By late November, the Orion constellation also makes its appearance, providing a celestial marker for the chilly winter season ahead.
Meanwhile, toward the southern night sky during the late evening can be seen the zodiac constellations of Aquarius, Capricornus, and Pisces in a region of the heavens known as the “Celestial Sea” because of the watery-themed constellations found there. Nearby constellations occupying this region of the sky include Cetus the Whale, Eridanus the River, and Piscis Austrinus the Southern Fish. Its brightest star, Fomalhaut, is sometimes called the “Lonely Star of Autumn” because it stands out in a region containing relatively few bright stars.
What constellations can be seen in autumn?
Together, all the star constellations mentioned above bring into view many spectacular deep-sky objects to supplement those visible throughout much of the year in the north circumpolar constellations. Let us begin our tour of the autumn sky by concentrating on five prominent constellations associated with autumn in the northern hemisphere sky.
Andromeda Constellation
The brightest star in the Andromeda constellation is Alpheratz (Alpha Andromedae), a blue-white star situated about 97 light-years away that shines at magnitude +2.06. The best time to see Andromeda in the UK is around 21:00 (9 p.m.) during November.
Andromeda is associated with the Andromedids meteor shower, also known as the Bielids after Comet 3D/Biela. Although spectacular Andromedid meteor storms occurred during the 19th century, the shower is now much weaker, with occasional activity still detected.
The constellation contains a large number of notable deep-sky objects, several of which are accessible with binoculars or amateur telescopes. Below are some details of a few of the most prominent Andromeda deep-sky objects.
Andromeda Galaxy (Messier 31, NGC 224)
The Andromeda Galaxy (M31) is the nearest large galaxy to the Milky Way and one of the principal members of the Local Group.
M31 is about 2.5 million light-years away and spans roughly 3 degrees of sky from end to end, or about six times the apparent width of the full Moon. Under sufficiently dark skies its bright central region can be seen with the naked eye, while binoculars reveal a much larger elongated glow.
The galaxy is thought to contain around a trillion stars, although estimates vary. It is accompanied by dozens of known satellite galaxies, including the prominent M32 and M110.
NGC 206
NGC 206 is a vast, bright star cloud in the disk of M31 and one of the largest star-forming regions in the Local Group. It contains numerous young, luminous stars and spans roughly 4,000 light-years.
Andromeda’s Cluster (Mayall II, G1)
This massive cluster lies far out in M31’s halo and is one of the brightest and most massive globular clusters in the Local Group. Mayall II is substantially more massive than a typical Milky Way globular cluster, leading astronomers to investigate whether it could be the stripped nucleus of a former dwarf galaxy.
Unlike M31 itself, G1 is not a casual binocular target. Its great distance means that even though the cluster is intrinsically enormous and luminous, observing it visually requires a reasonably large telescope and a good finder chart.
Aquarius Constellation
The Aquarius constellation‘s brightest star is Sadalsuud (Beta Aquarii), a yellow supergiant located roughly 540 light-years away with an apparent magnitude of about +2.9. The best time to view the constellation is around 21:00 (9 p.m.) during October.
Several meteor showers have radiants in Aquarius, including the Eta Aquariids and Delta Aquariids. Notable Aquarius deep-sky objects accessible to amateur observers include the following:
Messier 2 (M2, NGC 7089)
Located about 5 degrees north of Sadalsuud, M2 is one of the larger globular clusters in the Milky Way, spanning roughly 175 light-years. The cluster shines at around magnitude +6.3 and contains approximately 150,000 stars. At an age of roughly 13 billion years, it is among the ancient globular clusters of the Galactic halo.
Under a dark sky M2 can be detected in binoculars as a small hazy patch. A small telescope shows its bright, concentrated appearance, while larger apertures begin to resolve individual stars around its outer regions.
Messier 73 (M73, NGC 6994)
M73 is an asterism consisting of four stars that appear close together in the sky. Studies of their distances and motions indicate that the stars do not form a genuine physical star cluster.
Saturn Nebula (NGC 7009, Caldwell 55)
Located roughly one degree west of Nu Aquarii, this planetary nebula received its name because its extended structure reminded early telescopic observers of Saturn. Its vivid colors in astronomical images arise from glowing ionized gases energized by the hot stellar remnant at its center.
The Saturn Nebula is a much easier visual target than many planetary nebulae because of its relatively high surface brightness. In a small telescope it initially resembles a tiny bluish or greenish disk. More aperture and magnification begin to reveal its elongated shape, although the prominent ansae or “handles” seen in detailed photographs are considerably more difficult to observe.
Helix Nebula (NGC 7293, Caldwell 63)
Also known as the Eye of God, the Helix Nebula is one of the nearest planetary nebulae to Earth. Its large apparent size and low surface brightness make it a challenging visual target despite its proximity. Dark skies and low magnification help reveal its broad, ghostly ring, while larger telescopes show additional structure.
Capricornus Constellation
The Capricornus constellation‘s brightest star is Deneb Algedi (Delta Capricorni), a multiple star system located about 39 light-years from Earth with an apparent magnitude around +2.8. The best time to view Capricornus is around 21:00 (9 p.m.) during September.
Several minor meteor showers have radiants in Capricornus, while the best known is the Alpha Capricornids.
Messier 30 (M30, NGC 7099)
Among the most prominent deep-sky objects in Capricornus is Messier 30. Located about 27,000 light-years away, M30 is a compact globular cluster that can be detected with binoculars under suitable conditions and is readily visible through small telescopes.
M30 has undergone core collapse, leaving it with an exceptionally dense central region. Small telescopes show a compact hazy glow with a brighter center, while larger instruments can begin resolving some of its outer stars.
Pegasus Constellation
The Pegasus constellation’s brightest star is Enif (Epsilon Pegasi), an orange supergiant located roughly 690 light-years away with an apparent magnitude of about +2.4. The best time to view the constellation of Pegasus is around 21:00 (9 p.m.) during October.
Pegasus contains a number of galaxies and other deep-sky objects, although many require moderate or large amateur telescopes. One particularly accessible object is the globular cluster M15.
Messier 15 (M15, NGC 7078)
M15 is an ancient globular cluster located roughly 35,000 light-years from Earth. It contains hundreds of thousands of stars packed into a region about 175 light-years across and has an apparent magnitude of about +6.2.
Like M30, M15 has undergone core collapse, producing an extremely dense concentration of stars near its center. The cluster also contains numerous variable stars, X-ray sources and pulsars.
For observers, M15 is conveniently placed about 4 degrees northwest of Enif. Binoculars show it as a small fuzzy point, while telescopes reveal a strongly concentrated core surrounded by a granular halo. Larger apertures resolve increasing numbers of stars toward its outskirts.
M15 is also notable for containing Pease 1, discovered in 1928 and recognized as the first planetary nebula known inside a globular cluster.
Pisces Constellation
The Pisces constellation‘s brightest star is Eta Piscium, a yellow giant situated roughly 350 light-years away that shines at an apparent magnitude of about +3.6. The best time to view the constellation is around 21:00 (9 p.m.) during November.
Pisces is a large but relatively faint constellation. Although it contains numerous distant galaxies, most are challenging targets for modest amateur equipment. The principal exception for experienced observers is the face-on spiral galaxy M74.
Messier 74 (M74, NGC 628)
M74 is a classic grand-design spiral galaxy located roughly 32 million light-years away. Its two well-defined spiral arms contain numerous young star clusters and star-forming regions.
Despite having an integrated apparent magnitude of around +9, M74 has a low surface brightness because its light is spread over a comparatively large area of sky. This makes it one of the more challenging Messier objects for visual observers.
Look for the galaxy about 1.5 degrees east-northeast of Eta Piscium. A moonless, transparent sky is particularly important for M74, and low magnification generally gives a better chance of detecting its faint diffuse glow than pushing the magnification too high.