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, to be replaced in its lofty position by Andromeda and Pegasus, which first become visible just after dark in the east to northeast. By late November, the Orion constellation then makes its appearance and stays visible until February, providing a celestial marker for the chilly winter season ahead.
Meanwhile, towards 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” on account of the watery themed constellations that are found there. Nearby constellations also occupying this region of the sky includes Cetus the Whale, Eridanus the River, and Piscis Austrinus the Southern Fish, whose brightest star, Fomalhaut, is called the “Lonely Star of Autumn” as this first magnitude star is situated in a region containing only faint 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 that are almost always visible throughout the year in the north circumpolar constellations. Let us now begin our tour of the autumn sky by concentrating on the five main constellations associated with autumn in the northern hemisphere sky.
The brightest star in the Andromeda constellation is Alpheratz (Alpha Andromedae), a blue subgiant situated 97 light-years distant that shines with a magnitude of +2.06. The best time to see the constellation of Andromeda in the UK is at 21:00 (9 p.m.) during the month of November.
Andromeda is associated with the Andromedids meteor shower, also known as the Bielids, after the comet 3D/Biela. The shower was first documented in Russia on December 6, 1741, and although the shower has largely faded since then, some activity can still be observed during mid-November if conditions allow. The constellation contains a large number of notable deep sky objects, many of which are relatively easy targets for amateur telescopes and even binoculars.
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), the largest galaxy closest to the Milky Way, is counted as a member of the Local Group, a collection of about 30 or so galaxies that collectively are also an outlying member of the Virgo Super Cluster of Galaxies.
M31 is about 2.9 million light-years away and appears to be about six times as large as the full Moon, although the bright central bulge of the galaxy is visible only in small telescopes, while larger instruments will highlight the entire galaxy. It is estimated that M31 contains about 1 trillion stars, which is considerably more than the estimated 200 to 600 billion stars that make up our Milky Way galaxy.
M31 also has 14 or so satellite galaxies, including M32 and M110. Others include the spheroidal dwarf galaxies NGC 147, and NGC 185, as well as the Cassiopeia Dwarf Galaxy in the constellation Cassiopeia. Still others include the galaxies Andromeda Andromeda II, Andromeda III, Andromeda V, Andromeda VIII, Andromeda IX, and Andromeda X in the Andromeda constellation, and several others in the constellations Pegasus, Triangulum, and Pisces.
This enormous star cloud is located in a huge area that is free of the neutral hydrogen found in M31, and is one of the brightest and biggest star-forming regions in the entire Local Group of Galaxies. The region spans about 400 light years, giving it an apparent size of more than 4 degrees as seen from Earth.
Andromeda’s Cluster (Mayall II, NGC 224-G1)
This huge cluster is located about 130,000 light years from M31’s core, and is the brightest globular star cluster in the entire Local Group of Galaxies, with an apparent magnitude of 13.7. The cluster is about twice as massive as Omega Centauri, the most massive globular cluster in the Milky Way, and almost certainly contains an intermediate-mass black hole in its center. This cluster is so big and massive that it is entirely possible that it is not a true globular cluster, but rather the remnant of a dwarf galaxy that was tidally stripped by M31.
The Aquarius constellation‘s brightest star is Sadalsuud (Beta Aquarii) a yellow supergiant found 610 light years away of magnitude +2.87. The best time to view the constellation is at 21:00 (9 p.m.) during the month of October. Four meteor showers also originate in Aquarius, namely the March Aquariids, Eta Aquariids, Delta Aquariids, and Iota Aquariids. Notable Aquarius deep-sky objects that can easily be seen with binocular and small telescopes include the following:
Messier 2 (M2, NGC 7089)
Located about 5 degrees to the north of the star Sadalsuud, M2 is one of the largest known globular clusters, spanning an area of about 175 light years. The cluster shines with a magnitude of 6.3 and contains about 150,000 stars, all of which are between 12 and 13 billion years old. The brightest of these ancient stars, which are mostly yellow giants and red giants, have magnitudes of around 13.
Messier 73 (M73, NGC 6994)
M73 is an asterism made up of four stars that appear to be connected. However, studies performed in 2002 showed that the stars are not physically related, and that they are also moving away from each other in different directions.
Saturn Nebula (NGC 7009, Caldwell 55)
Located about one degree to the westward of the star Nu Aquarii, this pretty nebula is said to resemble the planet Saturn in small telescopes, hence the name. The green tint shown here is believed to be the result of the strong UV radiation emitted by the central star, which now shines with the luminosity of about 20 Suns.
Helix Nebula (NGC 7293, Caldwell 63)
Also known as the Eye of God, this large 2.5 light-year-diameter planetary nebula is located about one degree to the westward of the star Upsilon Aquarii. While small telescopes will only show the nebula as a faint patch of light, instruments with 6-inch and larger apertures will show the dark center, with a brightish star in the middle.
The Capricornus constellation‘s brightest star is Deneb Algedi (Delta Capricorni), a multiple star system located 39 light years from Earth with an apparent visual magnitude of +2.85. The best time to view Capricornus is at 21:00 (9 p.m.) during the month of September.
Five meteor showers also have their radiants in Capricornus: the Alpha Capricornids, the Chi Capricornids, the Sigma Capricornids, the Tau Capricornids, and the Capricorniden-Sagittarids.
Apart from some spectacular stars, Capricornus deep-sky objects of note are mostly limited to Messier 30 (NGC 7099). Located about 28,000 light years away, M30 is an easy target for small telescopes and even large binoculars. Like many other globular clusters in the Milky Way, M30 has undergone a collapse, or contraction of its core. Although the entire cluster spans an area of about 90 light years, 50% of the cluster’s mass is concentrated in a small area that spans only about 17.4 light years, or about 0.12 minutes of arc as seen from Earth.
The Pegasus constellation’s brightest star is Enif (Epsilon Pegasi), an orange super-giant found 690 light years distant with an apparent luminosity of +2.4. The best time to view the constellation of Pegasus is at 21:00 (9 p.m.) during the month of October. The July Pegasids are the only meteor shower associated with the constellation Pegasus.
Although the constellation Pegasus contains many noteworthy and spectacular deep sky objects, not many are visible with amateur equipment. Below are some details of one Pegasus deep sky object that can be observed by amateurs with modest equipment:
Cumulo de Pegaso – Messier 15 (M15, NGC 7078)
This cluster contains about 100,000 stars, and with an apparent magnitude of 6.2, it shines about 360,000 times brighter than our Sun. M15 is thought by most investigators to be about 12 billion years old, which makes it old enough to contain at least one known neutron star, designated M15 C.
The cluster also contains a fairly large number of know variable stars, and at least two known X-ray sources, designated Messier 15 X-1 and Messier 15 X-2, respectively. Furthermore, the cluster contains the planetary nebula Pease 1, which was the first planetary nebula to be discovered in a globular star cluster.
The Pisces constellation‘s brightest star is Eta Piscium, a yellow giant situated 294 light years away that shines with an apparent magnitude of +3.62. The best time to view the constellation is 21:00 (9 p.m.) during the month of November.
Apart from one meteor shower, the Piscids, which is associated with the constellation, Pisces contains several prominent asterisms, which were created by Johannes Hevelius and described in his book, Firmamentum Sobiescianum, published in 1690. These include Piscis Boreus (The North Fish), Piscis Austrinus (The South Fish), Linum Boreum (The North Cord), Linum Austrinum (The South Cord), and Testudo (The Turtle).
Although there are several spectacular deep sky objects in Pisces, none are particularly easy to observe with modest amateur equipment, with the possible exception of the spiral galaxy M74. While this beautiful galaxy is observable, doing so requires dark skies, excellent seeing conditions, and large aperture instruments.
Messier 74 (M74, NGC 628)
M74 is a textbook example of a classical grand design spiral galaxy, and is estimated to contain at least 100 billion stars. It also has the lowest surface brightness of all the Messier objects, which makes it the most difficult Messier object for amateurs to observe. Look for this galaxy about 1.5 degrees east-northeast of Eta Piscium, the most luminous star in the constellation.