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Astronomy for Amateurs
Chapter 5
Some Key Constellations
Now that we are ready to map the sky, let's look at some key constellations to help us find our way around. Since we are approaching this topic from the viewpoint of ancient Mediterranean astronomers, we will see only a few southern constellations.
The following black-on-white constellation maps are available thanks to the International Astronomical Union and Sky & Telescope magazine. Constellations are not just pictures in the sky but have established boundaries; any part of the sky is within a particular constellation. The IAU/S&T maps show the featured constellations' boundaries by highlighting the space within the boundary in white with the rest of the map in gray. The featured constellation includes names and Bayer designations of stars in the constellation, along with deep sky objects (celestial objects beyond the solar system) such as nebulae, star clusters, and galaxies. These are usually labeled with their Messier or NGC numbers (the Messier and New Galactic catalogs will be discussed later). The key to the deep sky objects is as follows:
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Open star cluster |
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Globular star cluster |
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Diffuse nebula |
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Planetary nebula |
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Galaxy |
These deep sky objects will be discussed later, but we will look at a few as we peruse their constellations.
The easiest place to find the IAU/S&T maps is Wikipedia in the articles about the constellations in question. You can download a PDF version optimized for 5 x 7 Kindle devices or tablets at Vocademy (IAU/S&T Constellations).
The dark-background maps were created using screenshots of Stellarium (a sky mapping program, app, and website: stellarium-web.org).
To see a larger version of any of the maps, right-click or long press the image and open it in a new tab.
The outlines of constellations are subjective, and different creators of star charts use different outlines. In 1952, H. A. Rey, an author of children's picture books famous for his Curious George series, published a book titled The Stars: A New Way to See Them. His fanciful reimaging of the constellations included faint stars to make outlines that looked more like their namesakes, albeit a bit of a reach. Nevertheless, modern renditions of the constellations frequently mix traditional outlines and Rey's caricatures. For example, Stellarium uses the traditional outline for Leo but Rey's outline for Gemini. The IAU/S&T maps use the traditional outline for Leo, with some of Rey's outline added in subdued lines. In the following constellation maps, I emphasize the constellations using mainly the traditional outlines or what I tend to see. I use these outlines because I am trying to show recognisable star patterns, not whimsical figures that require a stretch of the imagination. Nevertheless, Rey's renditions are a fun way to see the constellations, and I do recommend his book, especially for children. It's my earliest memory of my interest in astronomy.
Now, let's see those constellations.
Let's start in late Autumn looking high overhead around 9:00 in the evening.
Pegasus
Let's start with Pegasus (the mythical winged horse) because it aids in finding Pisces and, thus, the vernal equinox. Pegasus, also known as "the Great Square," forms a fairly conspicuous square high in the autumn and early winter sky. Notable features to help you identify Pegasus are the double triangle attached to the northwest corner and the constellation Andromeda attached to the northeast corner.
Pisces
Pisces (the fish) makes a large V straddling the southeast corner of Pegasus. Pisces isn't as conspicuous as Pegasus; its brightest star is only about fourth magnitude, so use Pegasus to help find it. A notable feature is the "circlet" on the northwest leg of the V. Picses is a fairly large but inconspicuous constellation, so think big when looking for it. The vernal equinox is near the western edge of Pisces.
Aries
Aries (the ram) is a small but easily spotted constellation just off the eastern leg of Pisces. It appears as no more than three second, third, and fourth-magnitude stars in a crooked line but is easy to distinguish, especially when it is rising relatively horizontally above the eastern horizon.
Andromeda
Notable for being the home of the Andromeda Galaxy, Andromeda (daughter of Cepheus) extends off the northeast corner of Pegasus. In fact, the star at the northeast corner of the Great Square is not part of Pegasus but is Alpha Andromedae.
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The Andromeda Galaxy is the closest full galaxy to us and is large and bright. Under good conditions, you can see the central portion as a faint fuzzy patch that disappears when you look directly at it (This is called averted vision. Your peripheral vision is more sensitive than your central vision, but your central vision is sharper and better at seeing color.). That fuzzy patch is only the center of the galaxy. If our eyes were sensitive enough, we would see the galaxy spanning a distance equal to five times the diameter of the Moon.
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Now, let's move on to winter.
Orion
Now, let's skip a few hours of right ascension to the east to Orion (the hunter), a fairly large, conspicuous constellation of bright stars centered on the celestial equator. Notable features include its hourglass shape resembling a man's torso and tunic, along with three bright stars in a row making Orion's belt. The northeast star of the hourglass is Betelgeuse (said to mean armpit in Arabic, but that is disputed), a bright, notably red star. The southwest star is Rigel (Greek for knee). The other stars of the hourglass are Bellatrix (left shoulder) and Saiph (right leg).
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Orion is conspicuous enough to be notable in its own right but is also notable for being the home of the Orion Nebula. This bright cloud of gas (mostly hydrogen) and dust is large and bright enough to be visible to the unaided eye. Look at the middle star of Orion's sword on a night with good seeing. That star looks a bit fuzzy because it is actually multiple stars embedded in the Orion nebula. The Orion Nebula is a diffuse nebula (having no particular form).
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There is a lot of hullabaloo in the popular media about Betelgeuse being about to explode into a supernova, one of the most powerful explosions in the universe. Keep a watchful eye on Betelgeuse because astronomers think it could explode anytime...within the next 100,000 years.
You may have noticed that Pegasus looks nothing like a flying horse, and Aries looks nothing like a ram, etc. Most constellations look no more like their namesakes than the George Washington Bridge looks like George Washington. Orion is one of several notable exceptions. You can clearly see his torso, belt, and sword. We will see some other constellations and asterisms that look like their namesakes below. You can make up your own asterisms if you wish, like the following one.
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Taurus
Taurus (the bull) is just northwest of Orion and next in line along the ecliptic after Aries. Notable features include two open star clusters (loose conglomerations of stars held together by their mutual gravity, also called galactic clusters). The brightest stars of the Hyades cluster appear as a rough V shape. The Pleiades (aka the seven sisters) is a star cluster about 10 degrees northeast of the Hyades.
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The Pleiades is a conspicuous star cluster often mistaken for the Little Dipper. It is commonly used as an impromptu vision test; if you have excellent vision (and excellent seeing), you can see all seven stars in the Seven Sisters. The Pleiades (Subaru in Japanese culture) is the namesake of the Subaru automobile.
Aldebaran is the brightest star in Taurus at magnitude 0.86 and is notably red. It appears on the southern leg of the V in the Hyades but is between us and the cluster, not part of it.
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Gemini
Over on Orion's other shoulder is Gemini (the twins). Notable features are the two bright first and second-magnitude stars, Castor and Pollux, and the roughly parallel lines of modest stars extending toward Orion.
Canis Minor
Canis minor (the Little Dog) is hardly worth mentioning, except it contains one of the brightest stars in the sky, Procyon, at magnitude 0.34. In the northern mid-latitudes, Procyon rises shortly before Sirius about 25 degrees to the north. Sirius is known as "The Dog Star," and Procyon is Greek for "before the dog."
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Canis Major
Canis Major (the Big Dog) lies southeast of Orion's right leg. It contains the brightest star in the sky (after the Sun, of course), Sirius.
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Sirius, Procyon, and Betelgeuse form a conspicuous triangle called the "Winter Triangle."
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Carina
Carina was once part of a giant constellation called Argo Navis (the ship of Jason and the Argonauts fame). Modern astronomers broke Argo Navis up into three constellations: Vela (the sail), Puppis (the deck), and Carina (the keel). Although most of Argo Navis is too far south to be seen from the Mediterranean area, Ptolemy listed it as one of his 48 constellations.
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Carina contains the second-brightest star in the sky, Canopus, at magnitude -0.74. It is about 35 degrees nearly due south of Sirius. For observers in the lower mid latitudes, when Sirius is high in the sky, Canopus makes a brief appearance low on the southern horizon.
Auriga
Now, let's head back north of Orion to Auriga (the goat herdsman). Auriga is a moderately conspicuous constellation making a potato shape extending off Taurus's northern horn. It is notable for containing Capella, one of the brightest stars in the sky at magnitude 0.08.
Not far from Capella is Epsilon (ε) Aurigae, the point star in a small triangle of third and fourth magnitude stars called "the Kids." Epsilon Aurigae is notable because about every 27 years, its brightness drops from about third magnitude to about fourth magnitude, with the dimming period lasting about two years. An old theory of why this dimming occurs was that Episilon Aurigae was orbiting a semitransparent supergiant star, so large and tenuous that the visible star was partially dimmed when behind the supergiant star. The current theory is that the visible star is orbited by another star embedded in a thick dust cloud. This cloud partially obscures the visible star when it moves between that star and the Earth.
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And now, on to spring.
Leo
Leo (the Lion) is one of the few constellations that has some resemblance to its namesake. The backward question mark asterism to the north of Regulus, the constellation's brightest star, is reminiscent of a resting lion. Regulus is one of the brightest stars in the sky at magnitude 1.39.
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Cancer
Cancer (the crab) appears as little more than two fourth- and fifth-magnitude stars between Gemini and Leo. Between these stars is an open star cluster called the Beehive Cluster. The Beehive Cluster appears as a faint luminous patch visible to the unaided eye under good seeing conditions.
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Virgo
Virgo (the virgin) consists of one of the brightest stars in the sky, Spica, at magnitude 0.97, in a sea of inconspicuous, mostly fifth- and sixth-magnitude stars.
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Virgo is notable for containing much of the Virgo Cluster, a gravitationally bound group of approximately 1,300 galaxies, of which 20 to 30 are visible in amateur telescopes.
Corvus
Corvus (the crow) stands out as four 3rd magnitude stars in the Virgo Void, southwest of Spica. The constellation appears as a slightly misshapen kite or maybe just a distorted rectangle. Nevertheless, it tends to stand out, so it gets a mention here.
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And then, on to Summer.
Libra
Nothing to see here; move along.
Think of Libra (the scales) as the eastern extension of the Virgo Void. However, since this list includes all the other zodiacal constellations, we might as well complete it with Libra.
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Centaurus
For observers in the northern mid latitudes, an innocuous curve of second- and third-magnitude stars peeks over the southern horizon in spring and summer. This is about all of Centaurus we get to see in the Mediterranean area.
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However, just above the southern horizon, if you live far enough south, a fuzzy star shows itself for a few hours each night. This is a globular star cluster (a dense spherical conglomeration of many thousands of stars) so bright that Ptolemy wrote about it, thinking it was a single star, notwithstanding that under good conditions, it appears as a fuzzy patch nearly as large as the Moon. Johann Bayer gave it the designation of omega (ω ), so it is thus called Omega Centauri. The modern New Galactic Catalog lists it as NGC 5139.
Of course, Centaurus is famous for containing Alpha Centauri, the closest star to our solar system (not counting its gravitationally bound companion, Proxima Centauri). Alpha Centauri was unknown to ancient Mediterranean astronomers, so it played no part in their astronomy.
Scorpius
Scorpius (the scorpion) is one of the most conspicuous constellations in the sky. Its scorpion tail of bright stars is unmistakable in the summer sky to the south.
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Its brightest star, Antares, shines a brilliant red at magnitude 0.96. Antares is Greek for "rival to Mars," as Ares is the Greek name for the god of war.
Being close to the center of the Milky Way, Scorpius is rich in deep sky objects. This includes two conspicuous open clusters, M6 (the Butterfly Cluster) and M7, sometimes called the Ptolemy Cluster (Messier numbers will be discussed later). Next to Antares is M4, a bright globular cluster. Scorpius contains another globular cluster, M80, and one other open cluster bright enough for most amateur telescopes.
Sagittarius
The main part of Sagittarius (the archer) is a conspicuous asterism called the Teapot.
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The spout of the Teapot points to the center of the Milky Way galaxy, where there is a conspicuous star cloud. The Milky Way bulges nodicibly in the Sagittarius Scorpiuis area. Being near the center of the Milky Way, Sagittarius is rich in deep sky objects. These include seven bright globular clusters, four conspicuous open clusters, and three prominent diffuse nebulae.
When Sagittarius is in the southwest, the Teapot appears to be pouring tea onto Scorpius's tail with steam (the Milky Way) rising out of the spout. With a bit of imagination, you can make a teaspoon asterism north of the Teapot, and if you don't mind including the constellation Corona Australis, you can add a lemon.
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Capricornus
Capricornus (the goat) is a large, modest constellation of third and fourth-magnitude stars resembling a fat boomerang. It is somewhat conspicuous simply because it is in an area devoid of other bright stars. It contains one globular cluster but is otherwise notable only for being part of the zodiac.
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Following our current path, we have gone full circle and are back to Autumn, with Pegasus again high overhead. Now, let's look more to the south.
Aquarius
Continuing Capricornus's mediocrity, we fill out the zodiac with Aquarius (the water bearer).
Aquarius has a few interesting deep sky objects, including two globular clusters, two planetary nebulae, and M73, a sparsely populated open cluster of only four bright stars…except in 2002, it was determined that they are four unrelated stars that just happen to line up from our viewpoint. Aquarius also contains a large, bright planetary nebula (a roughly spherical/bubble-shaped cloud of gas thrown off by a red giant star late in its life). This is called the Helix Nebula. It is nearly the size of the Moon in the sky, but is challenging to observe in small telescopes because, although intrinsically bright for such objects (magnitude 7.6), its brightness is spread out over a fairly large area.
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Piscis Austrinus
Piscis Austrinus (the southern fish) is an unassuming constellation in the stellar void occupied by Capricornus and Aquarius. It contains an isolated magnitude 1.16 star, Fomalhaut (corrupted Arabic meaning mouth of the fish). Fomalhaut is the Easter Island of stars, conspicuous by its lonesomeness, one of the brighter stars yet farthest from its nearest bright neighbor. Look for Fomalhaut in autumn evenings, shining by itself in the barrens vacant of notable stars to the south.
Grus
Just south of Pisces Austrinus is Grus (the crane), a curve of second-, third-, and fourth-magnitude stars that stands out among the fainter background stars. For observers in southern mid latitudes, Grus makes a brief appearance low in the southern sky in the summer and autumn months.
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Now, we need to backtrack to Summer and follow the Milky Way north from Sagittarius.
Aquila
North of Sagittarius, we find Aquila (the eagle). It's not remarkable except that it contains the bright star Altair at magnitude 0.76.
Saggita and Delphinus
North and northeast of Aquila are the small constellations of Sagitta (the arrow) and Delphinus (the dolphin). These two constellations are hardly notable except that, despite containing no stars brighter than third magnitude, they stand out against the fainter background stars. Besides that, they're cute. Delphinus and Sagitta are two of the few constellations that resemble their namesakes. Delphinus and Sagitta each contain a moderately bright globular cluster.
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Cygnus
Continuing along the Milky Way, we find Cygnus (the Swan), also called the Northern Cross. Cygnus is a prominent Summer and Fall constellation. In late December, the Northern Cross stands upright on the northwest horizon in the evening and is sometimes called the Christmas Cross.
The brightest star in Cygnus is Deneb (the tail), one of the brighter stars at magnitude 1.25. Cygnus contains several nebulae and two moderately bright open star clusters. These include the Veil Nebula (two arcs of gas thrown off by a supernova about 15,000 years ago) and the North America Nebula.
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The second-brightest star in Cygnus, Albireo, is interesting because, in a small telescope, it appears as two stars (a double star). Albireo is a favorite target of amateur astronomers because the two stars have strikingly contrasting colors. Albireo A is bright yellow, and Albireo B is deep blue. Astronomers have yet to determine if Albireo is a binary star (two stars gravitationally bound and orbiting one another) or is just an optical double (two unrelated stars lined up as seen from Earth).
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Lyra
Lyra (the lyre) is a small but conspicuous constellation containing the fifth brightest star in the sky, Vega, at magnitude 0.03. Lyra appears as a parallelogram of modest stars, with Vega off the northwest corner. It has two objects of interest to amateur astronomers. The planetary nebula known as the Ring Nebula (M57) lies between the two stars opposite to Vega in the parallelogram.
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Epsilon Lyrae, to the east of Vega, is called the "Double Double." To the unaided eye, Epsilon Lyrae appears as a single star. In binoculars, it appears as a double star. Under higher power, each star appears as a double star. All the components of Epsilon Lyrae are gravitationally bound, making a quadruple system.
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Vulpecula
Vulpecula (the fox) is so inconspicuous as to be invisible. However, it contains a favorite target for amateur astronomers, a planetary nebula called the Dumbbell Nebula (M27).
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Deneb, Vega, and Altair form the "Summer Triangle."
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Now that we are looking high in the sky, let's leave the Milky Way and circle the sky to the west.
Hercules
Let's go back to the west side of Cygnus, past Lyra, to Hercules (Hercules), the only northern constellation with a human namesake depicted in classical art with his head to the south. The body of Hercules is a conspicuous asterism called the Keystone. About 1/3 of the way from the northwest star of the keystone to the southwest star is a large, bright globular star cluster (M13) that is a favorite target for amateur astronomers.
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With excellent seeing and good eyesight, the Hercules cluster is just visible to the unaided eye.
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Boötes
Boötes (the herdsman, plowman, ox driver, or something) lies about two hours (30 degrees) west of Hercules. There is nothing particularly notable about Boötes except that it contains the fourth brightest star in the sky, Arcturus. At magnitude 0.05, Arcturus is notably reddish in color, being a red giant. To identify Arcturus, follow the handle of the Big Dipper and continue its curve toward the south until it intersects a bright reddish-yellow star. Oh, and how do you pronounce Boötes? It's something like Boh-oh-tis.
Corona Borealis
Corona Borealis (the northern crown) is a somewhat conspicuous semicircle of third- and fourth-magnitude stars with one star much brighter than the rest, the magnitude 2.24 Alphecca.
Canes Venatici
Canes Venatici (the hunting dogs) lies in an area devoid of notable stars except the constellation's brightest star, the magnitude 2.84 Cor Caroli. Canes Venatici is included in this list of notable constellations because it contains a favorite target of amateur astronomers, the Whirpool Galaxy (M51).
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Ursa Major
Now, from Canes Venatici, let's grab the handle of everyone's favorite constellation, the Big Dipper. Actually, the Big Dipper is a large, conspicuous asterism in the constellation of Ursa Major (the Great Bear). Many cultures traditionally see Ursa Major as a bear, but some see it as a plough (perhaps operated by Boötes). The handle of the Big Dipper is the bear's tail, elongated when Zeus swung the bear by the tail to fling it into the sky.
The two stars at the end of the bowl of the Big Dipper point to Polaris.
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The Big Dipper can almost be considered an open star cluster. Most of the stars in the Big Dipper (along with many fainter stars) are moving in the same direction at the same speed. However, they are not gravitationally bound and so are not technically a star cluster. Sirius is moving in the same direction at the same speed, but that is apparently a coincidence. If Sirius is part of the Ursa Major Moving Group, we are passing through it.
Mizar and Alcor make up yet another eye test. Can you see the fainter Alcor next to Mizar at the bend in the Big Dipper's handle? Can you separate the two stars? If you can, you can resolve stars that are 12 minutes of arc apart.
Ursa Major has two deep sky objects that are favorite targets of amateur astronomers: the galaxies M81 and M82. They are close enough together to be photographed in one field.
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M82 has a region near its center that appears to be an explosion of such a size that the word incredible is inadequate by several orders of magnitude. It is a region of star formation triggered by gravitational interaction with nearby M81.
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Ursa Minor
Ursa Minor (the Little Bear) is an inconspicuous constellation, but it is important because its second-brightest star, Polaris, is within a degree of the celestial north pole.
As discussed above, Ursa Minor is often used as an indicator of current seeing conditions. Under poor conditions, only two or three stars in the Little Dipper are visible.
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Cassiopeia
Using the pointer stars of the Big Dipper, continuing past Polaris, we just miss Cassiopeia (the queen of King Cepheus), back over in the Autumn sky. Depending on its current orientation, you may have noticed this as the conspicuous W or M in the sky.
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Perseus
Perseus (slayer of Medusa) appears as a cornucopia of moderate stars curving from Cassiopeia to Taurus. The western curve of stars points to the Pleiades. Between Perseus and Cassiopeia lies a favorite target of amateur astronomers, the double cluster (NGC 869 and 884).
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Algol (Beta Persei) is a type of variable star called an eclipsing binary. Its brightness drops from magnitude 2.1 to 3.4 approximately every 68 hours, remaining at the dimmer state for about 10 hours. Algol is a trinary system[1] in which the two brighter stars orbit each other on an orbital plane that lines up with the Earth. Every 2.86 days, the fainter of the two stars passes in front of the brighter star, partially obscuring it.
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That covers constellations that either stand out or are otherwise significant to the amateur astronomer. We may cover a few we missed later, but now we have a foundation to find our way around the sky as we progress.
I chose a few anchor constellations and described the key constellations around them or followed chains of key constellations across the sky. However, that is no substitute for getting out on clear nights and learning your way around the sky. Telling you how to do that is no easy task. I cannot tell you to go outside and look in a particular direction because what you see depends on the time of year and time of night. There are tools, such as planispheres (described below), to find what is where at a particular date and time, but it takes some experience to relate what the planisphere shows you to how that projects to the sky as you see it.
I recommend you go outside frequently on clear evenings and look around and see how the constellations relate. If you live in an urban area with bad seeing, you may have an advantage. Sometimes, under a dark, clear sky, you can be overwhelmed by the number of stars; finding constellations can be a challenge among all the background stars. Look for the conspicuous stars and constellations in the evening at the right time of year: Orion with Betelgeuse and Rigel, Capella, Sirius, and Procyon in the winter; Leo, Ursa Major, Hercules, Spica, and Arcturus in the spring; Scorpius, Sagittarius, Lyra with Vega, Cygnus and Altair in the summer; Pegasus, and Taurus in the autumn.
As you observe the sky, watch the movement and changes of the Moon. Learn to anticipate its position and phase as the days pass. Notice its relationship to the Sun as it passes through its phases. If planets are visible, note any changes in their positions against the background stars, along with changes in brightness over weeks and months.
The above maps are useful but focus on small portions of the sky. Below are some tools to help you get the big picture. As you look at the sky, be sure to note where the celestial equator, the ecliptic, and the celestial pole are located.
Other resources
Planispheres
A planisphere is an adjustable sky map. It has scales to set it for a time of year and time of night, showing the sky at the selected date and time in an oblong window. To make it work, the sky has to be heavily distorted on the map, so it takes a short while to relate it to the sky as you see it. Nevertheless, with a little experience and observation, it is a good tool to quickly orient oneself to the current sky.
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You can buy planispheres at bookstores and online. They are usually made of heavy paperboard and so are relatively inexpensive.
Sky charts
Heavens Above (heavens-above.com)
Heavens Above is a website hosted by the German Aerospace Center (DLR). The site mostly focuses on satellite tracking but has other resources.
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The interactive sky chart is a useful resource for orienting to the night sky at your place and time. It is a stereographic projection (the "dome" of the sky is flattened to a circle) showing the current sky or at the time and place of your choosing. It may take some time to learn to relate the chart to the sky as you see it, but it is very useful once you learn to orient yourself.
Stellarium (stellarium-web.org)
This is much more sophisticated than the Heavens Above chart. You can zoom in and out, rotate, etc. Click the time and date to select something other than the current time.
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Figure 92 Shows Stellarium zoomed out to show a stereographic projection as used by Heavens Above and the magazines below. However, it can be zoomed in and out to show whatever detail is desired. There are too many features to list here. Just go to the site and play with it.
Magazines
Sky & Telescope and Astronomy magazines print a current chart in each month's issue. They are about the same as the chart at Heavens Above, but portable.
Google Sky Map
Of course, you may already be familiar with Google Sky Map or something similar on your phone. By all means, use this to help find your way around the sky. However, in addition to using its full functionality, turn off the automatic direction-finding and orient the screen manually. This will help you learn to find your own way around the sky.
More information
Covering every constellation, notable star, and deep sky object therein would take volumes. Therefore, we have only scratched the surface here. If you are interested in learning about stars and deep sky objects constellation by constellation in exquisite detail, you can refer to Robert Burnham's Celestial Handbook. Some of the information in this three-volume encyclopedia is dated as it was published in 1978. It is out of print as a physical book but is available used and in electronic form from regular sources. It is also available at archive.org using the following link:
Robert Burnham's Celestial Handbook at archive.org
Terms learned in this chapter:
Binary star
Deep sky object
Diffuse nebula
Double star
Eclipsing binary
Galactic cluster
Globular cluster
Open cluster
Optical double
Planetary nebula
Stereographic projection
Supernova
Variable star
Zodiacal
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