The deep sky

This guide will help you better understand deep-sky objects and also provide lots of targets for your telescope.
By | Published: March 15, 2010
Eagle Nebula
Located in the constellation of Serpens the Serpent, the Eagle Nebula is a very luminous open cluster of stars surrounded by dust and gas. The three pillars at the center of the image, made famous by a Hubble Space Telescope image, are being sculpted by the intense radiation from the hot stars in the cluster.
T.A. Rector (NRAO / AUI / NSF and NOAO / AURA / NSF) and B.A. Wolpa (NOAO / AURA / NSF)
Deep-sky objects are celestial objects that exist outside our solar system. Three major types of deep-sky objects are nebulae, star clusters, and galaxies. The word nebula is Latin and means “cloud.” A nebula (plural = nebulae), therefore, is a cloud of gas and dust in space. Three types of nebulae exist: bright, dark, and planetary.

Bright (or diffuse) nebulae are frequently places of star formation. When stars begin to form, the region becomes so hot that the stars’ radiation excites the gas of a nebula, causing it to shine. This type of bright nebula is known as an “emission nebula.” The process by which the gas (hydrogen) is excited is called “ionization.” Neutral, non-ionized hydrogen is designated HI. Ionized hydrogen is represented as HII. For this reason, emission nebulae often are referred to as HII regions.

Orion Nebula (M42)
The Orion Nebula (M42) appears in a composite of four 60-minute exposures taken with a 6-inch f/9 refractor.
Jason Ware
Bright nebulae
If the stars are not hot enough to cause ionization of the surrounding gas, the starlight is reflected by the dust and can be seen as the second type of nebula — a “reflection nebula.” These nebulae look blue for the same reason our daytime sky appears blue — the light is being scattered throughout the nebula. A lot of nebulae have both emission and reflection components because some of the gas may be too far from the stars to be ionized by their radiation.

The first bright nebula discovered was the Orion Nebula (M42) in 1612. The first reflection nebula discovered was M78, also in Orion, in 1780. Other well-known, diffuse nebulae are M8, M16, M17, M20, and M43. M16 is actually a double object, a star cluster (NGC 6611) and a famous nebula known as the Eagle Nebula (IC 4703). M45, the Pleiades, also contains a reflection nebula.

A list of the brightest nebulae
Catalog name R.A. Dec. Size
NGC 281 00h53m  56°36′ 35′ by 30′
IC 63 00h59m  60°56′ 10′ by 3′
IC 1795 02h26m  61°59′ 12′ by 12′
NGC 1432 03h46m  24°09′ 60′ by 40′
NGC 1499 04h01m  36°38′ 160′ by 40′
NGC 1491 04h03m  51°19′ 6′ by 9′
NGC 1624 04h41m  50°26′ 5′ by 5′
NGC 1788 05h06m -03°21′ 2′ by 2′
NGC 1931 05h31m  34°12′ 4′ by 4′
NGC 1952 05h35m  22°02′ 8′ by 4′
IC 431 05h41m -01°31′ 8′ by 5′
IC 432 05h42m -01°34′ 10′ by 10′
IC 434 05h42m -02°19′ 90′ by 30′
NGC 2023 05h42m -02°19′ 10′ by 8′
NGC 1976 05h35m -05°28′ 90′ by 60′
NGC 1999 05h36m -06°44′ 2′ by 2′
NGC 2064 05h47m  00°01′ 10′ by 10′
NGC 2170 06h07m -06°23′ 2′ by 2′
NGC 2182 06h09m -06°21′ 3′ by 2′
NGC 2175 06h10m  20°29′ 40′ by 30′
NGC 2238 06h33m  04°58′ 80′ by 60′
NGC 2245 06h33m  10°10′ 2′ by 2′
NGC 2247 06h33m  10°21′ 2′ by 2′
NGC 2261 06h39m  08°43′ 2′ by 1′
NGC 2264 06h41m  09°54′ 10′ by 7′
IC 2177 07h04m -10°25′ 20′ by 20′
NGC 2467 07h52m -26°28′ 8′ by 7′
IC 4606 16h29m -26°37′ 60′ by 40′
NGC 6514 18h02m -23°00′ 20′ by 20′
NGC 6523 18h04m -24°20′ 45′ by 30′
NGC 6559 18h10m -23°59′ 15′ by 10′
IC 1274 18h11m -23°44′ 20′ by 5′
NGC 6590 18h17m -19°44′ 4′ by 3′
NGC 6611 18h19m -13°49′ 120′ by 25′
NGC 6618 18h21m -15°59′ 40′ by 30′
NGC 6888 20h13m  38°19′ 20′ by 10′
IC 1318A 20h17m  41°49′ 45′ by 25′
NGC 6914 20h25m  42°19′ 3′ by 3′
IC 1318B 20h28m  40°00′ 45′ by 20′
IC 5067 20h51m  44°21′ 25′ by 10′
IC 5076 20h56m  47°24′ 10′ by 10′
NGC 7023 21h02m  68°12′ 10′ by 8′
NGC 7129 21h42m  66°04′ 2′ by 2′
NGC 7380 22h47m  58°01′ 25′ by 20′
NGC 7538 23h14m  61°29′ 8′ by 7′
NGC 7635 23h20m  61°11′ 15′ by 8′
Horsehead Nebula
The Horsehead, also known as Barnard 33, is a cold, dark cloud of gas and dust, silhouetted against the bright nebula, IC 434.
David Hanon
Dark nebulae
Dark nebulae are clouds of dust and cold gas that can be seen because they obscure light from stars or bright nebulae behind them. The shapes of dark nebulae are among the strangest in the sky. Some dark nebulae, such as the Horsehead Nebula in Orion, are small and difficult to see even with large telescopes. Others, such as the Coal Sack in Crux, are large and easy to see with the naked eye.

The darkness of these objects is due to dust grains within the clouds and to the presence of extremely cold hydrogen molecules. The largest of the dark nebulae, called “molecular clouds,” are huge areas of star formation. The clouds must be cold or the radiation will heat up the individual atoms and prevent gravity from collecting them together. This would keep the gas from condensing into stars.

The first catalog of dark nebulae was prepared by American astronomer Edward Emerson Barnard (1857-1923). Use the following list to locate and observe the best of Barnard’s objects.

Great objects from Barnard’s catalog
Catalog name Common name R.A. Dec. Size
B33 Horsehead 05h41m -02.5 6′ by 4′
B42 Rho Ophiuchi 16h29m -24.3 12′ by 12′
B65/6/7 Pipe (stem) 17h21m -26.8 300′ by 60′
B72 Snake 17h24m -23.6 30′ by 30′
B78 Pipe (bowl) 17h33m -25.7 200′ by 140′
B86 Ink Spot 18h03m -27.8 5′ by 3′
B87 Parrot’s Head 18h04m -32.7 12′ by 12′
B142/3 Barnard’s E 19h41m  11.0 110′ by 80′
B348/9 Cygnus Rift 20h37m  42.2 240′ by 240′
We are looking down a tunnel of gas cast off by a dying star thousands of years ago. Look for elongated dark clumps of material embedded in the gas at the edge of the nebula, and also for the dying central star floating in a blue haze of hot gas. The colors represent three different chemical elements: helium (blue), oxygen (green), and nitrogen (red).
Planetary nebulae
A planetary nebula is the most common end product of stellar evolution. Stars about as massive as the Sun eject mass when they become red giants near the end of their lives. After about 10,000 years (a short period, astronomically speaking), the inner core of the star collapses and the star pulsates, ejecting much of its outer envelope. This leaves a small star called a white dwarf. The ejected envelope becomes a spherical shell of cool, thin matter expanding into space at 5-25 miles/second (10-40 kilometers/second) — a planetary nebula.

The star (now called the central star of the nebula) is extremely hot, reaching temperatures of 200,000 Kelvin. It also emits a large amount of ultraviolet radiation. This radiation ionizes the gas of the expanding planetary nebula and causes it to glow. Planetary nebulae have short lives, at least visually. As the shell expands, the white dwarf’s energy can excite the gas, but only out to a certain distance. So after 10,000 to 50,000 years, the planetary nebula stops glowing. It’s still expanding, but we can no longer see it. Most astronomers agree that a planetary nebula is the fate of our Sun in approximately 5 billion years.

On July 12, 1764, French comet hunter Charles Messier (1730–1817) discovered the first planetary nebula and made it number 27 on his famous list of objects that are not comets. M27 also is known as the Dumbbell Nebula. It took 15 years before the next planetary nebula was discovered. The object was the now famous Ring Nebula (M57), and it was found in January 1779 by another French astronomer, Antoine Darquier (1718–1802), who compared it to a “fading planet.” The German-born, English astronomer William Herschel (1738–1822) christened these objects planetary nebulae because they resembled — in shape and color — Uranus, the planet he had discovered only a few years before.

There are roughly 1,000 known planetary nebula in our neighborhood of the galaxy. A typical planetary nebula is less than one light-year across. Planetary nebulae usually have a blue, green, bluish-green, or greenish-blue color. Observers often report different colors for the same nebula. What color will a particular planetary appear to you? This is a little tricky to answer. The problem is that the wavelength of the light is close to the border between perceived green color and perceived blue color. Thus, even a very slight shift in a person’s color response could cause the color to appear either greenish or bluish.

Some of the finest planetary nebulae
Common name Catalog name R.A. Dec.
Blinking Planetary NGC 6826 19h45m 50°32′
Blue Snowball NGC 7662 23h26m 42°32′
Box Nebula NGC 6309 17h14m -12°55′
Bug Nebula NGC 6302 17h14m -37d06′
Cat’s Eye Nebula NGC 6543 17h59m 66°38′
Clown Face Nebula NGC 2392 07h29m 20°55′
Crescent Nebula NGC 6445 17h49m -20°01′
Dumbbell Nebula NGC 6853 (M27) 19h59m 22°43′
Eight Burst Nebula NGC 3132 10h07m -40°26′
Eskimo Nebula NGC 2392 07h29m 20°55′
Fetus Nebula NGC 7008 21h01m 54°33′
Ghost of Jupiter NGC 3242 10h25m -18°39′
Helix Nebula NGC 7293 22h30m -20°50′
Little Dumbbell Nebula NGC 650/651 (M76) 01h42m 51°35′
Little Gem Nebula NGC 6818 19h44m -14°09′
Owl Nebula NGC 3587 (M97) 11h15m 55°01′
Red Spider Nebula NGC 6537 18h05m -19°51′
Ring Nebula NGC 6720 (M57) 18h54m 33°02′
Saturn Nebula NGC 7009 21h04m -11°22′
Southern Ring Nebula NGC 3132 10h07m -40°26′
The Pleiades (M45)
Located 440 light-years from Earth in the constellation Taurus, the Pleiades cluster includes six naked-eye stars (and one variable star that may have once been naked-eye as well) along with thousands of other, fainter stars. The young stars are surrounded by the glow of their own light reflected by clouds of interstellar gas.
John Chumack
Star clusters
A star cluster is a group of stars loosely held together by gravity. There are three types of star clusters: associations, open clusters and globular clusters. Associations contain dozens of stars and resemble open clusters except that they are poorer in stars. Here, we’ll concentrate on the two other types of star clusters.

Open clusters. Located within the disk of the Milky Way Galaxy, open clusters also are known as “galactic clusters.” The Beehive (M44), the Pleiades (M45), and the Hyades clusters have numerous historical references. In the second century, Ptolemy mentioned Melotte 111 in Coma Berenices and M7 in Scorpius. Not until Galileo (1564-1642) trained his telescope on the Beehive, however, was it discovered that these objects are made of individual stars.

Sparse open clusters contain less than a hundred stars, while rich ones may have thousands. Open clusters form from nebulae. All open clusters are relatively young objects, generally no more than a few hundred million years old. We know the open clusters we observe are young because given enough time, the stars in an open cluster disperse due to gravitational interaction within the cluster. If we were able to follow a single open cluster from its formation, we would see the cluster disperse stars throughout its entire galactic journey. Approximately 1,700 open clusters are known.

Open clusters are pretty. This makes them fun to look at for beginning amateur astronomers. But many advanced amateurs spend a great deal of time observing open clusters as well. Compared with other deep-sky objects, open clusters are relatively large. This means you will generally be using low-power eyepieces, which provide wide fields of view. Some clusters are large enough and bright enough that binoculars will provide more satisfying views.

Take your time when observing an open cluster. Examine the field of view closely. Try to discern the members of the cluster as opposed to stars that only appear in the field of view. This is easy in many cases, but it can be a real trick when the cluster lies within the area of the Milky Way. Also, large telescopes sometimes can hinder the identification of cluster stars, causing confusion by making so many background stars visible.

Fifty bright open clusters
Catalog name Constellation Magnitude
M45 Taurus 1.6
M7 Scorpius 3.3
M44 Cancer 3.9
NGC 869/884 Perseus 4.4
NGC 2244 Monoceros 4.8
NGC 2362 Canis Major 4.8
M41 Canis Major 5.0
M47 Puppis 5.0
M39 Cygnus 5.3
NGC 2244 Monoceros 5.3
NGC 6633 Ophiuchus 5.3
M6 Scorpius 5.5
M35 Gemini 5.6
NGC 7686 Cassiopeia 5.6
M34 Perseus 5.8
M23 Sagittarius 5.9
M48 Hydra 6.0
NGC 1647 Taurus 6.0
NGC 1746 Taurus 6.0
NGC 1981 Orion 6.0
NGC 2264 Monoceros 6.0
NGC 2301 Monoceros 6.0
M67 Cancer 6.1
NGC 7160 Cepheus 6.1
M25 Sagittarius 6.2
M37 Auriga 6.2
NGC 1545 Perseus 6.2
M50 Monoceros 6.3
NGC 6940 Vulpecula 6.3
NGC 457 Cassiopeia 6.4
NGC 7243 Lacerta 6.4
M21 Sagittarius 6.5
M36 Auriga 6.5
M93 Puppis 6.5
NGC 129 Cassiopeia 6.5
NGC 654 Cassiopeia 6.5
NGC 752 Andromeda 6.5
NGC 663 Cassiopeia 6.5
NGC 1528 Perseus 6.5
M16 Serpens 6.6
M46 Puppis 6.6
NGC 1027 Cassiopeia 6.7
NGC 2343 Monoceros 6.7
NGC 2423 Puppis 6.7
NGC 7209 Lacerta 6.7
NGC 7789 Cassiopeia 6.7
M11 Scutum 6.8
NGC 7036 Cygnus 6.8
M103 Cassiopeia 6.9
M38 Auriga 7.0
This image of M13 was taken by the 3.6-meter Canada-France-Hawaii Telescope atop Mauna Kea.
CFHT / Coelum / Jean-Charles Cuillandre
Globular clusters
The other main cluster type is a globular cluster. These objects are spherical in shape and the greatest concentration of stars lies toward their centers. Globular clusters have many more stars than open clusters, ranging from about ten thousand stars to more than one million stars. Globular clusters are old. Most are in excess of 10 billion years old.

What really sets globulars apart, however, isn’t their age, but their location. Open clusters lie within the spiral arms (disk) of our Milky Way Galaxy. Globular clusters surround the Milky Way in a spherical distribution. This structure is often called the “halo.” Studying the distribution of globular clusters around the Scorpius-Sagittarius area correctly led Harlow Shapley (1885–1972) to conclude that the center of our galaxy must lie in that direction.

M22, a beautiful object in Sagittarius, was the first globular cluster to be discovered, in 1665. Twelve years later, the great southern globular Omega Centauri was observed by Edmund Halley (1656–1742). Today, about 200 globular clusters are known to surround our galaxy. Globulars have been observed around many other galaxies as well. As an example, the giant elliptical galaxy M87 in Virgo contains more than one thousand globular clusters.

In the entire deep-sky menagerie, no group excites new observers more than globular clusters. Even seasoned amateur astronomers consider globulars to be the most rewarding objects to observe. It’s easy to see why. Many are bright enough to be seen even from urban settings. But when observed from a dark site, these objects explode with detail. The use of higher magnifications brings whole new levels of detail to these clusters. Faint fuzz resolves itself into individual sparkling points of light that form intricate patterns.

The 25 brightest globular clusters
Catalog name Common name Constellation Magnitude
NGC 5139 Omega Centauri Centaurus 3.9
NGC 104 47 Tuc Tucana 4.0
NGC 6656 M22 Sagittarius 5.2
NGC 6397 Ara 5.3
NGC 6752 Pavo 5.3
NGC 6121 M4 Scorpius 5.4
NGC 5904 M5 Serpens 5.4
NGC 6205 M13 Hercules 5.8
NGC 6218 M12 Ophiuchus 6.1
NGC 2808 Carina 6.2
NGC 6809 M55 Sagittarius 6.3
NGC 6541 Corona Australis 6.3
NGC 5272 M3 Canes Venatici 6.3
NGC 7078 M15 Pegasus 6.3
NGC 6266 M62 Ophiuchus 6.4
NGC 6341 M92 Hercules 6.5
NGC 6254 M10 Ophiuchus 6.6
NGC 7089 M2 Aquarius 6.6
NGC 362 Tucana 6.8
NGC 6723 Sagittarius 6.8
NGC 6388 Scorpius 6.8
NGC 6723 M19 Ophiuchus 6.8
NGC 7099 M30 Capricornus 6.9
NGC 3201 Vela 6.9
NGC 6626 M28 Sagittarius 6.9
James R. Foster of Los Angeles, California, recorded this image of M33 with his 13.1-inch classical Cassegrain reflector at f/7.5. He made a 100-minute exposure on hypered Kodak Tech Pan 2415, plus 80-, 70-, and 70-minute exposures on hypered Kodak PPF 400. All exposures were later electronically combined.
James R. Foster
You might wonder how objects composed of up to a trillion or more individual stars could be so difficult to observe. Of course, the answer is distance. Galaxies are so far away that, except for a small number, they all appear small and faint. Even advanced observers regard observing faint galaxies as a challenge.

Today, we are indebted to the great American astronomer Edwin Hubble (1889–1953) for developing a simple classification scheme for galaxies. Hubble first mentioned this in a paper he wrote in 1922. Four years later, he expanded it and added some illustrations. Finally, in 1936, Hubble provided a better explanation of the classification scheme in his book The Realm of the Nebulae. And it was in this book that the famous “tuning fork” diagram first appeared.

Hubble’s scheme described several different main types of galaxies: ellipticals, spirals, and barred spirals. That’s three. An argument for a fourth Hubble type (off the tuning fork) could be made. The fourth class contains “irregular galaxies,” essentially every galaxy not on the fork.

Clusters of galaxies
After stars form, they arrange themselves into clusters. So do galaxies. Clusters of galaxies may be made up of a handful of galaxies or as many as a thousand. As with star clusters, clusters of galaxies are held together by mutual gravitational attraction. Please note that a galactic cluster and a cluster of galaxies are totally different. The first is a group of stars (also known as an open cluster). The second is a group of galaxies. The most famous cluster of galaxies is the one that contains our Milky Way. Known as the Local Group, it is composed of almost 40 members.

Two other well-known and well-observed clusters of galaxies are the Virgo cluster and the Fornax cluster. The Virgo cluster lies nearly 50 million light-years from Earth and is a bright cluster. A number of Messier objects are members of it, including M49, M49, M60, M84, M85, M86, M87, M88, M89, M90, M98, M99, and M100. In addition, more than one hundred NGC galaxies belong to the Virgo cluster.

Many observers think the Fornax cluster is the finest for amateur astronomers to observe. Eighteen galaxies make up the cluster. The brightest is NGC 1316 at visual magnitude 8.8. NGC 1399, magnitude 9.8, is the next easiest to see. NGC 1365 is third at magnitude 9.5. This galaxy is a barred spiral seen face on, with open spiral arms. In a wide-field, high-power eyepiece, up to ten members of this cluster of galaxies can be viewed.

Superclusters of galaxies
The largest separate structures in the universe, and some would say the structures that define the shape of our universe, are superclusters of galaxies. The Local Group, which contains the Milky Way, belongs to a supercluster of galaxies called the Local Supercluster. The Local Group is at one end and the Virgo cluster is near its center. The major axis of the Local Supercluster stretches 125 to 150 million light-years. Other nearby superclusters of galaxies include the Hydra-Centaurus Supercluster, which lies at a distance of about 150 million light-years and the Perseus Supercluster, which is roughly 220 million light-years away.
Galaxies brighter than 10th magnitude
Catalog name R.A. Dec. Magnitude
LMC 05h24m -69°45′ 0.91
SMC 00h53m -72°48′ 2.70
NGC 224 00h43m  41°16′ 4.36
NGC 598 01h34m  30°40′ 6.27
NGC 5128 13h25m -43°01′ 7.84
NGC 3031 09h56m  69°04′ 7.89
NGC 253 00h48m -25°17′ 8.04
NGC 5236 13h37m -29°52′ 8.20
NGC 5457 14h03m  54°21′ 8.31
Fornax 02h40m -34°27′ 8.40
NGC 55 00h15m -39°13′ 8.42
NGC 300 00h55m -37°41′ 8.72
NGC 205 00h40m  41°41′ 8.92
NGC 2403 07h37m  65°36′ 8.93
NGC 5194 13h30m  47°12′ 8.96
NGC 4594 12h40m -11°37′ 8.98
NGC 4736 12h51m  41°07′ 8.99
NGC 221 00h43m  40°52′ 9.03
IC 342 03h47m  68°06′ 9.10
NGC 4258 12h19m  47°18′ 9.10
NGC 6744 19h10m -63°51′ 9.14
NGC 1313 03h18m -66°30′ 9.20
NGC 3034 09h56m  69°41′ 9.30
NGC 4945 13h05m -49°28′ 9.30
NGC 5055 13h16m  42°02′ 9.31
NGC 6822 19h45m -14°48′ 9.31
NGC 4826 12h57m  21°41′ 9.36
NGC 4472 12h30m  07°59′ 9.37
NGC 1291 03h17m -41°06′ 9.39
NGC 1316 03h23m -37°12′ 9.42
Sculptor 01h00m -33°43′ 9.50
NGC 4486 12h31m  12°23′ 9.59
NGC 1068 02h43m -00°01′ 9.61
NGC 6946 20h35m  60°09′ 9.61
NGC 7793 23h58m -32°35′ 9.63
NGC 3627 11h20m  12°59′ 9.65
NGC 247 00h47m -20°46′ 9.67
NGC 2903 09h32m  21°30′ 9.68
NGC 4631 12h42m  32°32′ 9.75
NGC 4649 12h44m  11°33′ 9.81
NGC 3521 11h06m -00°02′ 9.83
NGC 4406 12h26m  12°57′ 9.83
NGC 3115 10h05m -07°43′ 9.87
IC 1613 01h05m  02°07′ 9.88
NGC 628 01h37m  15°47′ 9.95
NGC 4449 12h28m  44°06′ 9.99
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