Today, object finding usually consists of nothing more than pushing a couple of buttons. But what is there to find? Which objects are worthy of attention? The sky is filled with beautiful deep sky wonders, but the Messier list is the time-honored place for deep sky explorers to start. These 110 objects, discovered by Charles Messier and others in the eighteenth century, are a sampling of the best and the brightest. Once the Messier list has been conquered, most observers move on to the 8,000 objects of the New General Catalog. The "NGC" list of objects, originally published by John Dreyer in 1888, was partly based on work done by renowned amateur astronomer Sir William Herschel. The DSOs in the NGC range from Messier class in brightness and detail to ones that are challenges for the largest CATs. What can be expected of the various species of deep sky objects in either catalog?
Galaxies Galaxies, massive island universes that are the sisters of our Milky Way, are the objects of many an amateur astronomer's desires. Beginners, particularly, long to see the beautiful pinwheel-like spiral arms some galaxies display. Alas, that's not easy: galaxies are far, far away; they are the most distant objects in the cosmic zoo. The nearest large galaxy, M31 in Andromeda, is a staggering 2.3 million light-years distant. Because of their huge distances galaxies are almost always small and dim. They are also badly affected by light pollution, and, again, they are not helped at all by LPR filters. Many galaxies are visible from compromised urban and suburban sites, but just as fuzzy smudges. To have a prayer of seeing spiral arms with an SCT, no matter what its aperture, a dark site is a must.
What are the most visually stunning galaxies? M51, the Whirlpool Galaxy (Plate 56), is the place to go to see spiral arms visually. This is an interesting and fairly bright galaxy located near the Big Dipper asterism, within the borders of the small neighboring constellation Canes Venatici. Magnitude 8.0 M51 is prominent enough to be dramatically visible in 8-inch CATs, and its spiral structure is visible from less than perfect locations with a 5-inch telescope. Also interesting is the little irregular galaxy NGC 5195 just to the north of M51. A bridge of stars appears to connect the two galaxies and seems to be evidence of a recent interaction between the two ("recent" as in "millions of years ago"). This hazy pulled-off stream of stars is visible in 10- to 12-inch scopes from dark locations. From the typical suburban neighborhood? All that's seen of these two wonders is a pair of dim blobs, even in a C14.
M31is the most easily seen galaxy in the sky, visible to the naked eye among the stars of Andromeda even from urban observing sites. Beginners usually expect a lot from Andromeda, as M31 is usually known, since it is so bright (visual magnitude 3.5). They are also usually bitterly disappointed by this galaxy's appearance when they finally get a look at it. At first glance, M31 appears as nothing more than a bright, elongated blob.
Why? One reason is M31's sheer size. At 3 degrees across, it's impossible to fit this monster in one field of view even using long focal length eyepieces and an f/6.3 reducer/corrector. All that's in the field of even a low power eyepiece is the galaxy's round core. Another problem with Andromeda is its inclination. It is tilted only 6 degrees from our line of sight, so the arms aren't well seen even in CCD images. Nevertheless, M31 can be an amazing object for the experienced observer, showing off a couple of dark lanes; a giant cluster of stars; a pair of small satellite galaxies,
Plate 56. (M51) Traces of M51's delicate spiral arms are easily visible with an 8-inch SCT from dark sites. Credit: Author.
M32 and M110; and a huge retinue of globular clusters, the brightest of which are visible in an 8-inch SCT.
NGC 253, the Silver Dollar or Golden Galleon galaxy is, in some observers' opinion, one of the top two or three galaxies in the heavens for CAT users. It's bright (magnitude 7.1) even though it's an eyepiece-filling 25 arc minutes in size, and it displays a wealth of detail. Why doesn't it have a Messier number, then? Probably because its southerly declination of -25 degrees made it difficult for a mid-high northern hemisphere observer like Mssr. Messier.
M104, a magnitude 9.0 Virgo spiral is probably the best example in the sky of an edge-on oriented galaxy. Even 5-inch CATs reveal not only its thin sliver of a body but the huge central bulge that gives it its name, the Sombrero Galaxy. In addition to these features, an 8-inch SCT shows that M104 is bisected lengthwise by a dark lane of dust. In photographs and in really big CATs this lane has scalloped, irregular edges.
Nebulas Nebulas (nebulae) are the great clouds of dust and gas that lurk in interstellar space. Bright nebulas can be divided into four different and distinct types: emission nebulas, reflection nebulas, planetary nebulas, and supernova remnants.
Emission nebulas are great stretches of (mostly) hydrogen that pepper the Milky Way's spiral arms and which, when they contract due to gravitational effects and shockwaves from nearby supernovas, give birth to new generations of stars. Until stars are born, diffuse nebulas are dark objects—there's no light to "excite" them. When hot and massive young stars come to life in the midst of these clouds of gas and begin radiating torrents of ultraviolet light, nebulas begin to glow with the ruddy light of luminous hydrogen—think "neon tube." Diffuse nebulas are among the most beautiful objects in the heavens.
Reflection nebulas do not emit light on their own; they are composed mainly of dust rather than gas and "shine" by reflecting the light of nearby stars. For that reason they are blue instead of red. Some reflection nebulas contain enough excited hydrogen to show some red emission in images.
Planetary nebulas are entirely different from emission and reflection nebulas. Despite their name, they have nothing to do with planets, other than that most are round in shape. A planetary nebula is the corpse of a star. A star in the size range of our Sun does not explode violently as a supernova; instead it undergoes a lingering death, inflating to red giant size as it runs out of hydrogen fuel in its core. When fusion stops, what's left is the star's bare core, a "white dwarf" that forms the planetary nebula's central star. The nebula part of the planetary is composed of the outer layers of the star that were blown off during the red giant phase.
A supernova remnant is what's left over after the death of a large star, one that's exploded as a supernova. An expanding cloud of shockwave-disturbed gas with a tiny and dim neutron star or pulsar at its heart is all that remains of a once-glorious super-sun. Supernova remnants tend to be large and dim.
Diffuse nebulas and supernova remnants are as damaged by light pollution as galaxies. The largest ones, like the elusive California Nebula in Perseus, are actually harder to see from suburban and urban sites than the most challenging island universes. Fortunately, the suburban/urban amateur can always grab a light pollution reduction filter. A filter probably won't help much with the California or the Horse-head or the Cocoon or the Bubble or any of the really hard ones, but a UHC or OIII will increase the number of nebulas visible and the details in brighter ones. In the following paragraphs we will introduce some of the most interesting nebulas.
M42, the great glowing mass in Orion's sword (Plate 57) is the most wonderful nebula in the skies—for northern hemisphere CAT users, anyway. Some would say it's the most beautiful deep sky object of all. It's easily visible to the naked eye, and because of its "reasonable" size of 1 degree it's not too large to be appreciated in long focal length SCTs and MCTs. M42 is flanked by a small, detached, comma-shaped patch that has its own M number, M43. The Great Orion Nebula looks beautiful in all telescopes, from the largest to the smallest, and cuts through even heavy light pollution with aplomb. It's also home to some fascinating stars. Of particular note is the Trapezium, a small star cluster near M42's heart. A 90mm CATs shows a little square (trapezium) of four stars, and 6-inch and larger CATs regularly reveal two more members.
South-of-the-equator astronomers, in addition to getting a really good look at M42 (its southern declination places it high in the sky for some southern hemisphere observers), have another "great nebula" to marvel over, the Tarantula Nebula, NGC 2070, located in the far southern constellation of Dorado. This monstrous cloud stretches 40 arc minutes across the sky. It is not only larger than M42 in the telescope, it's larger in reality, but it's considerably farther away. If it were located at the same distance as the Orion Nebula, it would cover nearly 30 degrees—60 full Moons—of sky!
M78, surrounding a pair of dim stars not far from Orion's belt, is probably the best example of the reflection nebula species. It is fairly small at 8 arc minutes across, but that makes it show up easily despite a rather dim visual magnitude of 11.0. Don't expect to see the beautiful blue color visible in images, though. All the eye will make out is a dim gray smudge around two unimpressive stars.
Great Orion Nebula, M42, the most wonderful DSO in the northern sky. One of the first DSLR images taken by the author, using a C8 and Canon 400D Digital Rebel. Credit: Author.
M57, the justly famous Ring Nebula, is but one fine example of the multitude of beautiful planetaries that litter the Milky Way. This dead star is located in the small but prominent constellation Lyra (home to the bright star Vega) and is bright and unmistakable at magnitude 8.7 and a size of a bit more than 1 minute of arc in diameter. A 90mm MCT will reveal M57 without effort as a tiny spot of light. High magnifications and good seeing conditions can show at least hints of the Ring's "donut hole" in an ETX or a Questar, but it generally takes a C5 to show the donut shape clearly. An 8-inch SCT will display the ring shape very plainly and will show it is not round but somewhat elongated. An 8-inch will also make clear that the middle of the ring is not dark but a gray color. SCTs in the 12-inch class may show the Ring's central star, a magnitude 15 white dwarf, but not easily; not only is the star dim, but it is possibly variable and is masked by the thin nebulosity in the Ring's central hole.
M1, the first object in Messier's catalog, is the best and brightest supernova remnant in the sky. That said, it's not very bright in amateur telescopes and may be hard to spot in surburbia with smaller than 5-inch CATs. This object, which appears as a 1.5 x 1 arc minute 9th magnitude glow, is found in the prominent zodiacal constellation Taurus. Large SCTs, especially those equipped with OIII filters, may show hints of the strange tendrils that give this nebula its name. This expanding cloud of gaseous debris is the result of a supernova that exploded in 1054.
Globular Clusters Globular star clusters are incredibly ancient balls of stars. They are thousands of light years across and contain from thousands to millions of aged suns. They orbit the nucleus of our galaxy and are so old they were possibly witness to the birth of the Milky Way galaxy itself. Globulars are one of the best reasons for buying a larger aperture CAT. Even the brightest "globs" are mostly composed of magnitude 13 and dimmer stars, so at least a 6-inch telescope is recommended to revolve many stars in the brightest clusters, and considerably larger
Plate 58. (M13) Awesome globular cluster M13 delivers scads of tiny stars to both visual observers and astrophotogra-phers. C8 image with SBIG ST2000 camera. Credit: Author.
beauty of the sky's most incredible globular star cluster, Omega Centauri Credit: Author.
Centauri) This C8 astro-photo, done with 35mm film, only hints at the
Plate 59. (Omega apertures are needed to pick out the stars in dimmer globs. The old reliable 8-inch SCT is a decent globular hunter; under dark skies it provides good resolution on many clusters, showing stars in almost all the Messier globs.
M13(Plate 58) is the Great Cluster in Hercules. For northern hemisphere observers, this is it, the most beautiful globular of them all—that's what most amateurs say, anyway. At magnitude 5.8 and 20 arc minutes across, this object is undeniably prominent. However, although bright, it is not necessarily the easiest globular to resolve in small telescopes. Its stars are fairly tightly packed and can be difficult to separate in small telescopes.
M5, which lives not far from M13 in another "summer constellation," Serpens Caput is probably even better than M13. With a magnitude of 5.6, it's actually brighter than its more famous neighbor. Not only is it brighter, it's easier for small CATs to pick apart. It's larger, 23 arc minutes in size, and a little "looser" than M13, so a C5 will show many more stars more easily in this one than it will in Herc. You can usually pick out a few stars in M5 in an ETX on nights when M13 is nothing more than a featureless glow, even at high power. High power, by the way, is a good tool for the glob hunter. These objects take magnification well, and increasing the power almost always brings out more stars.
Omega Centauri, aka NGC 5139 (Plate 59), is the greatest glob. M13 pales beside it. Glowing at magnitude 3.9, it's visible from modestly dark sites as a "star." That's why it received the "Bayer Letter" Omega, an identifier usually reserved for stars. It is also huge, 53 arc minutes across, almost twice the size of the full Moon. This nearly indescribable beauty looks better in finder scopes than many globular clusters do in an SCT. Resolution? Resolving scads of tiny stars is a snap, even in the smallest CATs. The sad thing--for northern hemisphere observers—is that this is really a southern object. At -47 degrees south, it is invisible, or nearly so, from the more northerly parts of the United States and Europe.
M22 is a nice consolation prize for those denied the full beauty of Omega. It's still a southern object with a declination "address" of -23 degrees, but it's not insanely low for most northern hemisphere amateurs. This Sagittarius globular has a magnitude of 5.1 and a size of 23 arc minutes, so it looks good in any telescope. Own a 90mm ETX or Questar 3.5 and want to see globular cluster stars? This is the one.
Open ("Galactic") Clusters An open cluster is a nursery full of infant stars. Stars are born in clouds of gas, and when the gas dissipates, what is left behind is a close group of young sparklers. Their movements and gravity will eventually cause them to disperse, but for a time they present us with lovely groupings. Open clusters are as different from globular clusters as can be; they are composed of the very youngest stars, often suns no more than a few million years old. Globular stars count their ages in billions of years. Open clusters are essentially formless groupings, rather than well defined balls of stars like globs, but they make up for this shapeless-ness with luminosity. Galactic clusters are made incredibly lustrous by the presence of young, hot, massive blue and white stars, those of spectral types "O" and "B."
Most observers probably rate galactic clusters as the least interesting DSOs. In the eyepiece, it's often difficult to tell if what's visible is a true cluster or just a normal sprinkling of background stars. In the denser portions of the Milky Way, especially, it's hard to pick open clusters out from the general stellar background. Nevertheless, there are open clusters beautiful enough to impress the most die-hard glob fan. One advantage galactic clusters have over other deep sky objects is that many are bright enough to be relatively unaffected by light pollution.
For example, M37, located in the fall/winter constellation Auriga the Charioteer is a personal favorite. An integrated magnitude of 5.6 and a size of 21 arc minutes across mean it is bright and reasonably compact for a galactic. It is also insanely rich in stars. A telescope that can reach down to magnitude 12 (like a C5 or C6) will reveal at least 150 suns. This cluster is made even more beautiful by the presence of a lone reddish-orange star near its heart. Set off by the cluster's mostly blue and white stars, it provides a wonderful contrast that further enhances the great view this cluster provides.
M45, the Pleiades, are, like M31, hurt by size. This cluster is almost 2 degrees across, so a finder delivers a better view of it than an f/10 SCT. Nevertheless, it's still possible to get a nice view of the rich field of this naked eye group at CAT's lowest magnification. You can just barely squeeze all Seven Sisters into the field of your C8 by means of an f/6.3 reducer and a 35mm TeleVue Panoptic eyepiece. On a really good night, it's possible to (barely) glimpse the tenuous reflection nebula that surrounds Merope and several of the cluster's other stars. It used to be thought this was the gas left over from the cluster's formation, but it's now thought the Pleiades are just passing through a nebula-filled area.
M46, magnitude 6 and 27 arc minutes in size, is located in the southern constellation of Puppis. It is a nice, rich open cluster that's available to both northern and southern hemisphere observers, but what makes it a standout is the tiny planetary nebula NGC 2438 lurking among its stars. Just over 1 arc minute in diameter, the nebula needs high power to make it pop out from the cluster. An OIII can also help if the skies aren't what they oughta be. In the eyepieces of large scopes and in images NGC 2438 is revealed as a miniature Ring Nebula.
On crisp late fall nights don't forget to cruise over to M35 in Gemini. Like M46, it's impressive in itself. Its magnitude is 5.3, and its size is 28 arc minutes. Also like M46, it offers a bonus. In this case, the neighboring, much dimmer, and more distant galactic cluster, magnitude 8.6 NGC 2158. In the city it's hard to spot M35's little brother with anything smaller than a 12-inch, but under dark skies even a C5
will reveal a roundish glow, maybe sprinkled with a few stars at M35's side, 15 arc minutes to the southwest.
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