Deep-Sky Observing Special Interest Group

About this SIG

The AAAP Deep-Sky Special Interest Group is a special section of the club devoted to observers who enjoy observing the Deep-Sky, objects that lie beyond our solar system: galaxies, nebulae, planetary nebulae, star clusters, globular clusters, etc. The Deep-sky Interest Group is open to anyone interested in observing these objects, beginners as well as advanced amateur astronomers. For your enjoyment, each month our Web Site will list objects of the month to find with your telescope. We are currently working on lists of objects to be observed: Binocular Objects, Small Scope Objects, Challenge Objects, etc. Pins and certificates will be awarded for those completing the observing lists. These are similar to the Messier Certificates awarded by the Astronomical League. Other activities will be planned throughout the year, especially during the summer Wagman Star Parties and our Laural Mountian Star Cruise.

Chair

Bob Kepple
e-mail: AstroCards@aol.com

Objects of the Month

Objects in the following lists may be observed for about two months before and after the month they are listed. You may also observe objects in other categories. If you have a small telescope see how many objects you can find in the lists for larger scopes and, of course, individuals with larger instruments will have no trouble observing objects listed for smaller instruments. For those who would like to receive our observing certificates (or perhaps a pin) please note the date, time, size and type of telescope used, and write a brief description of each object observed. The descriptions will turned in to the Deep-Sky SIG Chairman at the end of the year. For March we are adding double stars to the list, please note the colors for the primary and the secondary of each double star.


Types of Deep-Sky Objects

Open Star Clusters

Star clusters are groups of stars, held together by their own gravity. There are two distinct types, Open and Globulars Clusters. Globulars are huge, round stars clusters, some of the larger globulars having as many as a million stars while open clusters have a much looser structure and a much sparser stellar population; they may have a few thousands stars or only a few hundred stars, even as few as a dozen stars.

Open clusters occur in the flat disk of our own Milky Way Galaxy, and are far more numerous than globular clusters. About 1,000 open clusters are visible through telescopes and there may be as many as 18,000 within our own galaxy. One of the nearest star clusters, the Hyades (in the constellation Taurus) is plainly visible to the unaided eye on a clear winter night. Other relatively close open star clusters, such as the Pleiades (also in Taurus) and the Praesepe (in Cancer) are just visible to the naked eye as fuzzy patches but become beautiful sights in binoculars. More distant open clusters are fine sights though small and medium size telescopes. The average open cluster ranges between 10 or 20 light years in diameter.



Globular Clusters

Globular Clusters contain typically hundreds of thousands of stars in spheres usually over 100 light years in diameter, therefore, they are much more conspicuous and visually impressive than open star clusters. A globular's star density usually is very high in its core and decreases gradually toward its periphery. They are the most ancient objects in the Galaxy being some 13 billion years old. Globular clusters are believed to be the first parts of the galaxy to form as giant clouds of gas collapsed into the disk. They are distributed in a halo around the center of our Galaxy. The constellations of Sagittarius, Scorpius, and Ophiuchus, lying toward the central bulge of our Galaxy, are exceptionally rich in globular clusters.

Although globular clusters are highly visible through small and medium size telescopes, a larger aperture is usually needed to resolve their stars. The two largest globulars are Omega Centauri and 47 Tucanae, both too far south to be seen from mid-northern latitudes. The brightest globular visible in the northern hemisphere is M13, the Great Hercules Cluster.



Galaxies

Our Sun, its solar system of planets, and all the stars in the night sky are members of our own galaxy known as the Milky Way Galaxy. Galaxies are gravitationally bound aggregations of millions, often billions, of stars, usually accompanied by massive clouds of gas and dust. They are similar to islands surrounded by oceans with a vast expanse of nearly empty speace between them and the next galaxy. When we peer out beyond the stars of our own Milky Way Galaxy, we see more galaxies everywhere in space from relatively nearby to the very edge of space and time. The further away we look, the further back in time we are looking because of the extremely long time it takes light to travel across space.

The distribution of galaxies in space is not uniform, rather, they tend to cluster into groups of galaxies. Galaxies are extremely distant objects, even the nearest large galaxy, M31, the Andromeda Galaxy, is 2.2 million light years from us, hence, they appear faint and small and relatively few of them offer much visual detail even in larger telescopes. In the eyepiece most galaxies appear merely as a stellar nucleus within a tiny, low-surface-brightness, ambiguously-edged halo. This halo is the combined light of billions of stars so far away that they can not be resolved. Even so, knowing what they are and how far away they are presents a mindboggling scenario.

Galaxies take on different shapes:
Elliptical Galaxies are spheroidal in shape and display no spiral pattern of other structure. There is usually a brighter central concentration thinning outward. They are classed from E0 to E7 with E0 being perfectly round to E7 the most elongated.

Lenticular Galaxies (S0 or SB0)) are a sort of transitional type resembling ellipticals showing a flattened central plane or disc but no spiral pattern.

Spiral Galaxies are beautiful objects containing spiral arms surrounding a central bulge or hub. Their main structural classifications are Sa, Sb, Sc. Sa galaxies have tightly wound spiral arms, Sb have moderately open spiral arms, and Sc have the more defined or open spiral pattern. Transitional types are indicated as Sab, Sbc, etc. When a central bar surrounding the nucleus is present a capital letter "B" is added, such as: SBa, SBb, SBc.

Irregular Galaxies (I or Irr) are amorphous intergalactic patches of gas and dust within which stars are presently being formed, these objects lack symmetry or smooth distrbution.



Nebulae

The word nebula derives from the Latin word for "mist" or "vapor." Nebulae are regions of gas and dust within the galaxy, the two basic types are dark and bright.

Dark nebulae are simply cold clouds of interstellar dust silhouetted against bright nebulae or against the Milky Way itself. They contain hydrogen and light-absorbing grains of graphites, silicates, ices, and possibly metals.

Bright nebulae are divided into four types based upon the process by which they shine: reflection, emission, planetary nebulae, and supernova remnants.

Reflection nebulae are bright nebulae that shine by light reflected from stars within them or nearby. They contain basically the same matter as dark nebulae but usually exhibit the same spectrum as that of the illuminating star. Because blue light is scattered more than red light, the reflection nebula appears bluer than the star. The stars in M45, The Pleiades Star Cluster, display a fine example of reflection nebulae.

Emission Nebulae are interstellar clouds of ionized gas glowing by what amounts to simple fluorescence. These nebulae are always near or around hot O and early-B stars because it is the ultraviloet radiation from these stars that ionizes the atoms in the gas. Regions of this kind are known as H-II Regions, an outstanding example is M42, the Great Nebula in Orion.

Planetary Nebulae are the remaining gaseous shells ejected from nova (dying stars). Their resemblance to planets (especially Uranus) when viewed through a small telescope was noted by Sir William Herschel who gave them this term in 1785. These objects are the death shrouds of red giant stars near the end stage of stellar evolution. As the star ages and becomes unstable the mechanism of atmospheric pulsations in the star's outer layers reaches escape velocity. The multiple shells or rings of most planetaries suggest that the ejection of the red giant's entire envelope usually requires several episodes. Having ejected sufficent material the remaining central star, no longer generating any energy by nucleosynthesis, shrinks to become a white dwarf. Well know examples of planetary nebulae are the "Ring" (M57) in Lyra and the "Dumbbell" (M27) in Vulpecula.

Supernova Remnants (SNRs) are the expanding debris clouds of supernova explosions from massive stars that eject most of their mass at very high volocites. The energy released from a supernova is a million times that of a Nova. The explosion can become as bright as an entire galaxy for a few days. After the explosion the star is either totally destroyed, or its central core collapses into either a very dense Neutron Star or a Black Hole. The most famous supernova remnant is the Crab Nebula (M1) in Taurus, seen in 1054 A.D. by Chinese astronomers.

Credits

We wish to thank Willmann-Bell, Inc., publishers of The Night Sky Observer's Guide, Volumes 1 and 2 for permission to use these books as a source of reference in the Deep-Sky SIG Web Site.