Welcome to Visual Astronomy
or
Astronomical Observing
Based mainly on observation with binoculars

By Alain Dussault

Deep sky or Celestial Objects. Definition of different objects types
What can be seen with binoculars

To cover Deep sky objects, I refer you to the following astrophographies sites:

http://astrosurf.com/neptune/astropix/index.html
http://www.galaxyphoto.com/
http://www.aa6g.org/astro.html

Diverse catalogues of deep sky object and other catalogues similar to Messier:
http://www.seds.org/messier/xtra/similar/similar.html

Celestial objects are classified in many categories, they are: open cluster, globular cluster, galaxies, nebulas, etc.  the following site has articles on the subject:

http://pegase.unice.fr/~skylink/doc_astro/glossaire/

On this site you would get a small description of each type of Deep sky objects with a photograph:

http://seds.lpl.arizona.edu/billa/twn//types.html

Open Cluster

Open clusters are irregular group of stars plus or minus condensed, assembling a few dozens to hundred, or millions of stars.  Those stars might be associated or be located in the same field of view.  They are then composed of stars more disperse which received the name association, like Melotte 20, which is the Alpha of Perseus association.

Open clusters are generally the most easily objects to find with binoculars, because their principal components are brilliant stars.  The most famous are The Pleiades (M 45), the Hyades (Melotte 25), the Beehive (M 44) and the double clusters of Perseus (NGC 869 and NGC 884).

There are many catalogues on open clusters  for binoculars, the main ones are:

Collinder (Ex:  Cr 399 - Coat Hanger)
King
Melotte (Ex: Mel 111)
Stock
Trumpler

These catalogues bear the name of those that have compiled them, and in general are objects for binoculars .

Globular Cluster

Globular clusters are immense swarm of stars in a spheroid form.  They are compact formations of from 10,000 to million of stars concentrated in a sphere of 100 to 150 light-years diameter.  They were formed when our galaxy was very young and are locate in galactic halo around our Milky Way galaxy, a vast region centered on the core of our galaxy.  There objects are very far from us.  Some can be seen with the naked eyes, and look like blobs in binoculars, like M13.

In a small telescope, we can resolve plus or minus some stars in globular cluster in using a high magnification.

Nebulas

The disk of our galaxy contains diffuse matter made of gas and dirt.  At certain place we find concentrations of this matter, they are nebulas.  Certain are obscure and absorb the light from the stars being, others shin like a neon sign.  There are many types of nebulas.

The dark nebulas are not visible in binoculars.  These dirt clouds are very dense and stop the light coming from stars situated in the background.  To be able to see them, you need at least an 8" telescope.

Emission diffuse nebulas are clouds of gas excited by ultraviolet emission of very hot stars.  These nebulas contains stars in formation and/or very young stars.  The most beautiful object f this type is the Great Orion nebula (M42-43), visible to the naked eyes, in the winter sky.

Reflection diffuse nebulas contains interstellar dirt which diffuse or reflect the light form near stars.  These stars are too cold to excite the gaseous component.  The brilliant stars of the famous open cluster Pleiades (M45), appears enclosed in a luminous halo in photographs.

The planetary nebulas, are so named, because in small telescope, they appear spherical and of greenish color.  They re composed of a gas extension around a star which has exploded in its evolution.  Sometime the central star, which excite the nebula,  is visible.  As we see it through an envelop of gas, transparent and luminous, the perimeter seem more brilliant.  that is why many planetary nebulas, have a ring form.  the Ring nebula (M57) is a good example.  

Finally certain diffuse nebulas are remnants form supernova explosion.  The Crab nebula  (M1) is typical.

Galaxies

Galaxies, those vast agglomeration of stars in the universe, comprise dwarf elliptical galaxies,  in dimension comparable to globular cluster, to giant spiral galaxies and even supergiant elliptical galaxies containing billions of stars.

The normal spiral galaxies are in the form of a flat disk, with a central globular core, surrounded by spiral arms.  We designate this type of galaxy by the letter S, followed by the letters "a", "b" or "c", depending how their spiral arms are open.   Galaxies of type SO are flat and bear no arms.  Our galaxy and the Great Andromeda galaxy (M31), are of type Sb.  This last galaxy is visible to the naked eyes.

The barred spiral galaxies looks like normal galaxies but have a luminous bar across their core, form which start the spiral arms.   They designated by the letter SB followed by a "a", "b" or "c", according to their arms opening.

The elliptical galaxies might of spherical form, or elongated like a football.  Their eccentricity is indicated by an rating from 0 to 7, after the letter E.  

Irregular galaxies, identified by a I or Irr, contains many dust and gas and young stars.  They do have have definite form.  Magellan clouds, are of this type.  These clouds are in the South hemisphere.

Particular galaxies are difficult to classify. They have a letter "p" with a particular qualitative.

Here is a list of deep sky objects, which are not Messier objects, but are visible in binoculars"

Next chapter, The Color of Stars and Variable Stars

Preceding chapter, The Planets (Jupiter, Mars, Venus, Saturn and Mercury)

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Last update August 1st, 1999.