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 Messier observed many objects in the night sky, including occultations, transits, eclipses, as well as sunspots and other meteorological events (Mallas, 3-4).  MessierÕs passion was comet hunting, as discovering comets was the way to gain fame in the field of astronomy at the time.  From 1764-17799, Messier discovered all the comets that were visible from Earth.  He observed more comets in his lifetime than had ever been observed before (Mallas, 1-3).

The Messier Catalog is a list of deep sky objects compiled in installments from 1774 through 1784 (Mallas, 1-15). These objects were believed to be nebulas, and Messier listed them as to avoid confusing these cloud-like objects with the comets for which he was avidly searching.  103 celestial objects, which appeared fuzzy and extended through the telescopes of the 18th century, were listed and given a number preceded by the letter M.  This is an object's Messier Number.  Six more objects were added in 1786. And the final entry, M110, was added early in the 20th century. It was later discovered that many of these objects were not true nebulae, but galaxies and star clusters.  A few errors in the catalog remain today.  M102 is a duplicate observation; M40 and M73 are asterisms, which are not true deep-sky objects; and M47, M48, and M91 are missing (Houstan, 444).

This list of objects works well as a starting place for beginning amateur astronomers, as they represent the majority of the brighter objects in the northern - hemisphere (Mallas, 1-15).

Open clusters, also called galactic clusters, are loose, irregular bodies that can contain several thousands of stars.  Open clusters are young star clusters where the stars move together and form a group.  They are concentrated in the disk of a galaxy, which is where interstellar mater and young stars are found.  Very few open clusters are found away from the galactic plane.  Astronomers use open cluster stars to determine the plane of the Milky Way galaxy (Cambridge, 243-4).
Stars in open clusters are linked weakly by gravitational forces.  Stars escape the pull of gravity about one per hundred thousand years.  From there, stars disperse into the galactic disk.  From this information, astronomers speculate that the life span of open clusters should not exceed one hundred million years.  When viewed though a telescope, open clusters look like a cluster of small stars (Cambridge, 243-4).

Globular clusters are the oldest of stellar clusters and are usually spherical in shape.  They are tightly packed stars in populations of up to several hundreds of thousands and can be in a volume 300 light years across.  Globular clusters are found in a spherical area of space surrounding galaxies called the galactic halo, but peak density, roughly 64%, is in the galactic center.  ÒEach cluster travels in an elongated orbit with a period of revolution of two hundred million years,Ó (Cambridge, 308). From observations of globular cluster orbiting the center of the Milky Way, we see the outer reaches of our galaxy and realize the true size of the Milky Way ( Kaufman, 263-4).
The globular clusters in the Milky Way galaxy, of which there are between 146 and 200, are very old (White, 24-8).  Their ages are between ten and fifteen billion years old.  They were formed first in the galaxy, and from this, they have not benefited from the interstellar matter of stellar nucleosynthesis or supernovae explosions.  The observations made of the elements that make up the globular clusters lend important information about the first stages of the chemical evolution of our galaxy (Cambridge, 308).  Globular clusters have lower amounts of metals, elements heavier than hydrogen or helium, than any other known celestial objects.  Post helium flash stars are typically the types of stars that are found in globular clusters (Cambridge, 308).  Most of the stars found in globular clusters are low-mass stars, as the higher mass stars have burned away as red giants.  The lower mass stars are left behind and are usually undergoing core hydrogen fusion (Cambridge, 308).  In the future, these low-mass stars will move into another state of their lives, the giant state.  As core helium fusion occurs and they devour their fuel, they die.  Both M31 in Andromeda and M87 in Virgo have great numbers of globular clusters (White, 24-8).

Diffuse nebula are Òan irregularly shaped cloud of interstellar gas or dust whose spectrum may contain emission lines or absorption lines characteristic of the spectrum on nearby illuminating starsÓ (Hopkins, 42).  These bodies are also known as reflection nebula.

The term planetary nebula refers to an expanding and usually symmetrical cloud of gas that surrounds certain hot stars.  Nearly one thousand planetary nebulas have been discovered.  The name denotes their disk-like resemblance to planets.  Radiation from the centrally located star excites the gaseous cloud, Òcausing fluorescence as in bright nebula.  The atoms of the (gas cloud) absorb the ultraviolet radiation from the star and re-emit it in radio, infrared and visible wavelengthsÓ (Cambridge, 274).  The phenomenon is a normal phase in the life of a low-mass star (between one and five solar masses), occurring between the red giant and white dwarf phases (Cambridge, 274).  The transition phase of the star from red giant to planetary nebula can be as quick as one thousand years but can last up to 10,000 years, and the transition phase between planetary nebula and white dwarf can have an equal transition period (Cambridge, 274).
The clouds that make up a planetary nebula expand at a rate of about 20 kilometers per second (Cambridge, 274).  From this information, it is inferred that an explosion creates a planetary nebula (Cambridge, 274).  ÒThe shell of a planetary nebula contains far more material than the shell thrown off by a nova. Therefore we can assume that the explosion forming a planetary nebula is very large and occurs very rarelyÓ (Meadows, 152).  To date, no explosion large enough to produce a planetary nebula has been observed.
 The temperature of the central star is hot, between 30,000 and 100,000 Kelvins (Cambridge, 274).  The mass of a planetary nebula is approximately 0.10 to 0.20 solar masses.  Tenuous gas is 10-20 g/cm3 , and dust is present (Cambridge, 274).

Supernova remnants are an expanding gas shell of gas from a supernova explosion, consisting of supernova projections and swept-up interstellar gas.  ÒA supernova is a star that temporarily brightens to an absolute magnitude of about ­15. A supernova explosion blows off all or most of the starÕs material at a high velocity, as a result in the final uncontrolled nuclear reactions of stars that reach an unstable state late in their evolutionÓ (Illingsworth, 331).  Young supernova remnants are generally optically faint, but emit large quantities of radio and x-ray waves.  The Crab Nebula, M1, is an example of a supernova remnant and is especially bright due to a central pulsar.  Older supernova remnants appear as rings of bright filaments, which are associated with radio and x-ray wave emission.  Compression by an expanding supernova remnant can trigger stellar birth in interstellar clouds and may have initiated the formation of our solar system.

A galaxy is an ensemble of stars of various masses and properties in a more or less dense interstellar medium, though this medium is virtually non-existent in elliptical galaxies.  Presently, more than a billion galaxies are observable, though they may possibly be the most under observed objects in the sky due to light pollution (Witkoski).  Galaxies are not randomly distributed, but form clusters.  ÒRegular clusters of galaxies contain at least a thousand members.  Irregular or open clusters contain no central nucleus of galaxies, as they appear to be simply a loose collection of galaxies without the symmetrical shape found in regular clusters.  Open clusters generally contain fewer than galaxies than regular onesÓ (Schweighauser).  The enormous distances of galaxies from Earth make it possible to study their evolution.  They are receding from one another with increasing velocities proportional to their distance, from which one can measure their red shift (Witkoski).  The galaxies with high redshift are the most distant from us and are visible at a younger state of their evolution.
 There are three ways to determine a galaxyÕs evolutionary progress.  The first is dynamic evolution, involving the way the galaxy is formed and how its components move and interact.  The second is the evolution of chemical composition.  The third is the evolution of luminosity and color; Òan ensemble of stars which are born, evolve, and die (hence changing luminosity and color), and which enrich the interstellar medium in chemical elements which they synthesizeÓ (Cambridge, 334).
There are four main morphological types of galaxies.  These are know as elliptical, spiral, peculiar, and irregular; in which elliptical and spiral are the two main types (Cambridge, 334).

Elliptical galaxies are massive and not especially luminous.  They tend to be rather red with low interstellar gas content.

Spiral galaxies are less massive and more luminous.  Their interstellar gas content is 1-10% of the total density of the matter of the disk.  Spirals contain three or four components:  a central region or bulge that looks like a small elliptical galaxy; a disk which is extended and flattened; spiral arms within the disk, in which the stars and gas are greatly concentrate; the fourth pertains only to barred spiral galaxies and that is a ÒbarÓ, a flattened structure that is between the bulge and the disk (Cambridge 334).  The outer parts of the disk are blue and the center bulge is yellow to red in color.  The bulge and the disk are surrounded by a tenuous, extended spherical region, which where globular clusters are found.
Spiral galaxies are further divided into categories that denote their outstanding characteristics.  SB denotes those galaxies with a central bar, while S refers to those without a bar.  Sa and SBa galaxies have tightly wound arms and relatively large central bulges.  Sc and SBc galaxies have loose arms and small central bulges.  Sb and SBb are intermediate between the two extremes (Mitton, 359).
10-15% of galaxies do not have a particular form and are thus known as irregular galaxies.  They have low mass, substantial luminosity, and a large concentration of interstellar gas.  Their dominant color is blue.

Spring is the best time to observe Messier objects, between March 19 and 30 is ideal.   A dark sky, low horizon and no moon are quintessential for optimal results.  Messier objects are visible only in the Northern Hemisphere (Nisqually).

The Cambridge Atlas of Astronomy; Cambridge University Press, NY. Ny; 1994.
    This was a very helpful and informative encyclopedia, it contains everything you need to know about general Astronomy.
Illingsworth, Vallery; The Facts on File Dictionary of Astronomy, Facts on File, NY, NY; 1979.
    This was useful to decipher technical astronomy terms.
Hopkins, Jeanne; Glossary of Astronomy and Astrophysics, University of Chicago Press; 1976.
    Helpful to decipher technical Astronomy terms.
Houstan, W.S.; "Deep Sky Wonders", Sky and Telescope; April 1991, Volume 81, pg.  444-6.
  Very helpful and informative, good history of the Messier Catalog.
Kaufmann, William J. III; Discovering the Universe , W.H. Freeman and Co.; 1996.
    This was the text we used for our Astronomy class. Useful general overview.
Kwok, Sun; ÒA Modern View of Planetary NebulaeÓ, Sky and Telescope; July 1996.
    Informative and helpful.
Machholz, Don; Ò Notes on Messier MarathonÓ, Astronomy ; March; 1980.
    Interesting, short letter to the editor- type article.
Mallas, John H., and Kreimer, Evered ; The Messier Album; Cambridge University Press; 1979.
    Gave good descriptions and pictures for the whole catalog.
Malin, David and Murdin, Paul; Colours of the Stars ; Cambridge University Press; 1984.
    Beautiful pictures, very useful information as well.
Meadows, A.J.; Stellar Evolution (second
    Very informative on the formation of stars.
Mitchell, Larry; ÒThe M31 ChallengeÓ, Sky and Telescope; November; 1997.
    Good information on M31 as well as how, and when to find it in the night sky.
Nisqually Valley Telescope MakerÕs Workshop,  "Just Messier"; 1994.
    Our greatest resource for locating Messier objects in the night sky.
Parker, Samantha; "The EagleÕs Nest", Sky and Telescope; February 1996.
    There were some beautiful pictures in here, of the awe inspiring M16.
Royal Astronomical Society of Canada, ObserverÕs Handbook 1998; University of Toronto Press Inc.; 1997.
    Good information for finding Messier objects in the night sky.
Schweighauser, Charles A.; Astronomy from A to Z: A Dictionary of Celestial Objects  and Ideas; Illinois Issues; 1991.
    Useful for learning Astronomy jargon.
Witkoski, Michael F.; ÒObserving GalaxiesÓ, Astronomy; April, 1980.
    Good information on observing galaxies, and when and where to find them.
White, Raymond E.; ÒGlobular Clusters: Fads and FallaciesÓ, Sky and Telescope; January, 1991.
    The best  low down on Globular clusters. 

June 9, 1998   by Amanda Robinson and Tacy Uldrich

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