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Comets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains and
frozen gases. They have highly elliptical orbits that bring them very close to the Sun and swing them
deeply into space, often beyond the orbit of Pluto.
Comet structures are diverse and very dynamic, but they all develop a surrounding cloud of diffuse
material, called a coma, that usually grows in size and brightness as the comet approaches the Sun.
Usually a small, bright nucleus (less than 10 km in diameter) is visible in the middle of the coma. The
coma and the nucleus together constitute the head of the comet.

As comets approach the Sun they develop enormous tails of luminous material that extend for
millions of kilometers from the head, away from the Sun. When far from the Sun, the nucleus is very
cold and its material is frozen solid within the nucleus. In this state comets are sometimes referred to
as a "dirty iceberg" or "dirty snowball," since over half of their material is ice. When a comet
approaches within a few AU of the Sun, the surface of the nucleus begins to warm, and volatiles
evaporate. The evaporated molecules boil off and carry small solid particles with them, forming the
comet's coma of gas and dust.

Once in a while we will be graced by the presence of a comet that is visible form our night sky. I was
fortunate enough to be able to image a few from my backyard observatory. I have posted images of
them here below:

On Jan. 2, 2006 Grzegorz Pojmanski at the Warsaw University
Astronomical Observatory discovered a faint comet on a photograph that
was taken on New Year's Day from the Las Campanas Observatory in La
Serena, Chile, as part of the All Sky Automated Survey (ASAS). A
confirmation photograph was taken on Jan. 4. Later a prediscovery
image of the comet dating back to Dec. 29, 2005 was also found.

Comet Macholz C/2004 Q2 - January 15, 2005 Images by Dave Trapani

This comet was not as image friendly as NEAT was above. Therefore, my images
are not the best quality as I was a little disappointed with them. I have included one
still image and an animation of about 30 minutes below:

1. This comet was discovered by Edwin Holmes on November 6, 1892 while conducting
regular observations of the Andromeda Galaxy (M31). The comet was undergoing an
outburst similar to the 2007 outburst when Holmes spotted it for the first time. 17P/Holmes
brightened to about magnitude 4 or 5 and then faded over several weeks.

1 - http://en.wikipedia.org/wiki/17P/Holmes#Discovery