- define and give examples of deep-sky objects
- discuss why galaxies are so challenging to observe
- explain the purpose of the Messier Catalogue
The night sky has always invited humans to gaze up
into the darkness and open up their imagination. While viewing hundreds
of points of light, mysterious planets moving amongst the stars and
satellites passing quickly by. However, there is another classification
of celestial bodies, which has captured the imaginations of amateur and
professional astronomers alike, primarily due to the fact that they are
so hard to see. This includes star-forming nebulae, nebulae that are
remains of ancient stellar explosions, star clusters and galaxies.
Collectively, they are called deep-sky objects
Double Open Clusters, NGC 869 & NGC 884
objects offer astronomers much more interesting views than stars.
Whereas stars are primarily tiny discs of light, with limited colours,
deep-sky objects have much more contour, shape and colour. All of the
beautiful astronomical photographs we see on calendars and on
television come from deep-sky objects.
galaxies are perhaps the most rewarding of deep-sky objects, they are
also some of the most challenging due to their extreme distance from
the Earth. Light arriving at Earth from a distant galaxy has traveled
millions of light years to reach us, so in a sense we are looking back
in time millions of years. Particularly distant galaxies are also
moving away from the Milky Way, and this can be seen by examining the
ancient light from galaxies. This gives us one way to describe the
nature of the universe as a whole, and is discussed in more detail in
The challenge lies in
finding dark enough skies from which to observe, and in being able to
collect enough light to observe details, such as spiral structure. To
do this, the largest of telescopes are constructed atop mountains,
above the clouds, where there is next to no light pollution and weather
is rarely a factor (see Unit 3, Lesson 3). Amateur astronomers must
exercise both diligence and patience while observing in order to view
galaxies using smaller telescope and less-then-ideal conditions. There
is more information on how to observe deep-sky objects in Unit 10,
The Cigar Galaxy (so-named for its shape) is one of the more popular galactic targets for
amateur astronomers. The lanes of dust seen in this image, however, are very hard to see with
all but the largest of telescopes.
Deep-sky objects fall into two catalogues which are
used throughout the world by both amateur and professional astronomers
to reference galaxies. The Messier Catalogue was created by Charles
Messier (1730-1817), an astronomer who was also an avid comet-hunter.
While scanning the skies for comets, Messier kept a record of “fuzzy
objects” which were not comets, specifically so he and others would not
confuse them with comets during future observations.
objects were discovered much later to be galaxies, clusters and
nebulae. His list of objects – 110 of them – are designated by the
letter M. M31 is the Andromeda galaxy discussed in Module 8, Tutorial
1, while the Cigar Galaxy pictured above is M82. Because Messier
observed from France, the Messier Objects are only found in the
The NGC is the New General Catalogue
of over 8,000 deep-sky objects, located throughout both hemispheres of
the sky. This catalogue was compiled much later, in the late 19th
century. While more thorough, amateur astronomers tend to prefer the
Messier Catalogue as its objects offer a tangible observing goal.
Did you know?
the most dedicated of amateur astronomers have undertaken the observing
task called a “Messier Marathon.” This means the astronomer must try
and find all 110 Messier objects within a single night. It requires
skill with his telescope, knowledge of the skies and dedication to
perform this single night accomplishment. Also, it is only possible to
perform this feat a few times a year due to the tilt of the Earth and
the changes in the visible part of the night sky.