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Whirlpool Galaxy The Whirlpool Galaxy (M51), was discovered in 1773 by Charles Messier, who cataloged objects in the sky, thus the "Messier Catalog" exemplifies two galaxies colliding with each other.
Hurricane Katrina (2005) Katrina was one of the most severe hurricanes in United States history. Notice the "spirals" of the cloud cover, which appear similar to spiral galaxies. Nautilus Spiral seashell The spiral shell of a sea Nautilus grows into larger and larger spirals. The progression of spiral dimensions changes in accordance with the Fibonacci series of numbers, i.e. 1 1 2 3 5 8 13 21 … . What is the next number in the Fibonacci series after "21"?

The spacing of the spirals in a spiral galaxy follows an intriguing phenomenon in Nature, the so-called "Golden Ratio", discussed by the ancient Greeks and Hindus and elaborated by the Italian mathematician Leonardo Fibonacci (~1170–1240 C.E.). The Golden Ratio (1.6…) is related to the "Fibonacci Series": 1 1 2 3 5 8 13… .  The ratio of two consecutive Fibonacci numbers approaches the limit of the "golden ratio" of 1.6… for Fibonacci numbers beyond 5.

The Whirlpool Galaxy is in reality two galaxies interacting with each other. The streaks of red in the image indicate hydrogen, which indicates regions where stars are born.

A galaxy is a group of stars. Some galaxies are composed of many millions of stars while other galaxies contain a trillion or more stars. Galaxies are classified by their shape. The more common galactic shapes are described below. The galaxy in which our Earth resides is called the Milky Way Galaxy, which is classified as a "Spiral" galaxy. Spiral galaxies resemble earthly hurricanes, having spiral arms that appear to rotate around a center bulge.

Galaxies appear in several forms other than spirals, including Elliptical, Globular, and Irregular. Although other galactic shapes appear in sky catalogues, these four shapes account for most of what astronomers have observed.
Studying galaxies, as well as other cosmic objects, helps us to understand Nature, which opens the door to resolving earthly issues. Studying the Sun and stars may well show us a path to producing boundless amounts of safe energy. Asteroids, the Moon, and perhaps, Mars may become repositories of recoverable minerals. We might also ponder the virtue of being awed and inspired by heavenly discovery.

1. Spiral galaxies resemble the shape of hurricane clouds. Most stars in the spherical center, or bulge, of a spiral galaxy are older stars, while younger stars inhabit the outer areas of a spiral galaxy. Our Sun and Solar System are whirling around in one arm of the spiral arms of the so-called "Milky Way Galaxy". The spiral arms contain dust, as well as stars. The dust in the arm in which our Earth resides is visible in the bright band of light that stretches across the evening sky.

2. Elliptical galaxies are usually shaped like an ellipse. In visible light, they appear smooth, revealing no features, such as individual stars or dust-laden gaseous clouds. Structural detail, however, is uncovered when these galaxies are viewed in ultraviolet and infrared light or in other wavelengths of electromagnetic radiation, such as x-rays or gamma-rays.
  Stars found inside elliptical galaxies are very much older than stars found in spiral galaxies. Elliptical galaxies usually display minimal star-formation. One could suggest that galaxies, as do human populations, age as the birth rate declines. M60 Galaxy M60:  is an elliptical galaxy about 60-million light years from Earth and about 120,000 light-years across. Notice NGC 4647, a spiral galaxy that is visible in the upper right corner of the figure (Australian Astro­nomical Observatory).
  An international collaboration discovered a super-massive black hole inside M60, the smallest known galaxy, which is paradoxically the locatin of one of the largest black holes ever discovered. An apparent close neighbor of M60 is a spiral galaxy that is much smaller than M60, NGC 4647 (upper right).

The Andromeda Nebula was first observed as a smudge of light (insetco5>, upper right) and consequently was called a "nebula". As telescopes improved, more and more detail was discovered. The former Andromeda Nebula is now known as the Andromeda Galaxy. Reality is what our eyes often do not see.
M5: Globular Cluster Somewhat confusing is that globular clusters and galaxies are very similar in appearance, i.e. they are groups of stars;  but globular clusters reside within the halos of galaxies. We might say, "a globular cluster is a galaxy within a galaxy."

3. Globular Clusters  are spherically shaped concentra­tions of stars: Clusters are found in the halos of galaxies; consequently, they are labeled as "clusters". The first globular cluster to be discovered is M22. German amateur astronomer Abraham Ihle discovered M22 in 1665. Still unknown is how globular clusters evolved and how they fit into the "grand scheme of things".

Large Magellanic Cloud The Large Magel­lanic Cloud is an irregular galaxy, having some resem­blance to a spiral galaxy. The Large Magellanic Cloud is in our galactic neighborhood, only 180,000 light years away from us.  (Photo by NASA/Ames Research Center in Public Domain) 4. Irregular: oddly shaped galaxies that do not resemble any of the more common forms listed above. Many of these irregular galaxies are among the oldest galaxies in our Universe. Younger galaxies seem to have more describable shapes. In visible light, irregular galaxies appear smooth, revealing no features, such as individual stars or dust-laden gaseous clouds. Structural detail, however, is uncovered when these galaxies are viewed in ultraviolet and infrared light or other wavelengths of electromagnetic radiation, such as, x-rays or gamma-rays. Stars found inside elliptical galaxies are very much older than stars found in spiral galaxies. A dense region of the larger of the two Magellanic Clouds shown here reveals many structural details.

A concise collection of photographs of several categories of galaxies can be seen at the Australian Astronomical Observatory.

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