Galaxies can be explained as massive systems of stars, dust, gas as well as other types of matter that are bound together by gravitational forces to form a single physical mass. As Binney and Merrifield (1998: p 34) explain, on the basis of extensive imaging surveys that have been carried out, it is estimated that there are more than 40 billion galaxies in the universe, that are spread out in a complex large-scale systems for example, clusters, super-clusters. There are also boundaries of huge empty spaces referred to as voids.
The sun is just part of galaxy called the Milky Way. This galaxy is a much flattened spiral galaxy that could be comprised by more than 400 billions stars. Through the use of telescopes, it is possible to view these galaxies. Gravitational Lenses Gravitational lenses can be described as galaxies and clusters of galaxies, which are so huge that they cause bends to the route of light objects towards their directions, thus they distort the forms of backdrop galaxies into curves as well as rings.
At times these gravitational lenses can even lead to photos of galaxies and quasars to be seen in symmetrical outlines around them. However such cosmetic arrangement is very rare and only occurs in out of a thousand elliptical galaxies. Galaxy merges It has been noted that galaxies comes together to create merges. In such a case galaxies can merge in two different ways. One is through forming stars, and the second way is by merging together. Basing on the present theories of how galaxies are formed, it is expected that they are must be extensive merging going on.
In fact, there are many examples to attests this point. However, it is very hard to consistently measure the extent of merging that is happening; it calls for huge samples and keen measuring machines and eyes. (http://www. galaxyzoo. org/science) Understanding these galaxies is a significant part of contemporary astronomy. Galaxies are basic units of matter within the space, and explaining the way they are formed as well as how they have achieved their present form is very critical for various aspects in astronomy.
Indeed it is significant to note that what is presently known regarding galaxies started with just a simple classification of how the galaxies appeared through the photos taken using huge observatory telescopes. From these photos, galaxies present a broad diversity of forms, or shapes and can basically be divided into various classes just like living things are classified. Classification of galaxy Visual galaxy classification has continued to be helpful at a moment when galaxies still are not better understood.
Classification of galaxies offers an outline for more studies and proposes a sound approach to studying these complex galaxies. Classification of galaxies Binney and Merrifield (1998 pp 36) tells us that, different schemes have been formulated to bring some order to galactic zoo; this has been done through pigeonholing galaxies in relation to one or various properties that include shape, luminosity and spectrum. This section has listed some galaxy classification. Hubble classification The well known and mostly used general scheme of galaxy classification is the Hubble classification.
In this classification, galaxies are classified according to how they appear. This system was formulated by Edwin Hubble, and it splits the galaxies into spiral (normal as well as barred), elliptical and irregulars. This scheme is characterized by the well known turning-fork figure. Elliptical galaxies are further grouped from spherical (E0) to highly elongated (E7) in relation to their increasing eccentricity. Generally, spirals vary from Sa (those with arms closely wound) to SBc (those with arms extensively spaced). Irregulars are referred to as Ir.
However, Alan Sandage added an extra category to this initial scheme. What was added is S0 for describing lenticular system that has a nucleus that is surrounded by a circle-like structure that does not have spiral arms. Accordingly Buta, Corwin and Odewahn (2002, p, 3) explains that, Galaxies are as well usually termed as ‘early’ for those designated as E and S0 or to be ‘late’ for those designated as Sb, Sb and Irr, in essence this is a remainder of early ideas that galaxies physically evolved alongside the Hubble order.
Sadly, this classification is contrary to that of the leading stellar population among these types, and it is also contrary to the early-late classification used in the Yerkes nomenclature. The figure bellow shows some of the galaxy classifications Source: http://www. daviddarling. info/encyclopedia/G/galaxies. html Morgan classification William Morgan also formulated a scheme of classifying galaxies that applies the integrated spectrum of stars and their shapes (apparent and real) together with the degree of the galaxy central concentration.
This scheme spells out the galactic spectral form as, a; af; fg; g; gk or k (the letter/s corresponds to the various integrated stellar forms). There are other form used for example, S for spiral; B for barred spiral; E for elliptical structure; L for low surface brightness and N for tiny bright nucleus. Numbers are used to classifying the inclination line of vision, thus, 1 is used for face on while 7 is used for edge-on (numbers 1 to 7 are used). Accordingly, Andromeda Galaxy classification is as kS5
De Vaucouleurs-Sandage classification In this classification, SA designates ordinary spirals, while SB designates barred spirals. Then a lower case s in brackets designates S-shaped spirals. Alternatively r will designate ringed shape. Lastly, various transitional phases have been incorporated between SA spirals and Magellanic irregulars Im. Thus using this classification, Andromeda Galaxy is classified as Sa (s) b. Van den Bergh (DDD) classification This classification comprises two parameters.
The first one is galactic type (this are Sa; Sb; Sc and Ir). The second parameter is the luminosity class (designated as I, II, III, IV and V). Just like the MKK structure of stellar luminosity classification, the notations S- as well as S+ are applied to designate sub-giant species that have low or high resolution, correspondingly. S (B) notation has created to designate objects that are intermediate between real spirals and the barred spirals. Conclusion
Galaxy classification is very useful since it provides a lot of insight information on physical aspects of galaxies. In classifying galaxies, there are various schemes that are used; however, the scheme formulated by Edwin Hubble though revised over the years still remains the most widely used. Classification of galaxy has laid down the foundation of future study in understanding the galaxies. Through correct classification we gain better understanding of these complex galaxies and indeed of our universe. Reference:
Binney, Joseph and Morrison Merrifield: Galactic Astronomy: Princeton: Princeton University Press, 1998: pp 34-37 Buta, Ronald; Howard Corwin and Stephen Odewahn: Galaxy classification: Cambridge University Press, 2002, p 2-4 Galaxy Zoo: The Science: Accessed from: http://www. galaxyzoo. org/science on 12/6/2009 Galaxy Zoo (2009): The Story So Far Accessed from: http://www. galaxyzoo. org/story on 12/6/2009 The internet Encyclopedia of science: Galaxy classification: Accessed from: http://www. daviddarling. info/encyclopedia/G/galaxies. html on 12/6/2009
University/College: University of Chicago
Type of paper: Thesis/Dissertation Chapter
Date: 8 October 2016
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