What Is A Galaxy?


A galaxy is a system of stars and interstellar matter which makes up the universe. The term galaxy derives itself from the Greek word galaxias, meaning "milky". Nature has provided a wide range of galaxies including faint and dwarfed objects and bright spiral-shaped galaxies. They exist in clusters that are further grouped into enormous clusters. Galaxies are different in shape due to the systems of their formation and how they evolve. The estimated number of observable galaxies range from 200 billion to 2 trillion or more containing stars that outnumber all of the grains of sand on the planet.

Types And Morphology Of Galaxies

Galaxies are divided into three general classes: elliptical, spiral, and irregular. The system of classification of galaxies was first proposed by Edwin Hubble in 1926. The classification depended on the visibility of the images observed on the photographic plate. However, Hubble’s classification may miss some of the important features including the star formation rate and activities within active galaxies.

Elliptical Galaxy

Elliptical galaxies were classified by Hubble on the basis of their ellipticity which ranges from E0 to E7 and has a rotational symmetry. They have an ellipsoidal profile giving them an elliptical appearance irrespective of the angle from which they are viewed. Elliptical galaxies have little interstellar matter and structure, and also have a low portion of open clusters. The rate of formation of new stars is greatly reduced due to the low portion of the open cluster. The galaxies are, therefore, dominated by old and more evolved stars. The stars forming the galaxies contain light elements since the formation of stars ends after the first burst. Many of the elliptical galaxies are formed due to the interaction of galaxies which leads to collisions and mergers with the galaxies capable of growing to enormous sizes compared to other classes of galaxies. Most of the enormous elliptical galaxies are found next to the center of the large galaxy clusters. Elliptical galaxies have red colors with their spectra suggesting that their light is emitted from mainly the old stars. The subclasses of the galaxies are distinguished by their shapes which are not necessarily three-dimensional shaped.

Spiral Galaxies

Spiral galaxies are circular in shape, resembling spiraling pinwheels. The galaxies are further divided into normal spirals and barred spiral. The arms of the normal spirals originate from the nuclear while the barred spirals have bars that straddle the nucleus while the arms unwind from the bar. The spirals galaxies are composed of rotating sets of stars and interstellar elements and older stars. The nucleus is sharp-peaked and has a smooth texture and can be small in size. The arms and the disk of the galaxies are blue in color while the central portions are red like in the case of the elliptical galaxies. The normal spiral galaxies are indicated by S followed by letters a, b, or c while the barred spirals are designated SB.

The two classes are further subclassed into three classes depending on the size of their nucleus. Sa galaxies are normal spiral galaxies with a tightly wound spiral arms which are visible. The galaxies have an amorphous bulge at the center. Examples of Sa galaxies include NGC 1302 and NGC 4866. Sb galaxies are the intermediate type of spirals galaxies having a medium-sized nucleus. The arms of the galaxies are relatively widespread than the Sa variety. The Sb galaxies contain stars, star clouds, and interstellar elements. Sc galaxies have a small nuclear and several open spiral arms. The spiral arms are protruding and contain several irregularly distributed star cloud and interstellar elements. The barred galaxies are designated SB followed by a, b, or c. the nucleus of the SBa galaxies are bright while the smooth spiral arms emanate at the end of the bar. The SBb galaxies also have smooth bars and relatively continuous and smooth arms which may start near the end of the bar. The SBc galaxy has both the bar and the arms resolving into star cloud.

Irregular Galaxies

The examples of the irregular galaxies are made up of highly irregular collections of bright areas. The galaxies do not have outstanding shape or the expected central nuclear and are bluer in color than the arms and disk of a spiral galaxy. A few of the galaxies are red in color. Examples of irregular galaxies include Irr I and Irr II.

Other types of galaxies include interacting galaxies which play an important role in the galactic evolution. The interacting galaxies may be formed as a result of a collision between galaxies that fail to merge. The stars of the interacting galaxies will not collide in the process but other elements like gas and dust will interact. Starburst galaxies form stars at an exceptional rate consuming their gas reserves in a time span less than the lifespan of the galaxy. Active galaxies contain active galactic nucleus which emits some energy instead of the star.

Properties Of Galaxies

Galaxies contain magnetic field within them. The magnetic fields are strong enough to drive mass inflow into the center of the galaxy. The magnetic fields can also affect the spiral arms leading to their modification which may affect the rotation of gas in the outer regions of galaxies. The normal large spiral galaxies vary in size with the average diameter ranging from 300,000 light-years. The weights of galaxies are also not clearly known but the total mass of the material within the radius from which the star can be detected is about 100,000 to 1 million times the mass of the sun. Stars and galaxies have abundant chemical elements including primordial gasses, hydrogen, and helium.

Cluster And Distribution Of Galaxies

Galaxies mostly clustered together in groups or in enormous complex. Some galaxies have companions while some are quite isolated. Galaxy clusters are classified into groups, irregulars, and spherical. A group cluster is composed of up to 50 galaxies of different types dating about five million light-years. Irregular clusters are loosely assembled mixed galaxies totaling about 1,000 galaxies or more while spherical clusters are crowded and are made up of exclusively elliptical galaxies. The spherical clusters are large and may contain up to 10,000 galaxies. The cluster of galaxies fills the sky although they are not easy to identify along a Milky Way. They are closely packed in the sky and are arranged in a manner to suggest some sort of organization. The clusters may interact with other structures to form a giant supercluster. A supercluster may be made of up to to 10 clusters and may span over 200 million light-years.


More in Environment