Galaxies and the Universe


For thousands of decades, astronomers have worked tirelessly to unlock the hidden mysteries of the heavens. On a dark, clear moonless night, apart from observing trillions of stars and planets scattered from end to end on a hemispherical two-dimensional surface- called “the night sky”, one would note another great source of wonder in form of a huge “fuzzy” faint band of light cutting across the sky. This is our Milky Way Galaxy which is home to the Sun and our solar system. Much of the present knowledge about the Milky Way dates back to the era of discovery of the first telescopes in the 1600s. However, it wasn't until the 20th century that moderb developments in technology allowed scientists to make full measurements of the Milky Way galaxy and other galaxies scattered throghout the universe.

Galaxies can be defined as giant self-contained gravitationally bound collections of stars that run to thousands of light-years across, each made up anywhere between a million (106) up to a million million (1012) stars. The closest easiest star to study by scientists is the Sun, due to its close proximity to the Earth. It is located at a distance of about 150 million kilometres (93 million miles) from the Earth and has a radius of about 700,000 kms (430,000 miles). Its mass is estimated to be about 330,000 times that of the Earth. The sun is known to generate approximately 4 x 1023 kilowatts (4 x 1033 ergs) of power per second, making it only an average star, with average size, mass, and brightness.

Current studies imply that galaxies could have formed shortly after the birth of the universe – during the big bang- about 14 billion years ago. They can be broadly classified based on their shape and morphology into: spirals, ellipticals and irregulars.

The Milky Way is orbited by two small satellite galaxies- the Large and Small Magellanic coulds, visible from the Southern Hemisphere, thus showing that larger galaxies often have smaller companions. The closest neighbouring galaxy to the Milky Way is the Andromeda Galaxy, one of the most-distant objects that can be observed by the unaided eye. Beyond Andromeda, there are also found to exist many other galaxies numbering to over 100 billion, each containing trillions of stars and nebulae, and occupying the space within our galactic neighbourhood and extending to the farthest corners of the observable universe. Galaxies are social beings existing in clusters containing a few tens over a hundred of members. One important feature of galaxies important in understanding our universe is the fact that galaxies are observed to be clustered into clusters of galaxies, which group to form even larger structures called super clusters, that inturn cluster to from the entire cosmic web.

The aim of this course is to equip the learner with necessary astronomical knowledge about galaxies: what they are, how they behave, what they are made up of and their composition, how they are classified and how they are arranged in the universe.


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