The Universe|how big is our Universe and Galaxies|

Hello Friends let learn about our universe, Galaxy etc. How big is our universe..... 

The Universe 

The Universe 

The Universe contains everything that exists, from the tiniest subatomic particles to galactic superclusters ( the largest structures known). Nobody knows how big the universe is, but astronomers estimate that it contains at least 125 billion galaxies, each comprising an average of 100 billion stars. The most widely accepted theory about the origin of the universe is the big bang theory, which states that the Universe came into being in a huge exploison -the Big Bang- that took place between 10 and 20 billion years ago. The Universe initially consisted of a very hot, dense fireball of expanding, cooling gas. After about one million years, the gas began to condense into localized clumps called protogalaxies. During the next five billion years, the protogalaxies continued condensing, forming galaxies in which stars were being born. Today, billions of years later, the Universe as a whole is still expanding, although there are localized areas in which objects are held together by gravity; for example, many galaxies are found in clusters. The big bang theory is supported by the discovery of faint, cool background radiation coming evenly from all directions. This radiation is believed to be the remnant of the radiation produced by the Big Bang. Small "ripples" in the temperature of the cosmic background radiation are thought to be evidence of slight fluctuations in the density of the early universe, which resulted in the formation of galaxies. Astronomers do not yet know if the Universe is "closed", which means it will eventually stop expanding and being to contract, or if it is "open", which means it will continue expanding forever.

Galaxies 

Galaxy

A Galaxy is a huge mass of stars, nebulae, and interstellar material. The smallest galaxies contain about 100,000 stars, while the largest contain up to 3,000 billion stars. There are three main types of Galaxy, classified according to their shape: elliptical, which are oval shaped; spiral, which have arms spiralling outwards from a central bulge (those who arms spiral from a bar-shaped bulge are called spirals); and irregular, which have no obvious shape. Sometimes, the shapes of the Galaxy is distorted by a collision with another galaxy.

The Milky Way Galaxy 

Milky Way Galaxy

The Milky Way is the name given to the faint band of light that stretches across the night sky. This light comes from stars and nebulae in our galaxy, known as the Milky way Galaxy or simply as ''the Galaxy''. The Galaxy is believed to be a barred spiral, with a dense central bar of stars encircled by four arms spiralling outwards and surrounded by a less dense Halo. We cannot see the spiral shape because the solar system is in one of the spiral arms, the Orion Arm( also called the Local Arm). From our position, the centre of the Galaxy is completely obscured by dust clouds; as a result, optical maps given only a limited view of the Galaxy. The Milky Way Galaxy is vast, about 100,000 light years across(a light year is about 9,460 billion kilometers); in comparison, the solar system seems small, at about 12 light hours across( about 13 billion kilometers). The entire Galaxy is rotating in space, although the inner stars travel faster than those further out. The sun, which is about two thirds out from the centre, completes one lap of the Galaxy about every 220 million years.

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Written by R Gowtham.