Universe, Galaxy and Milky Way

Universe

Everything that exists — from sub atomic particles to the largest structures ever found forms the Universe. It is believed that the Universe contains about 100 billion galaxies of which each one has an average of 100 billion stars. Scientists are of different opinion about the origin of the Universe. The Big Bang Theory is the widely accepted theory about the origin. According to this theory the Universe initially consisted of an extra large ball of compressed mass and very high temperature. The high pressure and temperature caused a huge explosion between 15 and 20 billion years ago, resulting in the scattering of the compressed mass. Thus formed billions of galaxies including our galaxy are called the Milky Way. It has not been proved yet whether the Universe is expanding further or contracting or remaining steady.

Steady State Theory

The Big Bang Theory is widely approved. The British scientists Thomas Gold, Fred Hoel and Herman Bondy together put forward the Steady State Theory about the origin. It states that the Universe remains steady without a beginning or end. But this theory was not much approved by others.

Expansion of the Universe

Edwin Hubble (1889-1953), an American astronomer determined the expansion of the Universe through his theory called Hubble's Law. It states that the speed at which a galaxy is receding is directly proportional to its distance from the Earth. With his colleagues, Hubble made a 100 inch telescope in Mount Wilson Observatory, which later helped to study about the unknown Universe. Hubble observed that the neighboring galaxies of the Milky Way are receding from the Earth and this proves that the Universe is expanding.

Galaxies

Galaxy is a huge mass of stars and other heavenly bodies assembled in the Universe by gravitational force. The size of the Galaxies differs according to the number of stars. The smallest ones are of about 1,00,000 stars while the largest Galaxies contain about 3,000 billion stars.

Galaxies are classified into three main groups according to their shape.

1. Elliptical Galaxy - oval shaped

2. Spiral Galaxy - with spiraling arms from a central bulge.

3. Irregular Galaxy - without an apparent shape.

The Milky Way Galaxy is included in the group of Spiral Galaxies. Sometimes the shape of the Galaxy is distorted due to the collision of two Galaxies. The changes occurring in their nuclei form super massive objects like black holes. Such changes occurred billions of years ago. Now they appear as compact, highly luminous objects called Quasars. These Quasars are very far away so that their exact nature is still unknown. Some Galaxies are isolated while others are active Galaxies and they emit extra large amount of radio energy. These radiations are thought to be caused by the presence of black hole.

The Milky Way

Our Galaxy which includes the Solar System is called the Milky Way. The Milky Way consists of about 150 billion stars. It is spiral shaped with a bulge at the centre. Four arms surround the central bulge spirally outwards and a less dense halo borders the Milky Way. The central bulge contains red and yellow stars. The spiral arms contain clouds of dust and gas in which hot, young, blue stars are seen. The four spiral arms are the Orion arm, Sagittarius arm, Perseus arm and Crux Centaurus arm. The Solar System is in the Orion arm. It is at a distance of 25,000 light years from the centre of the Milky Way. The Milky Way is about 1,00,000 light years across and 5,000 light years thick. The Milky Way is rotating in space. The speeds of movement of the outer stars are low when compared to the inner stars. The nearest Galaxy to the Milky Way is Andromeda.

Stars

Stars are self-luminous, hot bodies that occur in a cloud of dust and gas called nebula. There are notable variations in size, mass and temperature of Stars. The colour of the stars is determined by its surface temperature. It varies from about 3000°C to over 50000°C. The hottest Stars are seen blue and the coolest are red. The surface temperature of the Sun lies in between these extremes at about 6000°C and it appears yellow. The energy emitted during the process of nuclear fusion in the Star's core keeps them shining.

The life-cycle of a star

Stars are born in a cloud of dust particles and gas called nebula. It is formed as a huge ball on condensation of a region of nebulae having higher density. When matured it contracts under its own gravity increasing the surface temperature, and begins to glow. The outer layer gradually expands, cools to a reddish colour and it is the red giant. When the fuel begins to run out the outer layer drifts off as glowing gas shells called planetary nebulae. The explosion is called Supernova explosion. The core left behind cools and dims to form a white dwarf. When the shining stops completely it becomes a black dwarf.

Neutron Stars and Black Holes

Neutron Stars and Black Holes are formed after the Supernova explosion of the Stars. If the remaining core is less than three solar masses it contracts to form a Neutron Star. If the remaining core is greater than three solar masses it contracts to form a Black Hole. Neutron Stars contain subatomic particles called neutrons, weighing about a billion tonnes. They rotate rapidly emitting two beams of radio waves. The gravitation of the black holes is so extremely strong that not even light or radiation waves can escape from them. As the gravitation is strong, gases from other Stars are pulled in by the Black Hole. The gas accumulates to form a spiraling disc around the Black Hole. The disc spirals at a high speed and heats up sending out radiation. The spiraling of the disc to cross the boundary of the black hole makes it invisible to the visible Universe.

Figures of Stars

• Age: Between 1 million and 15 billion years.
• Mass: From 1/20 to 100 times the mass of the sun.
• Temperature Surface: 2800°C to 28000°C.
• Centre: more than 1,100,000°C.
• Diameter: Largest stars : about 1.6 billion kilometres
• Smallest stars: about 20 kilometres
• Composition: 75% hydrogen, 22% helium, and traces of other elements.
• Brightest star (excluding sun): Sirius
• Nearest star to the earth (excluding sun): Proxima Centauri (4.3 light years from the earth)