• All existing matters and space as a whole forms Universe. It was termed as Cosmos when first conceived as an orderly unit and its study is called as cosmology.
• It is believed to be expanding since its creation in the Big Bang about 13 billion years ago.
• Normal matters all that are visible (star, planet and galaxies) make up less than 5 % of the total mass of the universe rest are made of dark matters. These dark matters are not seen by the astronomers but can study their effects.


Big Bang Theory

• It is also called expanding universe hypothesis.
• Edwin Hubble, in 1920, provided evidence that the universe is expanding. As time passes, galaxies move further and further apart.
• Later Big bang theory was proposed by Georges Lemaitre in 1927.
• According to this theory billion of years ago cosmic matters were in highly compressed state and expansion started with primordial explosion which was bang in super dense ball. These exploded particles are still travelling at a speed of thousands miles per second and gave rise to our galaxies.

Steady State Theory

• The steady state theory was governed by Hermann Boudi and Thomas Gold.
• It is also known as theory of continuous creation. According to this theory universe has always existed and will always exist and will always look essentially the same, so there is no overall evolution thus balancing the average density despite the expansion.
• As old galaxies move apart the new galaxies are being formed.

Oscillating Universe Theory

• The Oscillating Universe Theory was advocated by Dr. Alan Sandage.
• This theory postulates that the universe not only expands but it also contracts. The time interval between the two phases is presumed to be billions of years. It is a mixture of both Big Bang an Big Crunch theory.


• A Galaxy is a large collection of stars, gas, dust, and dark matter bounded by gravitational force. At times they are so big that they are called as Island Universe.
• The studies related to the distant spaces with optical and radio telescopes indicate that about 100 galaxies are visible universe.

Structure and composition of Galaxies

• Elliptical Galaxies – Elliptical galaxies can be classified on the basis of their ellipticity, ranging from nearly spherical (E0) to highly elongated (E7). These have low portion of open clusters and low rate of new star formation.
• Spiral Galaxies – Spiral galaxies have a central nucleus with great spiral arms trailing round it resembling pin wheel. Andromeda Galaxy and Milky Way are the examples of such galaxy. The spiral arms are thought to be areas of high-density matter, or “density waves”.
• Irregular Galaxies – Irregular galaxies are youthful in nature with no sharp and boundary thinning out gradually, these galaxies contain large amount of gas and dust. This type of galaxy is the result of gravitational interaction or collision between formerly regular galaxies.


• The distribution of matter and energy was not even in the early universe. These initial density differences gave rise to differences in gravitational forces and it caused the matter to get drawn together.
• These formed the bases for development of galaxies. A galaxy contains a large number of stars. Galaxies spread over vast distances that are measured in thousands of light-years. The diameters of individual galaxies range from 80,000-150,000 light years.
• A galaxy starts to form by accumulation of hydrogen gas in the form of a very large cloud called nebula.
• Eventually, growing nebula develops localised clumps of gas. These clumps continue to grow into even denser gaseous bodies, giving rise to formation of stars.
• The formation of stars is believed to have taken place some 5-6 billion years ago


The following are considered to be the stages in the development of planets:
• The stars are localized lumps of gas within a nebula. The gravitational force within the lumps leads to the formation of a core to the gas cloud and a huge rotating disc of gas and dust develops around the gas core.
• In the next stage, the gas cloud starts getting condensed and the matter around the core develops into small rounded objects. These small-rounded objects by the process of cohesion develop into what is called planetesimals. Larger bodies start forming by collision, and gravitational attraction causes the material to stick together. Planetesimals are a large number of smaller bodies.
• In the final stage, these large number of small planetesimals accrete to form a fewer large bodies in the form of planets


• Solar System constitute of heavenly bodies revolving around an average star known as SUN.
• Our solar system consists of the sun (the star), 8 planets, 63 moons, millions of smaller bodies like asteroids and comets and huge quantity of dust-grains and gases. The Nebula from which our Solar system is supposed to have been formed, started its collapse and core formation some time 5-5.6 billion years ago and the planets were formed about 4.6 billion years ago.
• Eight Planets – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune
• Inner Planets – Out of the eight planets, mercury, venus, earth and mars are called as the inner planets as they lie between the sun and the belt of asteroids
• Outer Planets – the other four planets are called the outer planets.
• Terrestrial Planets – Alternatively, the first four are called Terrestrial, meaning earth-like as they are made up of rock and metals, and have relatively high densities.
• Jovian Planets – The rest four are called Jovian or Gas Giant planets. Jovian means Jupiter like. Most of them are much larger than the terrestrial planets and have thick atmosphere, mostly of helium and hydrogen.
• The terrestrial planets were formed in the close vicinity of the parent star where it was too warm for gases to condense to solid particles. Jovian planets were formed at quite a distant location. The terrestrial planets are smaller and their lower gravity could not hold the escaping gases.
• All the planets were formed in the same period sometime about 4.6 billion years ago. Till recently (August 2006), Pluto was also considered a planet. However, in a meeting of the International Astronomical Union, a decision was taken that Pluto like other celestial objects (2003 UB313)discovered in recent past may be called ‘dwarf planet’. Some data regarding our solar system are given in the box below

The Moon

• The moon is the only natural satellite of the earth. Like the origin of the earth, there have been attempts to explain how the moon was formed.
• In 1838, Sir George Darwin suggested that initially, the earth and the moon formed a single rapidly rotating body. The whole mass became a dumb-bell-shaped body and eventually it broke.
• It was also suggested that the material forming the moon was separated from what we have at present the depression occupied by the Pacific Ocean. However, the present scientists do not accept either of the explanations.
• It is now generally believed that the formation of moon, as a satellite of the earth, is an outcome of ‘giant impact’ or what is described as “the big splat”. A body of the size of one to three times that of mars collided into the earth sometime shortly after the earth was formed. It blasted a large part of the earth into space. This portion of blasted material then continued to orbit the earth and eventually formed into the present moon about 4.44 billion years ago


• A small rocky body orbiting the sun is termed as asteroid. Large numbers of these, ranging enormously in size, are found etween the orbits of Mars and Jupiter, though some have more eccentric orbits. The asteroid is categorized by their spectra, with most falling into three basic groups: carbonaceous (C-type), silicate (S-type), and metal-rich (M-type).


• A meteoroid is a small rocky or metallic body travelling through space and range in size from small grains to 1 meter-wide objects. When it enters the atmosphere to become visible is called as a meteor. It is also known as “shooting star” or “falling star.” One can see nearly 20 million of meteors in a day. On an average nearly each day nearly one to two reaches Earth.

The rings of Saturn

• The rings are made up of countless small particles, ranging in size from micrometers to meters and orbit about Saturn.
They are most extensive planetary ring system of any planet in the Solar System. The ring particles are made almost entirely of water ice, with a trace component of rocky material. There is still no consensus as to their mechanism of formation; some features of the rings suggest a relatively recent origin, but theoretical models indicate they are likely to have formed early in the Solar System’s history.

Dwarf planet

• A dwarf Planet is a planetary-mass object that is neither a planet nor a natural satellite. It orbits the Sun, and is massive enough for its shape to be in hydrostatic equilibrium under its own gravity, but has not cleared the neighborhood around its orbit.
• The term dwarf planet was adopted in 2006. Currently, the International Astronomical Union (IAU) recognizes five dwarf planets: Ceres, Pluto, Haumea, Makemake, Seden and Eris.
• Pluto as dwarf planet Pluto is called a “dwarf planet.” A dwarf planet orbits the sun just like other planets, but it is smaller. A dwarf planet is so small it cannot clear other objects out of its path. Similarly, Pluto is in a region called the Kuiper (KY-per) Belt. Thousands of small, icy objects like Pluto are in the Kuiper Belt. The orbit of which Pluto follows takes 248 days to revolve round the sun once and its oval in nature. There are moments when it is nearest to the sun causing the ice present on the planet to melts. Pluto having about one-fifteenth the gravity of Earth, its atmospheres altitude rises more than any other planet