ABOUT OUR UNIVERSE

BIG BANG The Big Bang is the prevailing cosmological

theory of the early development of the universe. Cosmologists use the term Big Bang to refer to the idea that the universe was originally extremely hot and dense at some finite time in the past and has since cooled by expanding to the present diluted state and continues to expand today.

Solar System The Solar System consists of the Sun and those

celestial objects bound to it by gravity, all of which were formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago.

Of the many objects that orbit the Sun, most of the mass is contained within eight relatively solitary planets whose orbits are almost circular and lie within a nearly flat disc called the ecliptic plane.

The four smaller inner planets, Mercury, Venus, Earth and Mars, also called the terrestrial planets, are primarily composed of rock and metal. The four outer planets, Jupiter, Saturn, Uranus and Neptune, also called the gas giants, are composed largely of hydrogen and helium and are far more massive than the terrestrials..

‘SUN’ the Star• The Sun is the star at the center of the Solar System and is about 109

times larger than the Earth. The Sun's color is white, although from the surface of the Earth it may appear yellow because of atmospheric scattering and is mainly composed of helium and hydrogen.

• As the Sun exists in a plasmatic state and is not solid, it rotates faster at its equator than at its poles whereas if planets are considered they rotate faster at their poles. After trillions of years stars turn into Red Giants.

• Red giants are stars with radii tens to hundreds of times larger than that of the Sun which have exhausted the supply of hydrogen in their cores and switched to fusing hydrogen in a shell outside the core.

PLANETS• A planet is a celestial body orbiting a star or stellar remnant that is

massive enough to be rounded by its own gravity. • The term planet is ancient, with ties to history, science, mythology,

and religion. The planets were originally seen by many early cultures as divine, or as emissaries of the gods. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects.

• In 2006, the International Astronomical Union officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticized, and remains disputed by some scientists.

• There are Eight planets in our Solar System and the other three are referred to as Dwarf Planets.

Dwarf PlanetsThe term dwarf planet was adopted in 2006 as part of a

three-way categorization of bodies orbiting the Sun, brought about by an increase in discoveries of trans-Neptunian objects that rivaled Pluto in size.

The IAU currently recognizes five dwarf planets—Ceres, Pluto, Haumea, Makemake and Eris. However, only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they fit the definition. Eris has been accepted as a dwarf planet because it is more massive than Pluto.

Dwarf Planet, Eris

Our Planet EARTH• The Earth is the third planet from the Sun, and the densest and

fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets. It is sometimes referred to as the World, the Blue Planet.

• At present, Earth provides the only example of an environment that has given rise to the evolution of life. Highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago and half a billion years later the last common ancestor of all life existed. The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and formed a layer of ozone in the upper atmosphere.

Comets• A comet is an icy small Solar System body that, when close enough

to the Sun, displays a visible coma (a thin, fuzzy, temporary atmosphere), and sometimes also a tail.

• These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei are themselves loose collections of ice, dust, and small rocky particles, ranging from a few hundred meters to tens of kilometers across.

• Comets have been observed since ancient times and have historically been considered bad omens.

• Halley's Comet or Comet Halley is the best-known of the short-period comets, and is visible from Earth every 75 to 76 years.

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