The universe is all of spacetime and everything thatexists therein including all planets, satellites, stars,galaxies, constellations, comets, dust particles, matter,energy. etc.
The units to measure the distances betweenanytwo heavenly bodies are:
-Astronomical Unit (AU)
1AU ----------> 1.5 * 10¹¹m.
-Light Year
1light year ----------> 9.46 * 10¹²km.
-Par Sec
1 par sec ----------->3.26 light year.

The family of sun, including all planets and theirsatellites, meteors, comets, asteroids, etc. is calledthe solar system.
We know,
Speed of light = 3 x 10⁸ m/s.
Time = 1 year.
We have,
Distance travelled by light in 1 year = speed of light * time
=3 * 10⁸m/s * 1 year.
= 3 * 10⁸m/s * 365 * 24 * 60 * 60 seconds.
= 9.46 * 10¹⁵m.
= 9.46 * 10¹²km.

Planets are the heavenly bodies which revolve aroundthe sun in ellipticalorbits.
Those planets which are present inside the asteroidbelt are called inner planets.
Those planetswhich are presentoutside the asteroidbelt are called outer planets.

The rocks and metal objects which are present in between the orbits of Mars and Jupiter that revolve around the sun are called asteroids or minor planets.

The pieces of stone or metal which enters the earth'satmosphere are called meteors.
Ameteorslasts for very short time and completelyfinished in atmosphere because the small rockpiecesburn and vaporizecompletelydue to the excessiveheat produced by the atmospheric friction.

The big meteors which do not burn completely andreach the surface of the earth are called meteorites.
Its types are:
-Stonymeteorites.
-Ironmeteorites.
-Stony ironmeteorites.

Any three purposes of launching artificial satellites are:
-For the communication of radio, TV and telephonesignals across the world.
-For weather forecasting.
-To study space and different regions of earth.
-For the study of cosmic rays and solar radiation.
-For spying or military purposes.

A galaxy is a vast system of stars, dust particlesand gases isolated from a similar system in space.
Its types are:
-Spiral galaxies.
-Ellipticalgalaxies.
-Irregular galaxies.

Our solar system is situated on Milky-waygalaxy.
It has the spiral shape.

Agroup of stars in thesky which resemble theanimal, god or materials is called a constellations.
Any four constellations are:
-Ursa major.
-Ursa minor.
-Orion.
-Cassiopeia.

A broom shaped shining object with a bright headis called a comet.
When a comet approachesthe sun, some of itsfrozen matter begins to evaporate into gases due tothe heart of the sun. These gases rush out by rapidvaporization in the form of jets carrying solid particleswith them. In this way, a long tail of a comet isformed due to the motion of comet and effect ofthe solar wind. Therefore, the tail of a comet becomelonger as it approaches the sun.

A fast moving star radiating radio frequency iscalled pulsar.
The star can't be seen but sends out regularrapid radio signals.

According to Big Bang theory, all the matter inthe universewas originallyconcentrated in one vastlump called a primeval atom. About 15 billions yearsago, this super dense and extremely hot body havingneutrons and protons exploded with Big Bang and allthe matter started flying in all directions through spaceleading to the formation of galaxies and other heavenlybodies.

a. Synodic month and sidereal month.

b. Asteroids and planets.

We all know, both earth and mars revolve around thesun in a fixed orbit. They have the specific numbersof satellites that revolves around them. So earth &mars are called planets.

Though there is the presence of water, oxygen,carbon dioxide on both Earth and Venus, life is notpossible in Venusbecause its surface temp. isveryhigh(about 865f) which is not suitable for the survival ofliving beings. But the temp. of earth isfavorableforliving beings. So, life is possible on Earth but not onVenus.

Pluto has been removed from the list of planets because:
-The size of the Plutois very smalt compared to theaverage size of other planets.
-The orbit of Pluto coincidewith orbit of Neptune.
-The orbit of Pluto doesn't lies on the same plane tothe plane of other planets.

When a comet approaches the sun, some of its frozenmatter begins to evaporatedinto gases due to the heatof the sun. These gases rush out by rapidvaporizationin the form of jets, carryingsolid particles with them.In this way, a long tail of comet is formed due tothe motion of the comet and effect of solar wind.

The tail of the comets are made up of ice and dustparticles. When the comet comes closer to sun, the icevaporizes and a long tail is formed due to high speed ofcomet. But when the comet movesaway from the sun,the ice doesn't vaporizes due to low temp. and finally taildisappears.

We know, star are the luminous huge heavenly bodieswithvery strong gravitational force. But the cometare the non-luminoussmall object revolving aroundthe sun with very less gravitational force. So, comet is not actuallya star.

A meteor lasts only for a few minutes becausethe small rock pieces burn and vaporize completelydue to the excessiveheat produced by atmosphericfriction.

Neptune: Planet
Milky Way: Galaxy
Venus: Planet
Enke: Comet
Andromeda: Galaxy
Phobes: Satellite
Ursa major: Constellation
Titan: Satellite

Ursa Major is seen insummerseasonin northensky.

The twelve special constellations of solar path arecalled zodiacs.
Any three examples are:
-Aries.
-Cancer.
-Leo.
-Pisces.

The heavenlybodies that have their own sources oflight are called stars.
Any two features of stars are:
-Theyare luminous.
-They consists of hydrogen, helium and dust particles.