The Universe, Milky Way Galaxy and the Big Bang

• The universe includes the sun, moon, stars, planets, and galaxies.
• The universe is the totality of everything that exists with those celestial bodies being referred to as heavenly bodies.

Origin of the Universe

• Scientists' prevalent theory regarding the origin of the universe is the Big Bang Theory.
• The Big Bang Theory states that billions of years ago, a large fireball was a result of a massive explosion from which, as the fireball cooled, smaller particles formed and grew into stars and galaxies.
• Astronomers are people who study the universe.
• Telescopes are the most important instruments for astronomers.

Milky Way Galaxy

• The Milky Way galaxy is home to Earth and the solar system.
• The Milky Way galaxy contains approximately 200 billion stars.
• Most of the stars within the Milky Way are not visible from Earth.
• Everything that is seen in the sky is part of the Milky Way galaxy, which is also known as Akash Ganga.
• The solar system is a small part of the Milky Way.

Solar System

• The solar system is formed by the sun, eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and other heavenly bodies, including asteroids, comets, and meteors.
• The sun is a star at the center of the solar system that is the source of heat and light.
• Planets do no emit light but instead reflect the light of the sun.
• Jupiter is the solar system's largest planet.
• Mercury is the smallest and closest planet to the sun
• Neptune is the farthest planet from the sun.
• Earth is the third planet and the only planet known where life exists.
• Mars consists of clay rich in iron, carbon dioxide, argon, and nitrogen, giving it red color and the name 'Red Planet'.
• Venus is called the 'Morning and Evening Star' due to its bright appearance from Earth.
• Saturn is the 'Ringed Planet,' with rings of ice pieces and dust particles.
• Uranus has a blue-green color due to cold gases in its atmosphere.
• Satellites are smaller heavenly bodies that revolve around planets.
• Satellites can be natural, such as the moon, or artificial.
• Satellites rotate on their axis, like planets do.
• Satellites need energy, with communication satellites needing between 1 and 1.5 kilowatts of electricity.

Planetary Movements

• All planets exhibit two types of movements: rotation and revolution.
• Rotation is the spinning of a planet on its imaginary axis.
• Revolution is the movement of a planet around the sun in a fixed orbit.