Celestial Bodies – Definitions
Nebula: It is a diffused mass of interstellar dust or gas or both, visible as luminous patches or areas of darkness depending on the way the mass absorbs or reflects incident radiation.
Galaxy: The universe contains over 100 billion galaxies. A galaxy is a group of billions of stars. Our own galaxy is called the Milky Way, and it contains about 300 billion stars (300,000,000,000) and one of these is our Sun. (Markenian-348: Largest known galaxy. It is 13 times larger than Milky Way.)
Constellation: Constellations are clusters of stars which appear in a specific form. Centaurus, Gemini, Leo, Hercules and Virgo are major constellations.
Stars: Some celestial bodies that have their own heat and light are called stars. The Sun is also a star.
The brightest star is Sirius, also called Dog Star, and the closest to the Earth is Proxima Centauri.
Stars are made up of vast clouds of hydrogen and helium gas and dust.
Planets: Some celestial bodies do not have their own heat and light. These are called planets. The word planet means ‘wanderer’.
Satellites: The word satellite means a smaller companion to anything. True to their meaning, these satellites revolve around their planets and also follow them in their revolution around the Sun.
The Moon, for example, is a satellite of the Earth. It moves around the Earth and also around the Sun along with the Earth.
Meteors: Small pieces of space debris (usually parts of comets or asteroids) that are on a collision course with the
Earth are called meteoroids. When meteoroids enter the Earth’s atmosphere they are called meteors or colloquially a shooting star or falling star.
Comets: Comets are the most exciting of the heavenly bodies. They are made up of loose collections of ice, dust, and small rocky particles. A well developed comet has a tail which is always turned away from the Sun. The head forms the bright portion of the comet. Halley’s Comet is one of the important comets. It visits the Earth regularly and is visible once in 76 years. It was last seen in 1986. As comets approach the Sun, they develop enormous tails of luminous material that extend for millions of kilometres from the head, away from the Sun.
Asteroids: Between the orbits of Mars and Jupiter, there is a swarm of small bodies which also revolve round the Sun. They are called asteroids.
