Why heavenly bodies are considered to the point sources even though they are huge in size
Answers
This is because most of the universal law of physics like gravitation and electrostatic force are applicable only for point size object and these forces act between the centre of two objects. So in order to use these laws we have to consider huge body as point size at their centre and then integrate all the points together to derive results for the whole huge body.
Stars
Stars are huge luminous balls of hot gas that act like the chemical factories of the universe. The closest star to Earth is the sun. Made up mainly of hydrogen and helium atoms, stars are driven by nuclear fusion reactions, which give off the huge amounts of energy that makes them shine. As the temperature of their core rises, to the order of several million degrees celsius, some of their atoms fuse together to form new elements - including the 92 elements that make up our bodies and everything around us.
Planets
Pluto and its moon Charon
Former planet: Pluto and its moon Charon
You would think that it was pretty easy to define a planet - a particularly large lump of rock orbiting a star. But in recent years the definition of planets has courted controversy, with the reclassification of Pluto from being the ninth planet of our solar system to that of a mere "dwarf planet" (a large asteroid). This is because although Pluto satisfies the requirements of being large enough to be pulled into a sphere by the force of its own gravity, while simultaneously orbiting a star, it fails on the last planetary requirement: it has not cleared the neighbourhood around its orbit - it is not big enough to dominate the other celestial bodies in its path.
Galaxy
The M51 whirlpool galaxy
The M51 whirlpool galaxy. Photograph: NASA/Hubble Space Telescope
Just like planets, moons and satellites, stars orbit in collections around a common centre of mass to form galaxies. The number of stars in a galaxy can range from as few as tens of millions to more than a million million, and these can be anywhere from 20,000 trillion to 20 quintillion (18 zeros) miles across. The term "galaxy" stems from the Greek, meaning milky, hence the name of our own galaxy: the Milky Way.