Is a star and a planet the same thing?

Answer 1

A star forms when a very big cloud of gas contracts under the influence of its own gravitational force. As this contraction takes place the object emits energy. This energy is called fall energy. As a result of this contraction the core gets denser and hotter. When the core reaches a temperature of about 3 million Kelvin, it starts to emit light, because of nuclear fusion ( Krane, K.S) reactions in the core. At this stage the gas cloud will stop its contraction because now the gravitational force is in equilibrium with the pressure build up by the hot gas. When this starts to happen you can say that a new star is born.

Planet formation

A planet on the other hand is build up out of the dust that surrounds a star. When a star is formed there is still a disk of gas surrounding it. As this gas cools, it condenses and forms solid grains. These grain particles accrete into large bodies called planetesimals, which then collide and accrete to make protoplanets. These protoplanets evolve into planets like the planets in our own solar system.

So the formation of a star is totally different from that of a planet. This is the main difference between a star and a planet. If an object has a mass of 0.084 times the mass of our own sun (85 times the mass of Jupiter) the core reaches a point where it can start the process of nuclear fusion in its core (see fig.1). If the mass is smaller than this, the lowest temperature to support nuclear fusion will never be reached and the object will never shine like a star. But can we call all of these objects planets? No, objects with a mass between 85*Mj (85 times the mass of Jupiter) and 13*Mj can't sustain nuclear fusion of elements like hydrogen (H) and helium (He) but can support the fusion of two protons into deuterium (D), early in their lifetime. These objects are called brown dwarfs. They form the transition between stars and planets.