The Sun
The Solar System was formed around 4.6 billion years ago from a large cloud of dust and gas, called a nebulaA cloud of gas and dust in outer space. If massive enough, these can collapse under gravity to form a protostar.. This collapsed under its own gravity, transferring gravitational potential energyThe energy stored by an object lifted up against the force of gravity. Also known as GPE. to kinetic energyEnergy which an object possesses by being in motion. in its particles. As the nebula collapsed it became denser, and rotated more rapidly. Collisions between particles caused kinetic energy to be transferred as internal energyEnergy stored in all materials, including energy due to the motion of particles and the chemical bonds between them. and thermal energyA more formal term for heat energy.. The core of the nebula began to form a hot, dense protostarThe early stage in the formation of a star, before nuclear fusion occurs..
Brian Cox explains how the Earth was formed
When the Sun鈥檚 core became hot enough and dense enough, nuclear fusionThe joining together of two smaller atomic nuclei to produce a larger nucleus. Radiation is released when this happens. Nuclear fusion happens in stars like our Sun, and in hydrogen bombs. reactions began. In these reactions:
- hydrogen nucleiNuclei is the plural of nucleus. The nucleus is the central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. join together to form helium nuclei
- energy is transferred by radiation
A star like the Sun is at equilibrium - gravity tends to pull it inwards, and radiation pressure from the nuclear reactions tends to expand it outwards. In other words, the gravitational collapse is balanced by the expansion due to fusion energy.