Sunlight. It is the large bright yellow circle we can see in the sky during the day.
Astronomy considers it one of many stars in our galaxy, the one which the Earth orbits. It is usually thought of as the centre of our Solar System, because it is the only star and the only source of energy, ultimately, for life as we know it.
At any one time the Sun is also in the centre according to geometry, since planets will be on all sides of it. This is important in astrology and in planning space travel. A spacecraft can actually surf on the solar wind which is particles flowing away from the sun out towards the system's edges.
Recently, some life has been discovered on the ocean floor that relies on chemosynthesis, driven by energy still in the Earth from its formation. Nuclear fusion and nuclear fission are used as a human energy source. Other than these, all life relies on the large-scale nuclear fusion that takes place in our Sun. On its surface we see it combining hydrogen into helium but it also forces many other heavier elements together nearer its core. Little is known about this as we can't get close.
Astrophysics considers our Sun to be a yellow dwarf star. It is in the middle of its life. In about a billion years it will have become a red giant, and will spread out as far as Earth's orbit. Long before that, all life on Earth will end as the energy from the sun literally bakes the planet like Mercury.
|Things in the Solar System|
|Sun | Mercury | Venus | Earth | Mars | Asteroid Belt | Jupiter | Saturn | Uranus | Neptune | Pluto | Comets|