At the very center of the Solar System there is a gigantic ball of fire, that we call our Sun. It was born around 4.6 billion years ago, when an enormous gas cloud, made up of hydrogen started to collapse under its own weight. The force of gravity brought particles closer and closer together, until they breached a critical threshold and ignited nuclear fusion, a process which continues in the Sun’s core to this day. It is what turns one material into another and every single piece of heavier matter in the universe was at some point formed in the heart of a star.
Nuclear fusion starts with two simple protons, which under immense heat and pressure are merged together. Usually they break apart after a while, but ever so often one of them will give off a positron and become a neutron. This starts a chain reaction, at the end of which helium is created. A helium atom is made up of two protons and two neutrons, so it has less mass than the original four protons. Thanks to Einstein we know that mass is equal to energy and thermodynamics state that energy or mass cannot be destroyed or created. This means that the fusion process releases energy. From there on even heavier elements are formed and as the complexity grows, so does the release of energy. This is where the light and heat of the Sun come from and it has been estimated that through its lifetime the Sun has burned up around the mass of one Saturn, one positron at a time.
Nevertheless, it is so big, that it actually comprises 99.8% of all of the mass in the Solar System and is roughly one million times larger than our Earth. Almost all of it is either hydrogen or helium and this gassy structure has always led scientists to believe, that it is brighter at the equator than its diameter at the poles. Similarly, because of the centrifugal force of its own rotation Jupiter is 7% wider across its equator. Incredibly, with the Sun this is not the case. It is an astonishing 1.4 million km across, but the biggest difference between its diameters are only 10 km and any variations due to the solar cycle are even smaller. In proportion this is so miniscule, that it makes the Sun the smoothest object ever observed in nature. There is only a single sphere in known existence, specially created out of pure silicon, that is smoother. Due to its smoothness, scientist can very accurately determine the number of atoms within it and thus provide a constant for the measurement of a single kilogram.