All matter is made of particles; these can be single atoms or atoms chemically joined to make molecules. Using this fact, matter can be classified into three broad groups: elements, compounds and mixtures. In an element, all the atoms are of the same type. If more than one type of atom is chemically joined, then a compound has been formed. If more than one type of atom or molecule is contained in the same substance, and the particles aren't chemically joined, this is a mixture.
The Nebular Hypothesis is the most accepted explanation of the origin of our solar system. It states that the Sun was formed by a massive cloud of dust and gas light-years across, and it was bigger than the actual sun itself. Most of the mass in the center formed the sun, and the rest expanded outward.