helium
Within a nebula, there are varying regions where gravity has caused the gas and dust to clump together. The gravitational attraction of these clumps pull more atoms into the clump. As this accretion continues, the gas pressure increases and the core of the protostar gets hotter and hotter. If the protostar gets dense enough and hot enough, a fusion reaction will ignite and the star lights up. The minimum mass for the formation of a star is about 80 times the mass of Jupiter. A star is a very large, very hot ball of gas which has hydrogen fusing into helium in the core. Stars spend the majority of their life fusing hydrogen into helium. When the hydrogen is nearly used up, the star can fuse helium into heavier elements. Throughout this process, a battle goes on in the core of the star between gravity trying to collapse the star and temperature-produced gas pressure pushing the material in the star outward. During the life of a star, there is a balance between the gas pressure pushing out and gravity pushing in.