what effects determine the size of a star
A Star's Size. A star begins its life in a contracting nebula of gas and dust. As the density of the material increases under the influence of gravity, compression heats the material. If the material gets hot enough, hydrogen atoms begin fusing together to form helium atoms, and a star is born. A star will spend most of its life, called the main sequence phase, fusing hydrogen into helium. Click the image to see the animation. It's logical that a large nebula (one with a large amount of mass) will form a large star, and a small nebula will form a small star. However, in addition to a star's initial mass, a star's size also depends on the balance between the inward pull of gravity and the outward push of the energy produced by fusion. The entire sequence of a star's evolution can be described in terms of the changing balance between these two opposing forces.