10-13 meters

More at 100 femtometers

Main sequence stars get their energy from the fusion of hydrogen into helium. The pressure and temperature at the center of stars like our Sun is enough to overcome the electromagnetic force keeping the bare hydrogen nuclei apart and helium nuclei are formed, with the release of lots of energy: One helium atom weighs less than four hydrogen atoms and the difference in mass is converted to energy. The release of this energy is enough to balance a star's gravitational contraction, and a star the size of our Sun can maintain this stable balance for 10 thousand million years or so. Stars more massive than our Sun can continue the fusion process beyond helium to carbon, nitrogen, oxygen, and beyond. The largest stars can create all the elements, particularly in the massive star's final catastrophic explosion as a supernova. The whole process of stellar evolution is quite well understood and is strongly dependent upon the mass of the star. It is the strong and weak nuclear interactions at the level of 1 to 10 femtometers (10-15 to 10-14 meters) that are responsible for the formation of stars 1 million kilometers (109 meters) in diameter and larger.

The primary source of energy in stars like our sun is called the Proton-Proton Chain. In a three step process, hydrogen is converted to helium:

Step 1
Step 2
Step 3

In order to produce the carbon nucleus, a star must be at least 4/10 as massive as the sun. (A smaller star will not be able to reach the internal temperatures and pressures required to fuse helium to higher elements.) The Triple-Alpha Process requires temperatures above 100 million K and very high densities of pure helium. These criteria are met only after most of the hydrogen core of a star has been converted to helium by the Proton-Proton Chain.

Triple-Alpha Process

From carbon12, stars as massive as the sun can produce oxygen16 and some neon20 by adding successive helium nuclei. For the production of heavier atoms, stars more massive than the sun are required.

Copyright © 2016 by Bruce Bryson