<P> These other elements are generated by stellar processes . In astronomy, a "metal" is any element other than hydrogen or helium . This distinction is significant because hydrogen and helium are the only elements that were produced in significant quantities in the Big Bang . Thus, the metallicity of a galaxy or other object is an indication of stellar activity, after the Big Bang . </P> <P> In general, elements up to iron are made in large stars in the process of becoming supernovae . Iron - 56 is particularly common, since it is the most stable element that can easily be made from alpha particles (being a product of decay of radioactive nickel - 56, ultimately made from 14 helium nuclei). Elements heavier than iron are made in energy - absorbing processes in large stars, and their abundance in the universe (and on Earth) generally decreases with increasing atomic number . </P> <Table> Most abundant nuclides in the Solar System <Tr> <Th> Nuclide </Th> <Th> </Th> <Th> Mass fraction in parts per million </Th> <Th> Atom fraction in parts per million </Th> </Tr> <Tr> <Td> Hydrogen - 1 </Td> <Td> </Td> <Td> 705,700 </Td> <Td> 909,964 </Td> </Tr> <Tr> <Td> Helium - 4 </Td> <Td> </Td> <Td> 275,200 </Td> <Td> 88,714 </Td> </Tr> <Tr> <Td> Oxygen - 16 </Td> <Td> 16 </Td> <Td> 9,592 </Td> <Td> 477 </Td> </Tr> <Tr> <Td> Carbon - 12 </Td> <Td> 12 </Td> <Td> 3,032 </Td> <Td> 326 </Td> </Tr> <Tr> <Td> Nitrogen - 14 </Td> <Td> 14 </Td> <Td> 1,105 </Td> <Td> 102 </Td> </Tr> <Tr> <Td> Neon - 20 </Td> <Td> 20 </Td> <Td> 1,548 </Td> <Td> 100 </Td> </Tr> <Tr> <Td_colspan="4"> </Td> </Tr> <Tr> <Td_colspan="2"> Other nuclides: </Td> <Td> 3,879 </Td> <Td> 149 </Td> </Tr> <Tr> <Td> Silicon - 28 </Td> <Td> 28 </Td> <Td> 653 </Td> <Td> 30 </Td> </Tr> <Tr> <Td> Magnesium - 24 </Td> <Td> 24 </Td> <Td> 513 </Td> <Td> 28 </Td> </Tr> <Tr> <Td> Iron - 56 </Td> <Td> 56 </Td> <Td> 1,169 </Td> <Td> 27 </Td> </Tr> <Tr> <Td> Sulfur - 32 </Td> <Td> 32 </Td> <Td> 396 </Td> <Td> 16 </Td> </Tr> <Tr> <Td> Helium - 3 </Td> <Td> </Td> <Td> 35 </Td> <Td> 15 </Td> </Tr> <Tr> <Td> Hydrogen - 2 </Td> <Td> </Td> <Td> 23 </Td> <Td> 15 </Td> </Tr> <Tr> <Td> Neon - 22 </Td> <Td> 22 </Td> <Td> 208 </Td> <Td> 12 </Td> </Tr> <Tr> <Td> Magnesium - 26 </Td> <Td> 26 </Td> <Td> 79 </Td> <Td> </Td> </Tr> <Tr> <Td> Carbon - 13 </Td> <Td> 13 </Td> <Td> 37 </Td> <Td> </Td> </Tr> <Tr> <Td> Magnesium - 25 </Td> <Td> 25 </Td> <Td> 69 </Td> <Td> </Td> </Tr> <Tr> <Td> Aluminium - 27 </Td> <Td> 27 </Td> <Td> 58 </Td> <Td> </Td> </Tr> <Tr> <Td> Argon - 36 </Td> <Td> 36 </Td> <Td> 77 </Td> <Td> </Td> </Tr> <Tr> <Td> Calcium - 40 </Td> <Td> 40 </Td> <Td> 60 </Td> <Td> </Td> </Tr> <Tr> <Td> Sodium - 23 </Td> <Td> 23 </Td> <Td> 33 </Td> <Td> </Td> </Tr> <Tr> <Td> Iron - 54 </Td> <Td> 54 </Td> <Td> 72 </Td> <Td> </Td> </Tr> <Tr> <Td> Silicon - 29 </Td> <Td> 29 </Td> <Td> 34 </Td> <Td> </Td> </Tr> <Tr> <Td> Nickel - 58 </Td> <Td> 58 </Td> <Td> 49 </Td> <Td> </Td> </Tr> <Tr> <Td> Silicon - 30 </Td> <Td> 30 </Td> <Td> 23 </Td> <Td> </Td> </Tr> <Tr> <Td> Iron - 57 </Td> <Td> 57 </Td> <Td> 28 </Td> <Td> </Td> </Tr> </Table> <Tr> <Th> Nuclide </Th> <Th> </Th> <Th> Mass fraction in parts per million </Th> <Th> Atom fraction in parts per million </Th> </Tr>

What are the two most common elements in the solar system