The birth of stars and planets
Like the giant galaxies in which they appear, stars and their planets form when clumps of gas and dust contract to much smaller sizes. The contraction is due to forces of gravity that pulls together the large interstellar gas clouds. The clouds are composed of about three quarter hydrogen and one quarter of helium. About 2 per cent of the cloud is made up of heavier elements. The same composition translates to stars, i.e. three quarter hydrogen, one quarter helium and about 2 percent heavy elements. The formation of stars is a fundamental cosmic process which makes galaxies visible and controls the evolution of common matter in the universe.
Scowen, P. et al (2010). From protostars to planetary systems: FUV spectroscopy of YSOs, protoplanetary disks, and extra solar giant planets. Retrieved on Dec 10, 2011 from http://www.astro.princeton.edu/~dns/Theia/Scowen_Planetdisks_PSF.pdf