Cosmic abundances of the elements

All chemical elements are present in about the same proportions in the Sun, in the nearby stars, and in the interstellar medium. Standing in contrast, planets have been deeply transformed by gravity and igneous differentiation. For this reason, their surfaces do not represent a fair sample of the cosmic abundances of the elements. However, there are still minor bodies in the Solar System (in particular in the asteroid belt) that have not been differentiated. Some of their fragments often reach the Earth; these primitive meteorites are called chondrites. Chondrites display the same elemental abundance ratios present in the Sun, at least for 60 elements of low volatility (loosely called “metals” by astronomers). The most volatile gases, such as hydrogen, helium, oxygen, and nitrogen, depleted in chondrites, were assumedly present in the same proportions as in the Sun, in the primitive mixture that formed the Sun and the planetary system 4.6 billion years ago.

The “cosmic abundances” are not essentially different elsewhere. About 98% of the total mass of the Universe seems to be 3/4 hydrogen and 1/4 helium. The next three most abundant elements are carbon, nitrogen, and oxygen; taken together, they make about one single percent of the mass. The other 87 elements, often called “metals” by astronomers, to the great dismay of chemists, represent a total of less than one percent of the mass.

The previous statement is an average, and there are small fluctuations. For instance, Snow and Witt (1995) review the evidence that carbon is more abundant by a factor of two in the Sun than in the nearby stars.