Let’s start with the question of what the solar atmosphere is made of. We can use the spectrum of a star’s absorption line to determine which elements are present. It turns out that the sun contains the same elements as the earth, but not in the same proportions. About 73% of the mass of the sun, the mass consists of hydrogen and another 25% of helium. All of the other chemical elements (including those we know and love in our own bodies like carbon, oxygen, and nitrogen) make up only 2% of our star.
The 10 most abundant gases in the sun’s visible surface layer. The composition of the outer sun layer is very different from the earth’s crust in which we live. (In the crust of our planet, the three most abundant elements are oxygen, silicon & aluminum). Although not like our planet, the composition of the sun is very typical of stars in general.
| Element | Percentage By Number Of Atoms | Percentage By Mass |
| Hydrogen | 92.0 | 73.4 |
| Helium | 7.8 | 25.0 |
| Carbon | 0.02 | 0.20 |
| Nitrogen | 0.008 | 0.09 |
| Oxygen | 0.06 | 0.80 |
| Neon | 0.01 | 0.16 |
| Magnesium | 0.003 | 0.06 |
| Silicon | 0.004 | 0.09 |
| Sulfur | 0.002 | 0.05 |
| Iron | 0.003 | 0.14 |
The fact that our sun and stars have a similar composition, consisting mainly of hydrogen and helium, was first demonstrated in 1925 in a brilliant thesis by Cecilia Payne Gaposhkin, the first woman to do a PhD in astronomy in the US.
However, the idea that the simplest light gases (hydrogen and helium) were the most abundant elements in stars was so unexpected and shocking that he assumed her analysis of the data must be wrong. At the time she wrote: “The enormous abundance that results from these elements of the stellar atmosphere is almost certainly not real.” Even scientists sometimes find it difficult to accept new ideas that are inconsistent with what everyone “knows” to be right. Before Payne Gaposhkin’s work, everyone assumed that the composition of the sun & stars would be very similar to that of the earth. Three years after her thesis, further studies showed beyond any doubt that the enormous abundance of hydrogen and helium in the sun is real. (And as we shall see, the composition of the sun and stars is much more typical of the composition of the universe than the strange concentration of heavy elements that characterize our planet.)
Most of the elements of the sun are in the form of atoms, with a small number of molecules, all in the form of gases: the sun is so hot that no matter can survive in liquid or solid form. In fact, the sun is so hot that many of the atoms it contains are ionized, that is stripp-ed of one or more of its electrons.
This removal of electrons from their atoms means that there are many free electrons & positively charged ions in the sun, making it an electrically charged environment that is very different from the neutral environment in which you are reading this text (call scientists this ionized gas is a plasma.)
In the 19th century, scientists observed a spectral line at 530.3 nanometers in the Sun’s outer atmosphere called the corona. This line had never been seen before, so it was believed that this line was the result of a new element found in the corona, quickly called coronium. It wasn’t until 60 years later that astronomers discovered that this emission was due to highly ionized iron, which stripped 13 of its electrons. This is how we discovered for the first time that the sun’s atmosphere had a temperature of more than a million degrees.
