Star Stuff

The elements that make up the stars also exist here on Earth. In fact, our Earth, and indeed all the planets, were created from the dust and gas produced by previous generations of stars that existed before our Sun and solar system formed. We truly are made of stardust!

Stars are made up almost entirely of hydrogen and helium. Here is a table of the most abundant elements in our Sun.

Element% by atoms
Most abundant elements in the Sun

It is not a trivial matter to determine the abundance of elements in the Sun. For most elements, astronomers have to look at the strength of spectral absorption lines in the photosphere. Some elements, like fluorine, chlorine, and thallium, require looking for their spectral lines inside of sunspots, which are cooler-than-average regions of the photosphere. Other elements require that we look at spectral lines in the solar corona, or capture and analyze the solar wind. And some elements we are simply unable to detect.

The region of the photosphere that is amenable to spectral study represents only about 2% of the mass of the Sun. Since the Sun’s formation 4.6 Gyr ago, some gravitational settling of heavier elements and diffusion of hydrogen towards the surface means the Sun is not uniform in composition. Fortunately, the relative abundances of the elements heavier than helium are probably similar throughout the Sun.

Lithium, the third element in the periodic table after hydrogen and helium, is the odd element out. It has a relative abundance in the solar photosphere that is only 1/170th that found in meteorites. The Sun’s original supply of lithium has largely been destroyed by the high temperatures inside the pre-main-sequence Sun, and today at the hot bottom of the Sun’s convection zone.

Light pollution is a problem here on Earth, but on the Sun we have a problem with “line pollution”. There are so many spectral lines that the weak signatures from some elements become difficult or impossible to isolate and measure. There is much blending of overlapping lines, and some elements—most notably iron which is the ninth most abundant element in the Sun—are “superpolluters” with hundreds to thousands of spectral lines from both excited and ionized states.

Sometimes, the spectral lines of interest are in a region of the electromagnetic spectrum (ultraviolet, for example) that can only be observed from space, and that creates additional challenges.

Notably, the noble gases helium, neon, argon, krypton, and xenon have no photospheric absorption lines that can be observed, and we must look to coronal sources such as the solar wind, solar flares, or solar energetic particles for information about their abundances.

Helium—the second most abundant element in the Sun—requires an indirect approach combining a theoretical solar model and observational helioseismology data to tease out its abundance.

The following elements are undetectable in the Sun: arsenic, selenium, bromine, technetium, tellurium, iodine, cesium, promethium, tantalum, rhenium, mercury, bismuth, polonium, astatine, radon, francium, radium, actinium, protactinium, and all the synthetic elements above uranium on the period table.

Interestingly, helium was discovered in the Sun before it was discovered on Earth! That’s why this element is name after Helios, the Greek god of the Sun.

The energy source that allows stars to shine steadily, often for billions of years, is fusion. Fusion in a star can only occur where both the temperature and pressure are very high. Usually (but not always!), this occurs in the core of the star. When the element hydrogen fuses into helium, a huge amount of energy is released in the process. Lucky for us, fusing hydrogen into helium is difficult to do in a one-solar-mass star. On average, any particular hydrogen atom in our Sun has to “wait” about five billion years before having the “opportunity” to participate in a fusion reaction!

In order for sustained fusion to occur in the core of a star, the star must have sufficient mass so that the core temperature and pressure is high enough. Present thinking is that the lowest mass stars where sustained fusion can occur have about 75 times the mass of Jupiter, or about 7% the mass of the Sun.


Lodders, K. 2020 Solar Elemental Abundances, in The Oxford Research Encyclopedia of Planetary Science, Oxford University Press
arXiv:1912.00844 [astro-ph.SR]

Lyman-Alpha Forest

The Lyman-alpha transition occurs when an electron in a hydrogen atom transitions from the first excited state (n=2) to the stable ground state (n=1), emitting an ultraviolet photon at 1215.67 Å. This and the other Lyman transitions to the ground state are named after American physicist and spectroscopist Theodore Lyman (1874-1954) who discovered and studied these spectral lines.

About 75% of the mass of our universe is hydrogen, so when we look at a very distant object, such as a quasar, the light from that distant object passes through a large number of tenuous hydrogen clouds between us and the distant object. The cooler hydrogen clouds absorb ultraviolet light at a wavelength of 1215.67 Å, so this wavelength is “removed” from the light from a distant object, as evinced by an absorption line in the spectrum of the distant object. But because the intervening neutral hydrogen clouds are moving at different speeds and cosmological redshifts, a number of different wavelengths have light removed (as seen from Earth), resulting in what is known as a Lyman-alpha forest. Analysis of the Lyman-alpha forest can tell us much about the neutral hydrogen clouds between us and any distant extragalactic source.

When a hydrogen cloud atom absorbs a 1215.67 Å ultraviolet photon, its electron jumps from the n=1 ground state up to the n=2 first excited state. However, excited electrons can’t stay in the n=2 state for long, and quickly return to the ground state again, emitting a photon of light at 1215.67 Å. So, why do we even see an absorption line? Yes, ultraviolet photons from the distant extragalactic source are removed from our line of sight by an intervening hydrogen cloud, but when ultraviolet photons are re-emitted, the photons radiate in all directions, and only a few travel towards us along our line of sight. The net result is an absorption line.

Further reading:
Lyman-alpha forest
Gunn-Peterson trough