Element Abundance in the Universe

What Is the Most Abundant Element in the Universe?

When a supernova like this one (Cassiopeia A) explodes, it returns hydrogen and helium to the universe, plus heavier elements, such as carbon, oxygen, and silicon.
When a supernova like this one (Cassiopeia A) explodes, it returns hydrogen and helium to the universe, plus heavier elements, such as carbon, oxygen, and silicon. NASA/JPL-Caltech/STScI/CXC/SAO

The element composition of the universe is calculated by analyzing the light that is emitted and absorbed from stars, interstellar clouds, quasars, and other objects. The Hubble telescope greatly expanded our understanding of the composition of galaxies and gas in the intergalactic space between them. About 75% of the universe is believed to consist of dark energy and dark matter, which is different from the atoms and molecules that make up the everyday world around us.

Thus, the composition of most of the universe is far from understood. However, spectral measurement of stars, dust clouds, and galaxies tell us the elemental composition of the portion that consists of normal matter.

Most Abundant Elements in the Milky Way Galaxy

This is a table of elements in the Milky Way, which is similar in composition to other galaxies in the universe. Keep in mind, elements represent matter as we understand it. Much more of the galaxy consists of something else!

 
ElementElement NumberMass Fraction (ppm)
hydrogen1739,000
helium2240,000
oxygen810,400
carbon64,600
neon101,340
iron261,090
nitrogen7960
silicon14650
magnesium12580
sulfur16440

 

Most Abundant Element in the Universe

Right now, the most abundant element in the universe is hydrogen. In stars, hydrogen fuses into helium. Eventually, massive stars (around 8 times more massive than our Sun) run through their supply of hydrogen.

Then, the core of helium contracts, supplying enough pressure to fuse two helium nuclei into carbon. Carbon fuses into oxygen, which fuses into silicon and sulfur. Silicon fuses into iron. The star runs out of fuel and goes supernova, releasing these elements back into space.

So, if helium fuses into carbon you may be wondering why oxygen is the third most abundant element and not carbon.

The answer is because the stars in the universe today are not first generation stars! When newer stars form, they already contain more than just hydrogen. This time around, stars fuse hydrogen according to what's known as the C-N-O cycle (where C is carbon, N is nitrogen, and O is oxygen). A carbon and helium can fuse together to form oxygen. This happens not just in massive stars, but also in stars like the Sun once it enters its red giant phase. Carbon really comes out behind when a type II supernova occurs, because these stars undergo carbon fusion into oxygen with almost perfect completion!

How Element Abundance Will Change in the Universe

We won't be around to see it, but when the universe is thousands or millions times older than it is now, helium may overtake hydrogen as the most abundant element (or not, if enough hydrogen remains out in space to far from other atoms to fuse). After a much longer time, it's possible oxygen and carbon may become the first and second most abundant elements!