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Abundance of elements in the Universe

The elements - namely ordinary (baryonic) matter made out of protons and neutrons (as well as electrons) - are only a small part of the content of the Universe. Cosmological observations suggest that about 73% of the universe consists of dark energy, 23% is composed of dark matter and only 4% corresponds to the visible baryonic matter which constitutes stars, planets and living beings. Dark matter has not yet been detected in a particle physics detector, and the nature of the dark energy is not yet understood.

Most standard (baryonic) matter is found in the form of atoms, although there are many other unusual kinds of matter, mostly plasma. Other forms of baryonic matter include white dwarves, neutron stars and black holes. Standard matter also exists as photons (mostly in the cosmic microwave background) and neutrons.

Hydrogen is the most abundant element in the known Universe; helium is second. However, after this, the rank of abundance does not continue to correspond to the atomic number; oxygen has abundance rank 3, but atomic number 8. All others are orders of magnitude less common.

The abundance of the lightest elements is well predicted by the standard cosmological model, since they were mostly produced shortly after the Big Bang, in a process known as Big Bang nucleosynthesis. Heavier elements were mostly produced much later, inside stars.

Helium-3 is rare on Earth and sought-after for use in nuclear fusion research. More abundant helium-3 is thought to exist on the Moon. Additional helium is produced by the fusion of hydrogen inside stellar cores by a variety of processes including the proton-proton chain and the CNO cycle.

Hydrogen and helium are estimated to make up roughly 74% and 24% of all baryonic matter in the universe respectively. Despite comprising only a very small fraction of the universe, the remaining "heavy elements" can greatly influence astronomical phenomena. Only about 2% (by mass) of the Milky Way galaxy's disk is composed of heavy elements.

These other elements are generated by stellar processes[1][2][3]. In astronomy, a "metal" is any element other than hydrogen or helium. This distinction is significant because hydrogen and helium (together with trace amounts of lithium) are the only elements that occur naturally without the nuclear fusion activity of stars. Thus, the metallicity of a galaxy or other object is an indication of past stellar activity.

These are the ten most common elements in the Universe as measured in parts per million, by mass[citation needed]:

Element Parts per million

Hydrogen 739,000

Helium 240,000

Oxygen 10,700

Carbon 4,600

Neon 1,340

Iron 1,090

Nitrogen 950

Silicon 650

Magnesium 580

Sulfur 440

All Others 650 Aluminium




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