Stars can rightfully claim the role of the primary form of matter in the Universe. At the current stage of cosmic evolution, this is an indisputable fact: over 97% of all matter is concentrated within stars.
As the renowned astrophysicist I.S. Shklovsky noted: “The fundamental evolution of matter took place and continues to take place in the interiors of stars. It was there that the melting ‘pot’ existed, driving the chemical evolution of the Universe and enriching it with heavy elements.”
Chemical Composition: Hydrogen and Helium
We can study the processes occurring in stars through their diverse stellar spectra. However, these differences are mostly caused by surface temperatures rather than the actual chemical composition of their outer layers.
The chemical composition of stellar atmospheres is characterized by the dominance of Hydrogen. Helium holds the second place, being approximately 10 times less abundant than Hydrogen. All other elements combined usually account for less than 1% of the total number of atoms. In our Sun, Hydrogen and Helium make up about 97–98% of its mass.
The Helium Paradox and «Fossil» Stars
From time to time, reports surface about stars that appear to be completely devoid of Helium. This is often an observational illusion: Earth’s atmosphere is opaque to many wavelengths, and we can typically detect Helium only in hot stars where it is ionized.
Conversely, some objects are abnormally enriched with Helium — up to 30% by weight. One of the most mysterious stars is 3 Centauri A. It is uniquely enriched with Helium-3, a light isotope that is millions of times rarer on Earth and in the rest of the Universe than Helium-4. Interestingly, the total Helium content in this star is very low (only 1.3% of the Hydrogen amount), yet 84% of that Helium is the rare Helium-3.
The Mystery of the «Surplus» Helium
Astrophysicists suggest that stars condense from nebulae composed mainly of Hydrogen and Helium. Inside the stars, Helium is constantly created through thermonuclear fusion.
However, calculations based on our Sun show that stellar fusion alone cannot explain the observed amount of Helium in the Universe. There is an excess of Helium that the theory of stellar synthesis cannot account for. This realization forces us to look back at the «pre-stellar» era of the Universe.
A Journey into the Past: Expanding Universe
To understand the origin of Helium, we must look at the Universe’s history. In the early 20th century, Vesto Slipher and Edwin Hubble discovered that galaxies are moving away from us. This led to Hubble’s Law, proving that the Universe is expanding.
Earlier, Soviet scientist Alexander Friedmann theoretically proved the non-stationary nature of the Universe based on Einstein’s General Relativity. If the Universe is expanding, it must have had a beginning.
The Big Bang and the First 30 Minutes
In 1927, Georges Lemaître proposed that the Universe began as a «Primeval Atom» or a «Cosmic Egg.» Later, George Gamow hypothesized that this «egg» consisted of densely compressed neutrons.
According to Gamow’s model, the synthesis of chemical elements occurred through a rapid succession of neutron captures during the first 30 minutes of the Big Bang. High temperatures made thermonuclear fusion possible before the Universe cooled down. However, this process stopped at Helium-4, as the temperature dropped too quickly for heavier elements to form.
Chronology of the Early Universe:
-
0.0001 seconds: Matter exists as hadrons and radiation in equilibrium.
-
First 100 seconds: Protons, neutrons, and electrons form. Nuclei of Deuterium, Helium-3, and Helium-4 are synthesized.
-
1 million years: The Universe cools enough (2700–3700°C) for the first atoms to form.
-
Today: The average temperature of the Universe is approximately -270°C.
The Chain of Primary Synthesis:
The first stage of chemical evolution in the pre-stellar Universe looked like this:
- n + р → d + γ;
- d + d → t + р и d + d → ³He + n;
- ³He + n → t + p;
- d + t → ⁴He + n.
Witnesses of the Giant Explosion
How do we know this happened? We have three «witnesses»:
-
Cosmic Microwave Background (CMB): The «afterglow» of the Big Bang, discovered in the 1960s.
-
Neutrino Sea: A theoretical background of neutrinos that is currently almost impossible to detect.
-
Helium: The abundance of Helium (about 25-30% of the mass of the Universe) is a direct confirmation of the Big Bang theory.
As physicists say, paraphrasing Archimedes: «Give us Helium, and we will build the Universe.»