Hydrogen and helium are the two lightest and most abundant elements in the universe, forming nearly all of the visible matter after the Big Bang. Together, they account for over 98% of the mass of atoms in the cosmos, with hydrogen making up about 75% and helium about 24%.
Hydrogen is the simplest element: one proton and one electron. It exists primarily as H₂ molecules in interstellar space and as atomic hydrogen in nebulae. Helium has two protons, two neutrons, and two electrons. It forms stable He₂ molecules only under extreme conditions, usually appearing as single atoms in gas clouds and stellar interiors.
These elements were forged in the first few minutes after the Big Bang during a process called Big Bang nucleosynthesis. As the universe cooled below 1 billion degrees Kelvin, protons and neutrons combined to form deuterium, then helium-4. This event produced the universe's initial helium abundance, which remains largely unchanged today.
Within the first 20 minutes after the Big Bang, temperatures dropped enough for nuclear fusion to occur without immediate disruption. Protons and neutrons fused into deuterium nuclei (one proton, one neutron), which quickly captured additional neutrons or protons to form helium-4 (two protons, two neutrons).
According to NASA's WMAP mission data, the primordial helium-4 abundance was approximately 24.5% by mass. This value aligns closely with predictions from the standard model of cosmology. Any deviation would suggest unknown physics in the early universe.
After the initial synthesis, no significant amounts of heavier elements formed until the first stars ignited. Stars act as factories for elements beyond helium through sustained fusion in their cores. For example, our Sun converts about 600 million tons of hydrogen into helium every second via the proton-proton chain reaction.
Hydrogen dominates in molecular clouds like the Orion Nebula, where dense regions collapse to form new stars. These clouds are typically 90% hydrogen by mass, with helium as the second most common component. Interstellar space contains roughly 75% hydrogen and 24% helium by mass, with trace amounts of lithium and beryllium.
Helium is also found in Earth's atmosphere at 5.2 parts per million by volume, but this is a tiny fraction of cosmic abundance. Most atmospheric helium comes from radioactive decay of uranium and thorium in Earth's crust, not from stellar processes.
On Jupiter, hydrogen makes up 90% of the planet's mass. Under immense pressure, it transitions into metallic hydrogen-a state that conducts electricity and may generate the planet's useful magnetic field. This phase is not found naturally on Earth.
| Element | Abundance in Universe (by mass) | Primary Formation Process | Key Location in Space |
|---|---|---|---|
| Hydrogen | ~75% | Big Bang nucleosynthesis | Molecular clouds, stellar cores |
| Helium | ~24% | Big Bang nucleosynthesis | Stellar interiors, nebulae |
Many assume helium is rare because it escapes Earth's gravity easily. While helium is scarce in our atmosphere, it is abundant in the universe. The misconception arises from confusing terrestrial availability with cosmic abundance.
Another error is thinking that all helium comes from stars. In reality, ~25% of helium in the universe originated in the Big Bang. The rest formed later in stars via hydrogen fusion. The amount of helium produced in stars depends on mass and age-low-mass stars like the Sun produce less helium than massive ones.
A lesser-known fact: helium-3, a rare isotope, is more abundant in lunar regolith than on Earth. Solar wind implanted it over billions of years. This isotope is valuable for potential fusion energy research, though it is not involved in Big Bang nucleosynthesis.
The Sun generates energy by fusing hydrogen into helium. In its core, temperatures reach 15 million Kelvin, enabling protons to overcome electrostatic repulsion and fuse. Over 10^38 protons are converted into helium each second.
Each fusion cycle converts about 0.7% of the initial mass into energy, following Einstein's equation E = mc². This accounts for the Sun's luminosity of 3.8 × 10^26 watts. Since the Sun began fusing hydrogen 4.6 billion years ago, it has converted roughly 100 times Earth's mass into helium.
Over time, the Sun's core becomes denser and hotter. Eventually, hydrogen fusion will cease in the core, leading to expansion into a red giant. At that stage, helium fusion begins, producing carbon and oxygen.
I once viewed the Orion Nebula through a 12-inch telescope during a dark-sky observing session. The emission lines from ionized hydrogen (H-alpha at 656.3 nm) glowed vivid red. This light came from ultraviolet radiation from nearby hot stars stripping electrons from hydrogen atoms, which then recombine and emit photons at specific wavelengths.
That night, I realized that the red glow I saw was not just visual-it was direct evidence of hydrogen's role in star formation. The same process occurs in distant galaxies, where H-alpha emissions map regions of active star birth.
Understanding what is hydrogen and helium reveals how fundamental these elements are to the structure and evolution of the universe. Their presence defines the life cycles of stars, shapes galactic chemistry, and sets the stage for all heavier elements.