The first stars likely ignited around 100 to 200 million years after the Big Bang, emerging from clouds of hydrogen and helium gas that had no heavier elements whatsoever. That places their birth roughly 13.6 billion years ago. Star formation then ramped up dramatically, peaked about 3.5 billion years after the Big Bang, and has been declining ever since.
The Dark Ages Before the First Stars
For the first 380,000 years after the Big Bang, the universe was too hot for even atoms to exist. Once it cooled enough for hydrogen and helium atoms to form, a long quiet period began. Astronomers call this stretch the Cosmic Dark Ages: roughly half a billion years when no stars, no galaxies, and no visible light sources existed anywhere. The universe was filled with a uniform fog of neutral hydrogen gas, slowly expanding and cooling in total darkness.
During this time, gravity was doing invisible work. Slight variations in the density of matter, tiny clumps that were fractionally denser than their surroundings, began pulling in more gas. Over hundreds of millions of years, these regions grew into gas clouds massive enough to collapse under their own weight. But collapsing isn’t the same as forming a star. The gas needed to lose heat and compress further, which required a cooling mechanism. In today’s universe, heavier elements like carbon and oxygen radiate heat efficiently and help gas clouds cool. The primordial universe had none of these elements. Instead, hydrogen molecules served as the only available coolant, allowing gas to shed enough heat to keep collapsing toward the extreme densities needed to trigger nuclear fusion.
The First Stars Were Nothing Like Our Sun
These first-generation stars, known as Population III stars, formed from gas that was entirely free of metals (in astronomy, “metal” means anything heavier than helium). Because hydrogen molecules are far less efficient coolants than heavier elements, the gas clouds that birthed these stars stayed hotter and needed to be much more massive before they could collapse. Theoretical models suggest many of these stars were enormous, potentially reaching 100 to 1,000 times the mass of our Sun. For comparison, most stars that explode in the present-day universe top out around 17 times the Sun’s mass.
That extreme size made them incredibly short-lived. A star’s fuel burns faster the more massive it is. While our Sun will shine for about 10 billion years, the largest first-generation stars would have exhausted their hydrogen fuel in just a few million years before exploding as supernovae. Those explosions were critically important: they scattered the first heavy elements (carbon, oxygen, silicon, iron) into the surrounding gas, seeding the raw material for the next generation of stars and, eventually, rocky planets like Earth.
No Population III star has ever been directly observed. Only stars below about 0.8 solar masses burn slowly enough to still be shining 13 billion years later, and it remains unclear whether any first-generation stars formed that small. The oldest known star in our galaxy, a dim star cataloged as HD 140283, has a measured age of 14.5 billion years, plus or minus 800 million years. That uncertainty makes it compatible with the universe’s age of 13.8 billion years, placing its birth in the earliest era of star formation.
How the First Stars Lit Up the Universe
The first stars didn’t just produce light. Their intense ultraviolet radiation began stripping electrons from the neutral hydrogen gas that filled the universe, a process called reionization. This era began roughly 300 to 500 million years after the Big Bang and lasted several hundred million years, ending about 1 billion years after the Big Bang. Early galaxies were the primary drivers of this process, with some contribution from the first supermassive black holes powering quasars. By the time reionization was complete, the fog of neutral hydrogen had cleared and the universe became transparent to ultraviolet light for the first time.
NASA’s James Webb Space Telescope has been pushing the boundary of how far back we can see into this era. One of the most distant galaxies confirmed so far, JADES-GS-z13-1, existed just 330 million years after the Big Bang. It was already producing stars and emitting bright hydrogen radiation, meaning star formation was well underway by that point. This galaxy’s light has traveled for over 13.4 billion years to reach us.
When Star Formation Peaked
Star formation didn’t happen at a steady rate. After the first stars appeared, the pace accelerated as gas accumulated into larger and larger galaxies. The rate of new star formation across the entire universe peaked about 3.5 billion years after the Big Bang, a period astronomers call Cosmic Noon. At that point, galaxies were churning out new stars far more rapidly than they do today.
After Cosmic Noon, the rate dropped off steadily. About 25% of all stars that have ever existed formed before this peak, and another 25% formed after the universe was already 7 billion years old. The decline follows a pattern similar to exponential decay, with the rate roughly halving every 4 billion years. Today, the universe still forms new stars, but at a fraction of its peak output. Most of the gas available for star formation has already been used up or heated to temperatures that prevent it from collapsing.
A Timeline of Star Formation
- 380,000 years after the Big Bang: The first atoms form. The Cosmic Dark Ages begin.
- ~100 to 200 million years: The first stars ignite from pure hydrogen and helium gas in small clumps of matter.
- ~300 to 500 million years: Early galaxies and stars begin reionizing the neutral hydrogen fog that fills the universe.
- ~330 million years: The earliest galaxy confirmed by JWST (JADES-GS-z13-1) is already forming stars.
- ~1 billion years: Reionization is largely complete. The universe is transparent.
- ~3.5 billion years (about 10 billion years ago): Star formation across the universe hits its all-time peak.
- ~9 billion years to present: Star formation declines steadily. Our own Sun formed about 4.6 billion years ago, well after the peak.
Our Sun, in other words, is a relatively late arrival. It formed from gas that had already been enriched by billions of years of earlier stellar generations, each one adding heavier elements to the cosmic supply. The iron in your blood, the calcium in your bones, and the oxygen you breathe were all forged inside stars that lived and died before the Sun existed.