This story comes from our special January 2021 issue, "The Beginning and the End of the Universe.” Click here to purchase the full issue.
Some 4.6 billion years ago, our Sun was born from a cloud of interstellar gas and dust.
It came from a giant molecular cloud — a collection of gas up to 600 light-years in diameter with the mass of 10 million Suns — which had been circling the Milky Way for who knows how many years. The pull of gravity caused some of this cloud to collapse, until it heated up enough to emit light.
That much astronomers know. But what caused this gas cloud to collapse in the first place remains the subject of vigorous debate.
Light in the darkness
Scientists have a firm grasp on the physics of how the Sun was born. Those atoms that formed the Sun in the giant molecular cloud — mostly hydrogen and helium — were moving slowly enough that they could collide and conglomerate into clumps of matter. They then linked up with other atoms, and eventually trillions of atoms joined in. After about 10 million years, the vast majority of these concentrated patches grouped together at the cloud’s center.
As the central mass grew, so too did the strength of gravity compacting it. This raised the pressure inside and heated it, causing it to emit infrared radiation. This clump of mainly hydrogen and helium was now a protostar — a phase that, for stars like the Sun, lasts about half a million years. The protostar continued to accrete mass as material from the cloud — which by this time had formed a disk around the central object — rained onto its surface.
As the emerging Sun packed on mass, the temperature and pressure of the protostar increased. Eventually, at a sweltering 9 million degrees Fahrenheit (5 million degrees Celsius), nuclear fusion kicked on in the protostar’s core. Once this happens, most stars quickly establish a balance between the inward pull of gravity and the outward push of radiation, and the star’s mass determines its final core temperature. For the Sun, that’s around 27 million F, or 15 million C. At this time, the Sun truly began to shine.