From the beginning to the end of the universe: the origins of our solar system
This story is from our January 2021 special issue, “The Beginning and the End of the Universe”. Click here to purchase the full issue.
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 across with a mass of 10 million suns – that had been circling the Milky Way for who knows how many years. The pull of gravity caused part of this cloud to collapse, until it warmed up enough to emit light.
This, many astronomers know. But what caused this gas cloud to collapse in the first place is the subject of vigorous debate.
Light in the Darkness
Scientists have a solid understanding of the physics of the birth of the Sun. The atoms that formed the Sun in the giant molecular cloud – mostly hydrogen and helium – were moving slow enough to collide and coalesce into clumps of matter. They then bonded with other atoms, and eventually billions of atoms joined them. After about 10 million years, the vast majority of these concentrated spots have clustered together in the center of the cloud.
As the central mass grew, the force of gravity compacting it also increased. This increased the pressure inside and heated it, causing it to emit infrared radiation. This cluster of mostly hydrogen and helium was now a protostar – a phase that, for stars like the Sun, lasts around half a million years. The protostar continued to accumulate mass as material from the cloud – which by then had formed a disk around the central object – rained down on its surface.
As the Sun emerged, the protostar’s temperature and pressure increased. Eventually, at a sweltering 9 million degrees Fahrenheit (5 million degrees Celsius), nuclear fusion kicked off in the protostar’s core. Once this happens, most stars quickly balance the inward pull of gravity and the outward push of radiation, and the star’s mass determines its final core temperature. For the Sun, it’s about 27 million F, or 15 million C. At that point, the Sun really started to shine.