The sun may have formed because a small galaxy passed by the Milky Way

By Leah Crane

NASA, ESA, and The Hubble Heritage Team STScI/AURA

A small galaxy called Sagittarius has been shaping the Milky Way for billions of years. Every time it has passed close to our galaxy, it has caused huge bursts of star formation that may even be responsible for the birth of the sun.

Tomás Ruiz-Lara at the Astrophysics Institute of the Canary Islands in Spain and his colleagues examined data from the Gaia space telescope to measure the ages of stars within about 6500 light years of the solar system.

They found three periods of increased star formation, occurring about 5.7, 1.9 and 1 billion years ago, as well as hints of another episode of star formation over the past 70 million years that seems to still be going on today. The timing of those episodes matches up with when simulations show that the Sagittarius dwarf galaxy, which orbits the Milky Way, passed at a distance of 26,000 light years away.

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“You have the Milky Way in equilibrium, mostly calm, and then when Sagittarius passed it was like throwing a stone in a lake,” says Ruiz-Lara. “It created these ripples in the galaxy’s density, so some areas became more dense and started forming stars more efficiently.”

Even though Sagittarius is relatively small – less than a tenth the size of the Milky Way – it seems to have had an outsized effect on the formation of stars here. Even our own solar system may owe its existence to the first close pass about 5.7 billion years ago.

“Maybe without Sagittarius the solar system wouldn’t exist,” says Ruiz-Lara. The timing works out, he says, but there is no way for us to know for sure.

These passes are likely to continue happening increasingly often as Sagittarius’s orbit around the Milky Way gets tighter. “It’s getting closer and closer, little by little over time, and in the end it will merge with the Milky Way,” says Ruiz-Lara.

Journal reference: Nature Astronomy, DOI: 10.1038/s41550-020-1097-0

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