The Sun and thousands of twins migrated across the Milky Way just in time

The Sun and thousands of twins migrated across the Milky Way just in time

By Jacek Krywko edited by Sarah Lewin Frasier

Our Sun was born 4.6 billion years ago near the crowded center of the Milky Way and then migrated about 10,000 light-years outward to the peaceful galactic suburbs it currently occupies. Now a couple of studies have been published today in Astronomy and astrophysics claims that the sun did not make this journey alone.

The telltale sign of the Sun’s galactic journey is its chemical composition, says Tokyo Metropolitan University astronomer Daisuke Taniguchi, a co-author on both studies. “Astronomers know that the Sun’s birthplace is closer to the galactic core than its current position,” explains Taniguchi. The dense inner regions of the Milky Way formed stars faster and accumulated heavy metals far faster than the outer edges – and a star with the Sun’s age and chemical components would not have been able to form in its current location. But getting there required crossing a dramatic border.

Observations of the Milky Way have revealed a vast rotating rod-like structure made of gas, dust and millions of stars that cuts through our galactic center. This bar creates a distinct gravitational phenomenon known as the corotation barrier that prevents inner galactic stars from migrating to the outskirts. Computer simulations suggest that only about 1 percent of the stars born in the Sun’s presumed original location could successfully break this barrier to reach our current neighborhood within a time frame of 4.6 billion years. And yet Taniguchi and his colleagues discovered that thousands of “solar twin” stars with a mass and a metallic makeup similar to that of the Sun were able to do so.

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To catalog these stellar migrants, the researchers turned to the European Space Agency’s Gaia satellite, an observatory that tracks the positions, motions and wavelengths of light from more than two billion stars. The researchers unearthed 6,594 solar twins within about 1,000 light years of Earth.

When the researchers looked at the age distribution in their catalog, they saw two distinct peaks: a narrow peak of stars around two billion years old that probably formed locally and another broad, massive grouping of stars between six billion and four billion years old that included our Sun—”a large population of stars that migrated from their birthplace to their current positions,” suggests Taniguchi.

Alice C. Quillen, a physicist and astronomer at the University of Rochester, who was not involved in Taniguchi’s study, cautions that there is a chance that the broad peak of solar twins could be an artifact generated by the way Taniguchi’s team picked this sample—a purely statistical illusion. “The range is distance-limited, and most of it will be stars entering the solar region,” says Quillen. This factor may favor stars with more elongated orbits, which tend to be older, because younger stars with circular orbits would not have come close to us yet.

But Taniguchi says his team addressed this bias, finding no strong effect of age on the distribution of orbital shape in solar twins.

His team suggests that the corotation barrier did not stop a migration of the Sun and its cohort because the barrier was not fully formed when it occurred. In fact, Taniguchi suggests, the growing galactic bar could have pushed the migration forward rather than limiting it. The Sun and thousands of twins could have been propelled by the combined gravitational forces of the forming bar, the spiral arm structure of the Milky Way, and most likely close passages of the nearby dwarf galaxy Sagittarius.

Rosemary Wyse, an astrophysicist at Johns Hopkins University who was not involved in the study, says the researchers’ argument is compelling, but adds that (as the study authors note) the exact timescales remain uncertain. “The field of galaxy dynamics is itself dynamic,” she says.

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