Our Sun, like many stars in the Milky Way, didn’t form where it currently resides. New analysis of stellar “twins” suggests a significant migration of stars occurred from the galaxy’s inner regions to the outer reaches over billions of years, and our Sun was among them. This discovery sheds light on the formation and evolution of the Milky Way’s central bar structure, a key component of galactic dynamics.
Currently orbiting around 25,000 light-years from the Milky Way’s center, the Sun’s composition and movement indicate it originated several thousand light-years closer to the galactic core. There, it formed alongside many stars of similar age in a region characterized by intense star formation. Researchers have identified multiple bursts of star formation throughout the galaxy’s history, including events approximately two billion and four to six billion years ago. “Data from the Gaia telescope supports the idea that these star formation bursts were triggered by episodic encounters between the Sagittarius dwarf galaxy and the Milky Way,” explain Takuji Tsujimoto of the National Astronomical Observatory of Japan and his colleagues.
Solar Twins as Historical Witnesses
To better understand the Sun’s journey and that of its contemporaries, Tsujimoto’s team undertook the most extensive analysis to date of solar twins – stars remarkably similar to our Sun in composition, size, and temperature. “These solar twins are among the best markers of galactic evolution because they have the same metallicity (Fe/H) but span a wide age range,” the astronomers state. The ratio of heavy elements like iron to lighter hydrogen in a star is generally determined by its birthplace: stars closer to the galactic center tend to have higher initial metal content and reach higher levels of heavier elements more quickly. “This results in a relationship between stellar age and metallicity that is characteristic of the galactic radius,” the team notes, allowing them to estimate a solar twin’s origin distance from the galactic center.
The researchers applied this method to reconstruct the past of 4,594 solar twins within approximately 950 light-years of the Sun, using data from the Gaia space telescope regarding their movement, position, and composition. “We found two striking features,” they report. The age distribution of these solar twins reveals two distinct peaks: a narrow one around two billion years, and a broader one between four and six billion years. The two-billion-year-old solar twins likely resulted from a star formation burst triggered by turbulence and gravitational influences from a close passage of the Sagittarius dwarf galaxy, and they largely formed in their current location.
The peak in the age curve at four to six billion years – the period when our Sun similarly formed – tells a different story. “Stars from this phase are almost exclusively those that have migrated outward from the inner stellar disk,” the astronomers report. “The solar twins in the age range of 4.5 to seven billion years show elemental compositions that nearly perfectly match that of the Sun, suggesting they formed in a very similar environment to the Sun.” Previous studies indicated the Sun’s birthplace was between 16,300 and 19,500 light-years from the galactic center, much closer than its current location. According to Tsujimoto and his team, the Sun was part of a mass migration of stars from this inner region of the stellar disk.
What Role Did the Central Bar Play?
This scenario presents a puzzle: like many spiral galaxies, the Milky Way has a central bar – a concentrated collection of stars and gas. This bar acts as a transport route for gas and dust, influences star formation, and affects stellar movement. It functions as a barrier, hindering the migration of stars from the galaxy’s inner regions outward. “As a consequence, fewer than one percent of stars forming at the same galactic radius as the Sun would be expected to reach the Sun’s current neighborhood over their lifetimes,” the astronomers explain. “Contrary to this expectation, our age distribution shows a clear hump in this age range instead of a dip.”
Tsujimoto and his colleagues believe the substantial outward drift of the four-to-six-billion-year-old solar twins suggests the galactic bar didn’t block this mass migration – and may have even facilitated it. This implies the Milky Way bar formed later than previously thought – around six to seven billion years ago, rather than the previously assumed minimum of eight billion years. “The epoch of bar formation could have triggered enhanced star formation in the inner disk and a subsequent radial migration,” the astronomers write. Several studies already suggest a later and more prolonged formation of the bar in the Milky Way, and this mass migration of the Sun and its “siblings” could support that theory. This also explains why our Sun is now so distant from its place of origin.
Source: Takuji Tsujimoto (National Astronomical Observatory of Japan, Osaka) et al., Astronomy and Astrophysics, doi: 10.1051/0004-6361/202658914
