Many stars in our Milky Way now shine far away from their places of origin – including our sun. Now, analyzes of solar twins in our neighborhood show that there must have been a mass migration from the inner region of the Milky Way outwards among these four to six billion year old stars. At that time, our sun, along with many of its contemporaries, also drifted to its current position, as astronomers determined. The interesting thing about it: The central bar of the Milky Way should actually have blocked this stellar migration, because it forms a barrier between the inner region of the galaxy and further outer regions of the star disk. This sheds new light on when the bar of our Milky Way formed and was fully formed.
Our sun now orbits the center of the Milky Way at a distance of around 25,000 light-years – but that wasn’t always the case. Analyzes of its composition and motion suggest that the Sun was formed several thousand light-years closer to the galactic center. There it was created together with many “contemporaries” in an area characterized by intense star formation. Studies suggest that there have been several bouts of such star formation in our galaxy’s past, including two and around four to six billion years ago. “Data from the Gaia telescope support the idea that these bursts of star formation 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. The Gaia mission’s star catalog also shows that, in addition to the sun, many other stars in the Milky Way are no longer where they were once born. However, reconstructing their origins and movements is not easy because of the eventful history of our galaxy.
Solar twins as contemporary witnesses
To learn more about the migration of our sun and its peers, Tsujimoto and his team undertook the most comprehensive analysis to date of solar twins – stars that are very similar to our star in composition, size and temperature. “Such solar twins are among the best markers of galactic evolution because they have the same metallicity (Fe/H) but a wide age range,” explain the astronomers. It is commonly assumed that the ratio of heavy elements such as iron to light hydrogen in a star is shaped by its birth environment: the closer to the galactic center a star forms, the higher its initial metal content and the faster it reaches higher proportions of heavy elements. “This results in a stellar age-to-metallicity ratio that is characteristic of the galactic radius,” the team said. This relationship can also be used to determine at what distance from the center of the Milky Way such a solar twin was formed.
Tsujimoto and his colleagues have now used this method to reconstruct the past of 4,594 solar twins, which are now within a radius of around 950 light-years around the sun. The basis for this was data from the Gaia space telescope on the movement, position and composition of these stars. “We found two striking features,” they report. Accordingly, the age distribution of these solar twins shows two clear peaks, a narrow, clearly separated one at around two billion years and a broad one at four to six billion years. As the astronomers explain, the solar twins, which are around two billion years old, are probably due to a burst of star formation that was triggered by turbulence and gravitational influences in a nearby passage of the Sagittarius dwarf galaxy. They were also primarily created on site.
This is different with the peak in the age curve at four to six billion years – the star formation phase in which our sun was also formed. “The stars from this phase are almost exclusively those that have migrated outwards from the inner star disk,” report the astronomers. “The solar twins, ranging in age from 4.5 to 7 billion years, show elemental compositions that match that of the Sun almost perfectly. This suggests that they were formed in a very similar environment to the Sun.” According to previous studies, the Sun’s birthplace was between 16,300 and 19,500 light-years from the galactic center, much further inward than today. According to Tsujimoto and his team, the sun was part of a mass migration of stars that migrated further outwards from this inner region of the star disk.
What role did the central beam play?
But this scenario has a catch: Like many spiral galaxies, the Milky Way has a central bar – a straight collection of a particularly large number of stars and gases. It forms a transport route for gases and dust, influences star formation and also the movement of stars. The galactic bar acts like a barrier that inhibits the migration of stars from the inner region of the galaxy to the outside. “As a consequence, less than one percent of the stars formed at the same galactic radius as the Sun are likely to have reached the Sun’s current neighborhood over the course of their lifetime,” explain the astronomers. “Contrary to this expectation, our age distribution shows a clear hump instead of a dip in this age range.” Accordingly, the galactic bar does not appear to have blocked the mass migration of solar twins from the inner region of the Milky Way – quite the opposite.
According to Tsujimoto and his colleagues, the mass outward drift of the four- to six-billion-year-old solar twins suggests that not only did the bar not form a barrier, it may have actually facilitated the migration. However, the Milky Way bar would then have to have formed a little later than previously assumed – around six to seven billion years ago instead of at least eight billion years ago, as previously assumed. “The bar formation epoch may have triggered increased star formation in the inner disk and subsequent radial migration,” the astronomers write. As they explain, there are some studies that suggest such a later and longer-lasting bar formation in the Milky Way. The mass migration of the sun and its “siblings” could now support this. At the same time, this explains why our sun is now so far away from its birthplace.
Source: Takuji Tsujimoto (National Astronomical Observatory of Japan, Osaka) et al., Astronomy and Astrophysics, doi: 10.1051/0004-6361/202658914