From the April 2019 issue

How do astronomers determine the Sun’s sibling stars and where the Sun originated?

Douglas Kaupa Council Bluffs, Iowa
By | Published: April 2, 2019
Young stars form in clusters but often disperse from their original birthplace over time. These bright stars are part of open cluster NGC 2547, which lies in the southern constellation Vela. Astronomers estimate it is between 20 million and 35 million years old.
Stars form from collapsing gas clouds that fragment and condense into thousands of pieces, each creating a star and a planetary system. The result is an open star cluster, whose constituents will ultimately disperse into the Milky Way Galaxy after a few hundred million years. If their original gas cloud was well mixed — as it is in most cases — all the stars in a cluster must have identical chemical compositions that reflect the mixture of ingredients unique to their parent gas cloud. Astronomers can measure detailed abundances of chemical elements in stars using spectroscopy, and thus they can look for groups of stars with similar chemical composition to reconstruct former clusters.

The cluster in which our Sun formed dissolved billions of years ago, and the Sun’s siblings — about 10,000 of them — are now spread throughout the Milky Way, a galaxy of at least 100 billion stars. Despite this difficulty, we have identified a few solar sibling candidates within a few hundred light-years of the Sun using chemical composition analysis. Once their present-day orbits around the galaxy are determined, astronomers can determine their place of origin by calculating the stars’ trajectories backward in time 4.5 billion years, which is the Sun’s age. In these calculations, all true solar siblings will have trajectories that bring them back to the same location.

The orbit calculations are uncertain because we don’t know exactly how mass is distributed in the Milky Way and how that has changed over such a long period of time. The Sun has orbited the Milky Way’s center about 20 times, each time passing through spiral arms, whose detailed properties are also uncertain.

Although the task seems insurmountable, astronomers are hopeful that within a few years, we will have enough data to find and characterize about 10 solar siblings out of a sample of a billion Milky Way stars. We are confident that once a significant number of solar siblings are identified using chemistry, a statistical analysis of their past orbits will allow us to calculate more precisely the original location of the Sun’s birth gas cloud, a key piece of information for investigating the conditions in which our star and planet formed.

Ivan Ramirez Carrascal 
Professor of Astronomy, 
Tacoma Community College, 
Gig Harbor, Washington