Astronomers have confirmed that a star in the constellation Hercules is a dead ringer for one of the Sun’s long-lost brothers.
Stars like our Sun form in groups. We see evidence for this throughout the Milky Way, most famously in the Trapezium Cluster in the Orion Nebula. But after stars begin to emit their nascent light, their gravitational interactions with other nearby "siblings" send them out from their birth cluster and into the expanses of our galaxy.
Even though they travel thousands of light-years from where they first formed, stars carry the signature of their birthplace in the detailed chemical composition contained in their atmospheres and their motions through space. Astronomers have been searching for the lost siblings of the Sun for some time, and they’ve found several candidates. Now, using these lines of evidence, they think they’ve confirmed one.
Ivan Ramírez (University of Texas, Austin) and his team used high-resolution spectroscopy obtained at the McDonald Observatory in Texas and Las Campanas Observatory in Chile to inspect the atmospheres of 30 suspected solar siblings. (These stars are not true "solar twins," stars that appear similar to the Sun with respect to virtually every observable property — including mass, luminosity, and composition — regardless of origin.) The team chose these 30 stars from previous studies that had highlighted them as potential solar siblings, based on motions, ages, and compositions.
Typically, star formation results in the formation of an "open cluster,” a group of young stars that have formed from the same gas cloud. The detailed chemical abundances of this gas cloud, as measured by traces of elements heavier than helium, are preserved within the young stars.
Open clusters only last for a few hundred millions years, their stars spreading out throughout the galaxy over time. The Sun itself is about 4.57 billion years old, so it’s had plenty of time to get lost.
Fortunately, astronomers don’t need a home address to identify solar siblings. By measuring the motions of stars through space, astronomers can "reverse" their motions and see which stars were near the Sun when it formed. You can imagine watching a video of an explosion in reverse: as you play the movie, things that are initially far apart begin to move closer to one another. The same method works here. However, it is important to model the Milky Way's gravitational field correctly, since it influences the motions of these stars. Ramírez and his team measured motions for 30 suspected siblings and were able to "rewind the tape" on each of them.
Next, the team measured the detailed atmospheric composition of each suspected sibling. In order to be a match, the star needs to not only have been close to the Sun about 4.6 billion years ago, but it also needs to have the same age as the Sun and have similar abundances of iron, silicon, oxygen, and other heavier elements.
After this test, only two of the 30 candidates showed a match to the Sun's chemical composition, and only one, HD 162826, was close to the Sun at the time it formed. Thus, this makes HD 162826 the best "solar sibling" candidate to date. This star is about 15% more massive than the Sun, making it one of the Sun's big brothers. At 110 light-years it is also relatively nearby, shining at magnitude 6.7 in the constellation Hercules.
The team found that, instead of painstakingly looking at as many elements as possible, the most useful tactic is to measure the abundances of a handful of elements that vary greatly among stars that otherwise have similar compositions. One of these elements, barium, should be easily observable with medium-resolution spectra, the team says.
Although HD 162826 is slightly more massive than the Sun, Ramírez’s team notes that many of the solar siblings are likely to be low-mass M dwarfs, since these are the most common type of star made during star formation. Current capabilities likely won’t be able to identify these little brothers — M dwarfs have crowded spectra that are difficult to analyze, and they’re also inherently dim. But with the launch of ESA’s Gaia, astronomers will have precise measurements on the motions of millions of nearby M dwarfs, which will help isolate which of these were formed alongside our Sun.
Reference: I. Ramírez et al. "Elemental Abundances of Solar Sibling Candidates." Astrophysical Journal, in press.