A newly compiled 3D map has revealed more than 1 million binary star pairs located within 3,000 light years of Earth, highlighting the ubiquity of these celestial objects.
Remember that sublime moment in Star Wars when an introspective Luke Skywalker gazes upon a double sunset on Tatooine? To our eyes, that’s some seriously exotic stuff, but binary star systems are actually quite common, representing at least half of all Sun-like stars in the Milky Way. That said, a hefty portion of these include “wide binaries,” in which distances between stellar companions exceeds 10 AU, or 10 times the average distance from the Earth to the Sun (it’s also a comparable distance between the Earth and Saturn).
New research published in the Monthly Notices of the Royal Astronomical Society provides a census of these wide binaries, at least those to within 3,000 light years of Earth. The new paper, led by astrophysicist Kareem El-Badry, a PhD student at the University of California, Berkeley, chronicles the relative locations of 1.3 million binary pairs spread across a good chunk of the Milky Way, which measures more than 100,000 light years in diameter. Jackie Faherty from the American Museum of Natural History in New York worked with El-Badry to produce a stunning video fly-through of the newly mapped binary pairs.
To compile the new 3D atlas, El-Badry used data gathered by ESA’s Gaia space telescope, which has been in orbit at the Earth-Sun Lagrange point—that sweet spot between two large objects that allows spacecraft like Gaia to stay put—since 2013.
Finding binary stars parked closely to each other is a relatively straightforward process (you need a spectrometer), but finding wide binaries is another thing entirely. That’s where Gaia comes in, with its ability to measure the position and proper motion of nearby stars, which it’s done for millions of objects. That said, it can’t really track stars further than 3,000 light years away, hence the limited scope of the new census.
Wide binaries are “easy to study with the Gaia spacecraft, because at wide separations the two stars can be spatially resolved as two distinct points of light on the sky,” El-Badry explained in an email. “At closer separations, binaries are unresolved, so other methods (like spectroscopy) are needed to detect them.”
El-Badry, along with colleagues from the Max Planck Institute for Astronomy and Boston University, developed a computational technique to pinpoint pairs of stars moving together through space and at the same distance from Earth. For this analysis, El-Badry used data from the Gaia release of Dec. 3, 2020, which included nearly 2 billion stars.
Importantly, this technique can lead to false positives, as some stars only appear to have companions, with objects moving through space in concert with each other, but by total coincidence. (The authors call these “chance alignments.”) El-Badry figures that 1.3 million pairs identified in the study have a 90% chance of being true binary stars. That’s a high degree of uncertainty, and an area in which this research could be improved.
The new catalog also includes a sizable number of white dwarfs, the hot, dense remnants of former stars. Around 1,400 systems listed in the catalog consist of two white dwarfs, while 16,000 systems consist of one white dwarf and another type of star. The vast majority of stars in the catalog are main sequence stars, which are still in the main phase of their existence.
El-Badry and his colleagues also found that around 25% of all Sun-like stars have companions located further away than 30 AU, which is roughly the distance between the Earth and Pluto. Distances between 30 to 50 AU are common, but the team also managed to document binaries at extreme distances, including some pairs separated by an entire parsec, or 3.26 light years. Those were outliers, however, as most binary stars were found to be located to within 1,000 AU of one another. At such distances, it’s fair to wonder if these pairs can even be considered companions, but El-Badry said double stars exist across an enormous range of physical separation.
“The closest binaries have separations smaller than the Earth-Moon distance, and orbital periods of only a few minutes,” he explained. “The widest have separations up to a few light years, and orbital periods exceeding 100 million years. That means that at the widest separations, the two stars orbit each other only about once per orbit around the Milky Way, and have only ever completed a few tens of orbits since they formed. Of course, the gravitational acceleration at such wide separations is extremely weak, but not completely negligible.”
Another interesting observation is that many binary pairs have a similar mass. This is odd, particularly given the distances involved between some of these objects.
“One surprising result from our study is that even at separations of thousands of AU, there is a strong excess of ‘identical twin’ binaries in which the two stars have almost equal mass [to] within a few percent,” said El-Badry. “This was not expected, since conventional star formation theory predicts that at these distances, the two stars form basically independently, so their masses should not be strongly correlated.”
This observation could speak to star formation theories, with binary pairs forming together in the same stellar nursery, and then slowly drifting apart over time. As El-Badry pointed out, “how this population formed will take more work, both in terms of theory and observations.”