We are surrounded by double stars, a new 3D map suggests

A newly compiled 3D map has revealed over a million pairs of binary stars located within 3,000 light years of Earth, highlighting the ubiquity of these celestial objects.

Remember this sublime moment in Star wars when an introspective Luke Skywalker watches a double sunset over Tatooine? To us, these are seriously exotic things, but binary star systems are actually quite common, making up at least half of all Sun-like stars in the Milky Way. That said, much of these include “wide binaries,” in which the distances between stellar companions exceed 10 AU, or 10 times the average distance from Earth to the Sun (this is also a comparable distance between Earth and Saturn).

New search published in the Monthly Notices of the Royal Astronomical Society provides a census of these large binaries, at least those within 3000 light years of Earth. The new paper, led by astrophysicist Kareem El-Badry, a doctoral student at the University of California at Berkeley, chronicles the relative locations of 1.3 million binary pairs spread across much of the Milky Way, which measures over 100,000 light years in diameter. Jackie Faherty of the American Museum of Natural History in New York City worked with El-Badry to produce a stunning video overview of the newly mapped binary pairs.

To compile the new 3D atlas, El-Badry used data collected by ESA’s Gaia Space Telescope, which was in orbit at the Earth-Sun point of Lagrange – which sweet spot between two large objects that allow spaceships like Gaia to stay put – since 2013.

Finding binary stars that are close to each other is a relatively straightforward process (you need a spectrometer), but finding wide binaries is quite another thing. This is where Gaia comes in, with its ability to measure the position and correct movement of nearby stars, which is done for millions of objects. That said, it can’t really track stars more than 3,000 light years away, hence the limited range 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, the binaries are not resolved, so other methods (like spectroscopy) are needed to detect them.”

El-Badry, along with colleagues at the Max Planck Institute for Astronomy and Boston University, developed a computational technique to locate pairs of stars moving together in space and at the same distance from Earth. For this analysis, El-Badry used data from the Gaia publication of December 3, 2020, which included nearly 2 billion stars.

Importantly, this technique can lead to false positives, because only some stars appear have companions, with objects moving in space in concert with each other, but by total coincidence. (The authors call these “random alignments.”) El-Badry estimates that the 1.3 million pairs identified in the study have a 90% chance of being true binary stars. This is a high degree of uncertainty and an area in which this research could be improved.

The new catalog also includes a significant number of white dwarfs, the hot and dense remains of ancient stars. About 1,400 systems listed in the catalog consist of two white dwarfs, while 16,000 systems consist of a white dwarf and another type of star. The vast majority of the 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 about 25% of all Sun-like stars have companions farther than 30 AU, which is roughly the distance between Earth and Pluto. Distances between 30 and 50 AU are common, but the team has also been successful in documenting binaries at extreme distances, including some pairs separated by an entire parsec, or 3.26 light years. These were outliers, however, as most binary stars were within 1000 AU of each other. At such distances, it’s fair to wonder if these pairs can even be considered companions, but El-Badry said double stars exist in a huge 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 larger ones have separations of up to a few light years and orbital periods exceeding 100 million years. This means that at the widest separations, the two stars orbit only once per orbit around the Milky Way, and have only completed a few dozen orbits since their formation. Of course, the gravitational acceleration at such wide separations is extremely small, but not completely negligible.

Another interesting observation is that many binary pairs have similar mass. It’s strange, especially considering the distances involved between some of these objects.

“A surprising result of 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] at a few percent, ”El-Badry said. “This was not expected, because conventional star formation theory predicts that at these distances, the two stars form essentially independently, so their masses should not be strongly correlated.

This observation could speak to theories of star formation, with binary pairs forming together in the same stellar nursery, then slowly separating over time. As El-Badry pointed out, “training this population will require more work, both in terms of theory and observations.”