A newly composed 3D map has revealed more than 1 million binary star pairs located within 3,000 light-years from Earth, highlighting the ubiquity of these celestial objects.
Remember that sublime moment in Star Wars when an introspective Luke Skywalker stares at a double sunset on Tatooine? In our view, that’s some seriously exotic stuff, but binary star systems are actually very common, representing at least half of all the sun-like stars in the Milky Way. That said, a good chunk of this includes “wide binaries,” in which distances between star companions are greater than 10 AU, or 10 times the mean distance from the Earth to the sun (it’s also a similar distance between Earth and Saturn) .
New Research published in the Monthly Notices of the Royal Astronomical Society gives a count of these broad binaries, at least up to 3,000 light-years from Earth. The new paper, led by astrophysicist Kareem El-Badry, a PhD student at the University of California, Berkeley, describes the relative locations of 1.3 million binary pairs spread across much of the Milky Way, which spans more than 100,000 light years in diameter. . Jackie Faherty of the American Museum of Natural History in New York teamed up with El-Badry to create a stunning video fly-through of the newly mapped binary pairs.
To compile the new 3D atlas, El-Badry used data collected by ESA’s Gaia space telescope, which has been orbiting Earth-Sun Lagrange point – that good place between two large objects that allow spacecraft such as Gaia to remain stuck – since 2013.
Finding binary stars parked close together is a relatively easy process (you need a spectrometer), but finding wide binary stars is quite another. That’s where Gaia comes in, with its ability to measure the position and correct motion of nearby stars, which it does for millions of objects. That said, it can’t really track stars over 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 great distances, the two stars can be resolved spatially as two different points of light in the sky,” El-Badry explained in an email. “Tighter separations do not resolve binaries, so other methods (such as spectroscopy) are needed to detect them.”
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El-Badry, together with colleagues from the Max Planck Institute for Astronomy and Boston University, developed a computational technique to locate star pairs that move together through space and are the same distance from Earth. For this analysis, El-Badry used data from the December 3, 2020 Gaia release, which included nearly 2 billion stars.
Importantly, this technique can lead to false positives, as only with some stars to show up to have companions, with objects moving through space in harmony with each other, but by total coincidence. (The authors call this “accidental alignments.”) El-Badry estimates that 1.3 million pairs identified in the study have a 90% chance of being true binary stars. That is a great deal of uncertainty, and one area in which this research could be improved.
The new catalog also includes a significant number of white dwarfs, the hot, dense remnants of former stars. About 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 the stars in the catalog are main sequence stars, 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 also managed to document binaries at extreme distances, including some pairs separated by a whole 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 is reasonable to wonder if these pairs could even be considered companions, but El-Badry said binary stars exist over a vast range of physical separation.
“The closest binaries have separations less than the Earth-Moon distance and orbital periods of just a few minutes,” he explained. “The widest have separations of up to a few light years and orbital periods of over 100 million years. That means the two stars at their widest distances orbit the Milky Way only about once per orbit around the Milky Way and have completed only a few dozen orbits since they formed. Of course, the gravitational acceleration at such long distances is extremely weak, but not entirely negligible. ”
Another interesting observation is that many binary pairs have a similar mass. This is strange, especially considering the distances between some of these objects.
“A surprising result of our study is that even at a distance of thousands of AU, there is a strong excess of ‘identical twin’ binaries in which the two stars have almost equal masses. [to] within a few percent, ”said El-Badry. “This was not expected, as conventional star formation theory predicts that at these distances the two stars will form independently of each other, so their masses should not be highly correlated.”
This observation could speak to star formation theories, where binary pairs form together in the same stellar nursery and then slowly drift apart over time. As El-Badry noted, “How this population is formed will take more work, both in terms of theory and observations.”