Thousands of stars of many colors on a black background
The Hubble Space Telescope captured this view of Milky Way stars in the constellation Sagittarius in 2017. Credit: ESA/NASA

Data from a galaxy-mapping satellite have revealed that the solar neighborhood is teeming with stars that travel in pairs. Using data from the Gaia mission, researchers have identified 1.3 million binary pairs (2.6 million stars) within about 3,200 light-years of the Sun on the basis of the stars’ positions and trajectories. From this discovery, researchers learned that roughly half of all Sun-like stars are gravitationally bound to a companion star and that the paired-up stars prefer to be separated by about the distance between the Sun and Pluto.

Previous catalogs had identified roughly 200 binary pairs through independent measurements and about 100,000 unconfirmed candidates. Not only is this 3D atlas a significant leap forward in binary star identification, but it also will help astronomers better understand how stars form and dynamically evolve, pinpoint the ages of some stars and their planets, and study how stellar companions might affect the formation of planetary systems.

“Just like single stars, many binaries have planets around them,” said Kareem El-Badry, an astronomy doctoral student at the University of California, Berkeley, and lead researcher on the catalog. “From studying the population demographics of binaries in this catalog—such as their distributions in periods, eccentricities, separations, masses, things like that—we learn details about how the star formation process happens, which is intimately related to how planets form.”

One Star, Two Star, Close Star, Far Star

When it comes to objects in space, the distance to that object is one of the most challenging, and most important, measurements to take. For example, a star that is very close to Earth but very dim could look the same as a star that is very bright and very far away. But knowing whether it’s one or the other is very critical for understanding how that star, and any planets it may host, formed and evolved.

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 Stellar parallax is the most precise way to measure the distance from Earth to a star (see video at right), and it can be done only for stars that are close enough to Earth for their movement to be seen. About 30 years ago, the European Space Agency’s (ESA) Hipparcos space astrometry mission used this method to measure the distances to roughly 2.5 million stars in the Milky Way, and astronomers have relied on those data as starting points to calculating distances to objects farther away. In the past few years, however, ESA’s Gaia mission has been picking up the torch. Gaia is systematically measuring the positions, distances, and motions of nearly 1 billion stars in our galaxy and beyond, creating an unprecedented 3D map of Earth’s cosmological home.

El-Badry’s team used these data to search for nearby binary stars, that is, two stars that formed at the same time from the same cloud of gas and dust and gravitationally interact with each other. In Gaia data, binary stars would appear to be the same distance from Earth, move through the galaxy with the same directions and speeds, and be separated from each other by no more than about 3 light-years. (Beyond that separation, gravity isn’t strong enough to keep the pair together.) The researchers also evaluated the chances that a pair had one star that was a random background star, that a pair might have a third star, and that pairs might be part of a cluster of stars, and they filtered out pairs that had a high likelihood of any of those.

Out of about 65 million stars and about 2 quadrillion possible pairings within about 3,200 light-years of the Sun, the team identified 1.3 million pairs of stars with a high likelihood of being gravitationally bound (see video below). Of those pairs, nearly 900,000 are pairs of hydrogen-burning stars like the Sun, about 16,000 have one hydrogen-burning star and one white dwarf, and nearly 1,400 have two white dwarfs. The results, which were published in Monthly Notices of the Royal Astronomical Society in February, support the long-held theory that most Sun-like stars have a binary companion and that binary companions are typically separated by 30–50 times the Earth-Sun distance (an astronomical unit, or AU), roughly the distance from the Sun to Pluto.

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 This study showed that “for stars about the mass of the Sun, something like 25% of them have a companion beyond about 30 AU,” El-Badry said. “And then from other work, we think that another 25% have a companion closer than 30 AU. So when you add it up, about half of Sun-like stars have a stellar companion.”

Twins, White Dwarfs, and Planets

“Another thing we can look at is what is the distribution of mass ratios in the systems. Do the stars like to be about the same mass or very different?” El-Badry explained. “One of the most surprising and exciting results that’s come out of [this work] is that there is an unexpected excess population of binaries where the two stars are almost exactly the same mass. We call them identical twins.” This result is surprising because at such wide separations, the stars should form independently of each other, and so their mass ratios should be more randomized. The team suspects that twin stars were actually born much closer together and drifted apart over the years. These dynamics could influence the stability of a hypothetical planetary system, the researchers say.

Spacecraft images of binary stars arrayed in a gridded collage
Around half of all Sun-like stars are one of a pair. Above, a collage of some of these binary star pairs as seen by the Gaia spacecraft. Credit: ESA/Gaia/DPAC

Moreover, this catalog could help astronomers determine the ages of hydrogen-burning stars, which is also very difficult to do. “It’s relatively easy to measure age of a white dwarf, at least compared to how hard it is to measure the age of a normal star,” El-Badry said. For pairs that have a hydrogen-burning star and a white dwarf, the white dwarf’s age can be assumed for all components in the system, including exoplanets.

That’s exactly how the team, led by astronomer David Martin, used this catalog to study planet candidate TOI-1259Ab, first discovered by the Transiting Exoplanet Survey Satellite (TESS). “The TESS satellite has discovered thousands of candidate transiting exoplanets,” Martin said. “What we did was cross match the two catalogs, restricting the Gaia binaries to special ones containing at least one white dwarf. From that, we came up with this candidate, TOI-1259, which has a planet transiting a main sequence star and a white dwarf companion. The fact that it has a white dwarf companion came from [El-Badry’s] catalog. Such systems are rare and can tell us a lot about how stellar and planetary systems evolve.” With the white dwarf’s age calculated to be 4.1 billion years, TOI-1259Ab has one of the oldest-known ages for an exoplanet. Martin, at The Ohio State University in Columbus, was not involved with the creation of the binary star catalog.

“Really, if you find any planet transiting any star, you should cross match it with [the] catalog to see if there is any type of stellar companion!”

Martin plans to keep working with the new catalog. “Right now, we are working on expanding this work from this first candidate to other similar systems that contain both a planet and a white dwarf,” Martin said. There are at least 300 more of those types of systems in El-Badry’s catalog. “That, of course, is just a targeted use.…Really, if you find any planet transiting any star, you should cross match it with [the] catalog to see if there is any type of stellar companion!”

El-Badry hopes that other astronomers use these data to help calibrate relationships between a star’s age and its easily measured characteristics like rotation, in addition to studying population-level demographics for these systems. Future releases of Gaia data will increase the precision with which they can calculate the positions and motions of stars, El-Badry said, letting scientists detect binary stars that are very close together and also detect movement caused by an unseen companion, like a black hole.

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer


Cartier, K. M. S. (2021), 1.3 million pairs of stars surround the Sun, Eos, 102, Published on 31 March 2021.

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