A massive 3D atlas of previously unknown “binary” stars within about 3,000 light years of Earth—all 1.3 million of them—has been devised by a 26-year old Ph.D. student while on a Zoom call.

Kareem El-Badry, an astrophysics Ph.D. student from the University of California, Berkeley, rose at 3 a.m. on December 3 last year to attend a Zoom call so he could be among the first to get access to a massive data archive just made available from the Gaia satellite.

Launched in 2013, that stargazing satellite’s mission is to conduct a census of around a billion stars in the vicinity of the Solar System to reveal their composition, formation, evolution and also their trajectories.

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While the Zoom call with 100 other astronomers progressed, El-Badry—originally from Roseburg, Oregon—downloaded about 10GB of data, ran pre-programmed queries on it to extract the information he needed, then created a 3D map of 1.3 million binary stars.

Binary stars are two stars that orbit each other (or, rather, a common center of mass). The idea of a Solar System containing two Suns might sound strange, but binary stars actually make up at least half of all Sun-like stars.

“I was working while on Zoom,” said El-Badry. “But it’s not like someone was presenting and I was ignoring them to work on my binaries—most people on the call were working or going into breakout discussion about different projects.”

Since Gaia is a European project, the data releases happen during the day in Europe. “Astronomers around the world had been itching to get their hands on the new data, so many got up at odd hours,” said El-Badry. “For me, it was mostly just excitement at getting to see the new data.”

The new catalog of nearby binary stars has been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.

Before the pandemic the same astronomers would get together in person for hackathon-style explorations of the Gaia data before a new data release. “For the last data release in 2018, I met up at 5 a.m. with 100 astronomers in a conference room at the Flatiron building in NYC—at the Simons Foundation’s Center for Computational Astrophysics—waiting for the data to drop, and the excitement was quite palpable,” said El-Badry. “The atmosphere isn’t quite the same over Zoom and Slack, but astronomers were still excited.”

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Why a new map was needed

His 3D map is a timely update. The last compilation of nearby binary stars, assembled using data from the now-defunct Hipparcos satellite, included only about 200 likely binary stars.

“This is just a massive increase in sample size,” said El-Badry. “And it is an increase in what kinds of evolutionary phases we find the binaries in. In our sample, we have 17,000 white dwarfs alone. This is a much bigger census.”

White dwarfs are stars that are on the verge of death. In El-Badry’s atlas there are 1,400 binary star systems that consist of two white dwarfs and around 16,000 that consist of a white dwarf and another type of star. That’s crucial because the age of white dwarfs can be accurately calculated, which in turn helps to age other stars and planets in the same star system.

Our Sun will likely end up as a white dwarf in about five billion years.

Check out this fly-through video of the atlas (above), which takes the viewer on a trip through the Milky Way Galaxy toward Earth, traveling through a 6,000-light-year-diameter cloud of more than 1 million binary stars—all mapped by El-Badry.

The video was a collaboration with Jackie Faherty, a scientist and educator at the American Museum of Natural History in New York City. After reaching Earth, the video switches to a view into the future as pairs of binary stars move through space.

What Gaia is doing

“What Gaia is essentially measuring is how each star moves on the plane of the sky, both because of its own motion through the Milky Way, and because of our constantly shifting perspective as the Earth moves around the Sun,” said El-Badry. “As one might imagine, these measurements get more precise the longer Gaia looks at each star.”

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The data he used was based on 34 months of observations, with each of around 1.8 billion stars getting observed about 100 times.

It’s fairly tedious work for the satellite, but the data it gathers is uniquely revealing. What happens to the data is also unique. Unlike many other large science projects, Gaia’s data is made publicly available as soon as it’s ready. “Anyone with an internet connection can get a first-look at the data and have a chance of finding something new,” said El-Badry.

How common are binary stars?

About half of all stars with masses similar to the Sun are binaries. The fraction of stars that are binaries increases with stellar mass, so all stars bigger than 10x the mass of our Sun are binary stars, while only one in 10 of stars that are 10% of the Sun’s mass are binary stars. “It’s also worth noting that besides binaries, there are also triples, quadruples, and even higher-order multiples,” said El-Badry.

Most stars that you can see with the naked eye are more massive than the Sun since massive stars are brighter and can be seen to larger distances. “I would estimate that something like 60% of all naked-eye stars are binaries,” said El-Badry. “In many cases —say, if one component of the binary is much brighter than the other)—it’s not obvious that a star is a binary until you do a careful analysis.”

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How and why so many star systems are binaries

It’s not precisely known why binary star systems form, but astronomers have a basic understanding of how. “The gas clouds out of which stars form are often unstable and susceptible to gravitational fragmentation,” said El-Badry, who will move to Boston in the fall to start as a postdoctoral researcher at Harvard. “If a gas cloud or protostellar disk fragments into two as it collapses under its own gravity, the two fragments can each become a star.”

Until Gaia could precisely measure the distances and motions of millions of nearby stars, the only way to find binaries was to look for stars close together in the sky. That’s very difficult because stars that look very close from Earth could actually be unrelated and hundreds to thousands of light-years from one another, but just happen to lie along the same line of sight from Earth.

Wishing you clear skies and wide eyes.

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