Some 90s trends have made a very welcome comeback; Others no one needs to see again. But unlike the frosted tips, baggy Batik shirts, landline phones and pogs that have (hopefully?) been consigned to the trash can, binary stars are resurfacing as worth studying. And not too soon.
In short, binary stars are vital in the quest to understand stellar evolution. They could also improve our understanding of exotic astrophysical events such as supernova explosions, which signal the birth of black holes and neutron stars. But for many years, binary star research was as neglected as that old Tamagotchi in your drawer.
Fortunately, the binary star renaissance is now underway, confirms Harvard and Smithsonian Center for Astrophysics (CfA) researcher Kareem El-Badry.
“I think a lot of astronomers viewed binaries as a solved problem and less exciting than other areas in astrophysics, like galaxy formation, cosmology, or exoplanets,” El-Badry told Space.com. “This is an old field, so after 200 years of study, most of the important questions seemed to be answered about 15 years ago. Now binary stars are coming back stronger than the trend in the 90s!”
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He attributes the return of binary stars to decades of cosmic data collected by Gaia, the European Space Agency’s star-mapping space telescope.
Gaia was launched in 2013 with a mission to create detailed measurements of a billion stars in the Milky Way galaxy and beyond. By mapping the motions, brightnesses, temperatures and compositions of these stars, Gaia aims to create the most precise three-dimensional model ever of our cosmic backyard.
The most recent data drop from Gaia occurred on October 10, 2023, and included data on 1.8 billion stars. This, incredibly, was just a taste of what was to come with the fourth major data drop from the space telescope, called Gaia Data Release 4 (DR4). Gaia owes its power to a technique called “astrometry”, which is almost adapted to the study of stars along with their companions.
“Astrometry measures distances to stars and detects their ‘wobble’ across the sky due to binary companions,” El-Badry said. said. “The sheer volume of the Gaia data set means it contains many things that have never been seen before: Gaia gave us orbital solutions for binaries, about 50 times more than any previous mission.”
When two become one
Despite our familiarity with (and understandable bias towards) single-star systems, double star systems are more common in the Milky Way than one might imagine, thanks to being orbiting such a lonely stellar body. About 50% of Sun-sized stars are found with a binary partner, rising to around 75% for larger stars.
“More than half of all stars are binary stars!” El-Badry said. “So, if you want to understand the evolution of ‘typical’ stars, you have to take into account the existence of their companions.”
These binary systems offer scientists the opportunity to study unseen phenomena around single stars such as the Sun. This includes what happens during mass transfer from a star to its binary partner, events that lead to a star being expelled from the binary system. The systems can also help us understand what would happen if stars came together and merged into a single star.
In general, Gaia data can also reveal the types of distances binary stars orbit each other, providing a better picture over a wider range of binary distances than other missions, even if those distances are very wide.
“Binaries give us the chance to measure stellar masses directly through the star’s gravitational effects on each other, which we wouldn’t be able to do otherwise,” El-Badry said. he explained. “This showed that there is an extreme population of binary stars where the two stars have almost exactly the same mass, even if they are separated by hundreds to thousands of times the distance between the Earth and the Sun.”
But Gaia has been effective not only in terms of the amount of binary stars it has physically revealed. The ESA spacecraft also provided better estimates of the distances of binary stars as a whole; El-Badry said this had previously been a “major hurdle” in astrophysics.
Gaia data can also tell us something about binary systems in which a star is orbited by something more exotic, such as a white dwarf, a neutron star, or a black hole. These systems emit gravitational waves as their components rotate around each other.
“Binaries also allow us to detect dark objects that we cannot normally see, such as black holes, through their gravitational effects on luminous companions,” El-Badry said. El-Badry added that this could help astronomers better understand how often normal stars have a black hole companion. and how these highly exotic systems can form.
This renaissance in binary star research coincides with a reignited interest in understanding how these systems form; This interest was sparked by the recent detection of gravitational waves resulting from the merger of black holes and neutron star binaries.
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The future looks bright for the duo stars, and El-Badry is actually hopeful that a few trends from the ’90s could create a similar revival one day.
“Hopefully we won’t have to use flip phones, write too many emails and the Spice Girls will all come back together,” the CfA researcher said. “I don’t really want low-rise jeans and curly hair being associated with ‘being ugly’ in every teen movie to make a comeback.”
El-Badry’s review of a decade of Gaia binary star data has been published in the arXiv paper repository.