<\/p>\n Using computer models to rewind the paths of stars near the sun, a team led by University of Colorado Boulder astrophysicist Michael Shull<\/a> found that two blue-white stars, Beta Canis Majoris and Epsilon Canis Majoris, passed within 30 light-years of the sun about 4.5 million years ago.<\/p>\n You may like<\/p>\n Today, the stars mark the front and rear legs of the constellation Canis Major (the Great Dog), more than 400 light-years from the solar system. But roughly 4.5 million years ago, when the stars brushed past the solar system, they were younger, hotter and brighter. Their passage also may have overlapped with the time when Lucy \u2014 a remarkably complete fossil<\/a> of an early human ancestor discovered in Ethiopia in 1974 \u2014 walked the Earth.<\/p>\n “They weren’t headed straight toward us…but that’s close,” Shull told Live Science. “If Lucy and her friends had looked up and had noticed the stars, the two brightest stars in the sky would have been not Sirius, but Beta and Epsilon Canis Majoris.”<\/p>\n The findings were published Nov. 24 in The Astrophysical Journal<\/a>.<\/p>\n A lingering mystery <\/p>\n Today, the solar system drifts through a diffuse patchwork of more than a dozen nearby wisps of gas and dust called the local interstellar medium, made primarily of hydrogen and helium and extending roughly 30 light-years from the sun.<\/p>\n Astronomers think these clouds were sculpted over several million years by shock waves from exploding stars in the Scorpius-Ophiuchus region, a rich cluster of massive stars about 300 light-years from Earth, which compressed interstellar gas into the thin local clouds seen today.<\/p>\n \n A map of the local interstellar clouds just outside our solar system. Blue arrows show the directions that these clouds are moving. The yellow arrow shows the direction of the sun’s motion. (Image credit: NASA\/Adler\/U. Chicago\/Wesleyan; https:\/\/svs.gsfc.nasa.gov\/10906<\/a>)<\/p>\n Observations dating back to the 1990s, including from NASA<\/a>‘s now-retired Extreme Ultraviolet Explorer space telescope, revealed<\/a> that this region is unusually ionized. Helium atoms, in particular, have been stripped of their electrons at nearly twice the rate expected relative to hydrogen.<\/p>\n That imbalance has puzzled astronomers, because helium requires more energetic radiation to ionize than hydrogen does. This makes its elevated ionization difficult to explain with the sun’s radiation alone, which does not extend that far beyond the solar system.<\/p>\n You may like<\/p>\n To investigate the mystery, Shull and his colleagues calculated the properties and ultraviolet output of Beta and Epsilon Canis Majoris, using measurements from the European Space Agency<\/a>‘s Hipparcos<\/a> satellite, which mapped the positions of more than a million stars across a four-year mission that ended in 1993.<\/p>\n Knowing how far away the two stars are today \u2014 about 400 light-years \u2014 and how fast they are moving allowed the team to trace the stars’ paths backward through time to reconstruct their close pass by the solar system roughly 4.5 million years ago.<\/p>\n “It’s like a dance floor”<\/p>\n The team’s models show that the wispy local clouds would have been intensely ionized by the two stars’ radiation \u2014 at levels up to 100 times stronger than those seen today. Over time, the gas would have slowly returned to a more neutral state through a process called recombination, in which free electrons reattach to ions and become atoms.<\/p>\n “It’s like a dance floor,” Shull told Live Science. “You’ve got protons and electrons dancing around, and sometimes they’re dancing together and sometimes they’re popping apart.”<\/p>\n But that process takes time, and continued exposure to radiation from other sources continues to keep the gas partially ionized, Shull said. Astronomers had previously identified other sources of ionizing radiation, including three nearby white dwarf stars<\/a> \u2014 G191-B2B, Feige 24 and HZ 43A \u2014 compact stellar remnants that are known to emit strong ultraviolet light. Researchers also pointed to the Local Bubble<\/a> \u2014 a vast, supernova-blown region of hot gas that extends about 1,000 light-years around the solar system.<\/p>\n “More energetic photons preferentially ionized helium,” he said. “That’s the bottom line.”<\/p>\n![\"Map](\"https:\/\/www.newsbeep.com\/ie\/wp-content\/uploads\/2025\/12\/Tkw58LDvsrfV4MxRQTP2Pi.jpg\")
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