Astronomers Find 4.4-Million-Year 'Scar' in Nearby Interstellar Clouds After Close Pass of Two Massive Stars

Astronomers report that the sun experienced a close cosmic encounter about 4.4 million years ago when two very hot, massive stars swept through our neighborhood and left a detectable ionization “scar” in the local interstellar clouds just outside the solar system.

The discovery comes from detailed dynamical modeling of the motions of the sun, the local interstellar clouds (a complex of gas and dust extending roughly 30 light-years), and two bright stars now located about 400 light-years away in the constellation Canis Major: Epsilon Canis Majoris (Adhara) and Beta Canis Majoris (Mirzam). By tracing trajectories backward, the team found the stars likely passed within ~30 light-years of the sun, close enough to strongly affect the surrounding gas.

"It's kind of a jigsaw puzzle where all the different pieces are moving," said team leader Michael Shull of the University of Colorado Boulder. "The sun is moving. Stars are racing away from us. The clouds are drifting away."

What the Passage Did

Both intruder stars are about 13 times the mass of the sun and much hotter—surface temperatures reach roughly 25,000 °C (about 45,000 °F), compared with the sun’s ≈5,500 °C (≈10,000 °F). Their intense ultraviolet radiation stripped electrons from atoms in the local interstellar clouds, a process called ionization. This left hydrogen and helium atoms positively charged and produced the ionized signature — the "scar" — the researchers detected.

Previously observed measurements showed an unusually high ionization fraction in these clouds: roughly 20% of hydrogen atoms and about 40% of helium atoms were ionized. The new modeling indicates the close passage of Epsilon and Beta Canis Majoris is the most plausible explanation for that elevated ionization, with additional contributions from other local sources.

Other Contributors

The team argues the ionization was likely boosted by at least four additional ultraviolet/X-ray sources: three nearby white dwarf stars and the so-called local hot bubble — a low-density cavity in the interstellar medium thought to have been carved out by the supernovae of some 10–20 earlier stars. Those ancient explosions heated surrounding gas, causing the bubble to emit ionizing radiation that would have further affected the local clouds.

Timescales and Future Risks

Ionization is not permanent. Over time the atoms will recombine with free electrons and return to a neutral state; the researchers estimate that recombination could take on the order of a few million years. Epsilon and Beta Canis Majoris themselves are much shorter lived than the sun and are expected to end their lives as supernovae within the next few million years. At their current distances (~400 light-years), those explosions would be spectacular in the sky but are not expected to pose a lethal threat to Earth.

For scale, the closest approach (≈30 light-years) equals roughly 175 trillion miles (≈281 trillion km). By galactic standards this was a relatively short passage in a galaxy that spans some 100,000+ light-years.

The study detailing these results was published at the end of November in The Astrophysical Journal.

Key takeaway: A close passage of two massive, ultraviolet-bright stars about 4.4 million years ago likely ionized a significant fraction of the nearby interstellar clouds, leaving an observable "scar" that helps explain puzzling ionization levels measured today.