Free Astronomy Magazine May-June 2025

27 MAY-JUNE 2025 to study the millimeter-wavelength flare activity of Proxima Centauri. Proxima Centauri’s small size and strong magnetic field indicate that its entire internal structure is convec- tive (unlike the Sun, which has both convective and non-convective lay- ers), making the star much more ac- tive. Its magnetic fields become twisted, develop tension, and even- tually snap, sending streams of en- ergy and particles outward in what is observed as flares. “Our Sun’s ac- tivity doesn’t remove Earth’s atmos- phere and instead causes beautiful auroras because we have a thick at- mosphere and a strong magnetic field to protect our planet. But Proxima Centauri’s flares are much more powerful, and we know it has rocky planets in the habitable zone. What are these flares doing to their atmospheres? Is there such a large flux of radiation and particles that the atmosphere is getting chemi- cally modified, or perhaps com- pletely eroded?” said MacGregor. This research represents the first multi-wavelength study using mil- limeter observations to uncover a new look at the physics of flares. Combining 50 hours of ALMA ob- servations using both the full 12- meter array as well as the 7-meter Atacama Compact Array (ACA), a total of 463 flare events were re- ported at energies ranging from 10 24 to 10 27 erg, and with a brief du- ration ranging from 3 to 16 sec- onds. “When we see the flares with ALMA, we see the electromagnetic radiation–the light in various wave- lengths. But looking deeper, this radio wavelength flaring is also giv- ing us a way to trace the properties of those particles and get a handle on what is being released from the star,” says MacGregor. To do so, the team characterized the star’s so- called flare frequency distribution to map out the number of flares as a function of their energy. Typically, the slope of this distribution tends to follow a power law function: smaller (less energetic) flares occur more frequently, while larger, more energetic flares occur less regularly. Proxima Centauri experiences so many flares that the team detected many flares within each energy range. Furthermore, the team was able to quantify the asymmetry of the star’s highest energy flares, de- scribing how the flares’ decay phase was much longer than the initial burst phase. Radio and millimeter-wavelength observations help constrain the en- ergies associated with these flares and their associated particles. MacGregor highlighted ALMA’s key role: “The millimeter flaring seems much more frequent. It’s a different power law than we see at the opti- cal wavelengths. If we only look at optical wavelengths, we’re missing critical information. ALMA is the only millimeter interferometer sen- sitive enough for these measure- ments.” ! A rtist’s concept of a stellar flare from Proxima Centauri. [NSF/AUI/NSF NRAO/S. Dagnello]