The stakes of this question extend far beyond stellar physics. Space telescopes such as Kepler and TESS have revealed that magnetically active stars frequently unleash superflares, explosive releases of magnetic energy exceeding 10^33 erg, roughly the energy of the largest solar flares on record and often much greater. On the Sun, large flares are usually accompanied by CMEs, eruptions that carry billions of tons of plasma into interplanetary space and drive the most severe space weather at Earth. If young solar-type stars behave the same way, their planets would be bathed not only in intense X-ray and ultraviolet radiation but also in dense magnetized plasma, shocks, and energetic particles, conditions that could strip atmospheres, alter chemistry, and influence whether a rocky world can ever become habitable.

Yet confirming a CME on another star is extraordinarily difficult. Unlike the Sun, distant stars cannot be spatially resolved, so astronomers must infer eruptions from disk-integrated spectra and light curves. The most productive technique searches for transient Doppler shifts in chromospheric lines, especially H-alpha. Cool filament or prominence material erupting toward the observer produces a blue-shifted absorption or emission feature, exactly as seen in solar eruptions. Other diagnostics include coronal dimming, a temporary fading of X-ray or ultraviolet emission caused by the evacuation of coronal plasma, and radio bursts such as type-II and type-IV events that trace shocks and energetic electrons. Each method has limitations, and a single signature is rarely enough to prove that material actually escaped the star.

The breakthrough came from dedicated monitoring of two nearby young solar-type stars: EK Draconis, a G1.5V star roughly 50 to 125 million years old, and V889 Herculis, a G0V star only about 30 million years old. Using the KOOLS-IFU spectrograph on the 3.8-meter Seimei Telescope in Okayama, Japan, Namekata and collaborators captured a spectacular event on April 5, 2020, when EK Draconis produced a superflare of about 2 x 10^33 erg accompanied by a clear blue-shifted absorption component in H-alpha, reaching velocities up to 510 kilometers per second. The feature decelerated in a way consistent with the star’s surface gravity, indicating that cool material had been ejected from near the stellar surface, the hallmark of a filament eruption.

A second event on April 10, 2022 revealed the complementary geometry. During a superflare of roughly 1.5 x 10^33 erg, the H-alpha line showed a blue-shifted emission component moving at 400 to 690 kilometers per second, interpreted as a prominence erupting above the stellar limb. Because the feature appeared in emission rather than absorption, and because its velocity was too high to be explained by flare-related line broadening, the natural interpretation is cool material rising beyond the edge of the visible disk. Together, the two events demonstrate that both on-disk filament eruptions and off-limb prominence eruptions can be detected in unresolved stellar spectra, mirroring the two ways solar eruptions are observed.

Critical to these interpretations is the solar-stellar connection. By analyzing solar filament eruptions as if the Sun were an unresolved point of light, the so-called Sun-as-a-star approach, researchers have shown that the disk-integrated H-alpha signature of a solar eruption closely resembles the EK Draconis events, including the blue-shifted absorption and a post-flare dimming-like feature. This calibration provides a powerful check against false positives. Data-driven modeling strengthens the case further: pseudo two-dimensional magnetohydrodynamic simulations reproduce the observed H-alpha dynamic spectra, showing that cool erupting material embedded in an expanding magnetic loop can decelerate under gravity while the surrounding structure continues to expand outward and evolve into a CME.

Do these eruptions actually escape? Several lines of evidence say yes, at least sometimes. The observed velocities approach or exceed the escape velocity of EK Draconis, with velocity dispersions suggesting top speeds of 660 to 1080 kilometers per second. On the Sun, the CME front typically propagates several times faster than its filament core, implying stellar CME fronts of several thousand kilometers per second. Moreover, an empirical solar criterion based on the velocity-length-scale relation, which separates CME-associated from confined filament eruptions, places the EK Draconis events well above the threshold. The inferred masses of the cool erupting material, ranging from about 10^17 to 10^20 grams, are vastly larger than typical present-day solar filament masses and broadly consistent with extrapolations of solar mass-flare energy relations.

Not everything fits the simple solar extrapolation, however. The kinetic energies of the stellar events fall below some naive solar scaling relations, a discrepancy that has two competing explanations. Physically, the strong overlying magnetic fields of active stars may suppress or decelerate eruptions, confining even enormous events. Observationally, H-alpha traces only the cool core of an eruption, whose projected velocity is expected to be slower than the surrounding coronal front, so the inferred kinetic energies should be treated as lower limits. Resolving this controversy is one of the central challenges for the coming decade, and it will require multi-wavelength observations that can trace the hotter, faster components of eruptions.

Frequency matters as much as size. A five-year monitoring campaign of EK Draconis and V889 Herculis detected fifteen H-alpha superflares, including four with blue-shifted signatures of eruptions, yielding a lower-limit association rate of about 27 percent and an eruption frequency of roughly 0.21 events per day for EK Draconis. Combining this rate with the estimated masses gives a CME-driven mass-loss rate of order 10^-13 to 10^-12 solar masses per year, comparable to the expected steady wind of a young solar-type star. If confirmed, transient eruptions would be a major player in the spin-down and angular momentum evolution of young stars, not a curiosity.

The implications for planets are profound. CME impacts can compress magnetospheres, disturb ionospheres, and drive atmospheric escape, particularly on weakly magnetized worlds like early Mars and Venus, while energetic particles from CME-driven shocks can both destroy ozone and synthesize greenhouse gases and prebiotic molecules. Modeling of the young solar analog kappa^1 Ceti suggests that CME impact probabilities for early terrestrial planets may be as high as 30 percent. The review points to a future built on coordinated campaigns spanning optical, X-ray, ultraviolet, and radio wavelengths, with missions such as ESCAPE, LAPYUTA, XRISM, NewAthena, and the SKA poised to fill critical gaps. For the first time, astronomers have an empirical foundation for understanding the space weather that young Earths endured, and perhaps that made life possible.

Subject of Research: Observational evidence for coronal mass ejections from young solar-type stars and their impact on planetary space weather

Article Title: Coronal mass ejections from young Suns: an observational view through the solar–stellar connection

Article References: Namekata, K. (2026). Coronal mass ejections from young Suns: an observational view through the solar–stellar connection. Astrophysics and Space Science, 371(9), Article 110. https://doi.org/10.1007/s10509-026-04641-w

Image Credits: AI Generated

DOI: 10.1007/s10509-026-04641-w

Keywords: coronal mass ejections, superflares, young solar-type stars, EK Draconis, stellar activity, space weather, H-alpha spectroscopy, solar-stellar connection, exoplanets, stellar mass loss, filament eruptions, magnetic fields

The scale of the problem is difficult to overstate. AGB stars shine with luminosities of 100 to 1000 times that of the Sun and have effective temperatures below 3000 kelvin, giving them radii of several hundred solar radii. Red supergiants are even more extreme: hotter, between 3450 and 4100 kelvin, with radii that can exceed 1000 solar radii. Despite their vast size, their atmospheres are astonishingly thin and dynamic. Convection, the process that transports heat from the interior to the visible surface by bulk motion of hot gas, is not a local phenomenon in these stars. In the Sun, convection manifests as millions of small granules covering the photosphere. In giant envelopes, the pressure scale height is so large that individual convective plumes grow to sizes comparable to the stellar radius itself. Observations with optical interferometers have now directly imaged these giant cells on stars such as Betelgeuse, Antares and the AGB star π1 Gruis, revealing a handful of enormous bright and dark patches that evolve on weekly to yearly timescales.

Understanding such convection demands a fully three-dimensional, time-dependent treatment, because the process is non-local, non-linear, and spans wildly disparate length scales. Traditional one-dimensional stellar evolution models rely on mixing length theory, a prescription invented by Böhm-Vitense in 1958 that compresses all of convection into a single tunable parameter. For the Sun and solar-like stars, calibrated mixing lengths work reasonably well. But in the coolest and most luminous stars, where the convective plumes span a large fraction of the star, turbulence becomes supersonic, and radiative opacity varies by orders of magnitude, the one-dimensional approximation breaks down dramatically. The review emphasizes that only global simulations, often called star-in-a-box models, which encompass the majority of the convective envelope plus the near circumstellar environment, can capture the physics at play.

Two numerical codes anchor this effort. The first, CO5BOLD, developed over decades and first applied to global giant-star simulations by Freytag and collaborators in 2002, solves the coupled equations of compressible hydrodynamics and non-local radiative energy transport on a three-dimensional Cartesian grid embedded in a fixed, spherically symmetric gravitational field. It uses a finite-volume approach with directional splitting and an approximate Riemann solver of Roe type, along with a short-characteristics method for radiative transfer built on Rosseland mean opacity tables that merge high-temperature OPAL data with low-temperature PHOENIX data. The code can be extended to magnetohydrodynamics and can handle rotation through a corotating frame. The second code, Athena++, developed by Stone and colleagues, takes a different route: it employs adaptive mesh refinement, task-based dynamic execution, and a Godunov-type shock-capturing scheme, solving the time-dependent, frequency-integrated radiative transfer equation directly over discrete ordinates, with radiation fully coupled to the gas energy and momentum equations. For red supergiants, Athena++ simulations use spherical polar coordinates covering 70.6 percent of the face-on hemisphere, with an accurate gravitational potential that includes the mass of the simulated envelope itself, a detail that matters because these loosely bound atmospheres are anything but negligible in mass.

What do these simulations actually show? The picture that emerges is spectacularly turbulent. The surfaces of simulated AGB and red supergiant stars are dominated by a few giant convective cells, with wide upflows surrounded by narrow, turbulent downdrafts, evolving on timescales from weeks to years. Non-stationary convection in the deep interior generates sound waves. These waves travel outward, slow down and compress as the temperature and sound speed drop, and their amplitudes grow with the steep decline in density until they steepen into shocks. Just above the visible surface, the resulting shock network is extraordinary: turbulent pressure can exceed twenty times the gas pressure, shock waves propagate with average Mach numbers of 3 to 4, and local peaks reach Mach numbers of 8 to 10. Individual shocks above the photosphere can span 200 to 250 solar radii, a width comparable to the star’s own radius, at temperatures around 2500 kelvin. These shocks intermittently levitate gas to distances where dust can condense, providing the crucial first step in the wind-acceleration mechanism that drives the massive mass-loss rates of 10 to the minus 6 to 10 to the minus 4 solar masses per year observed in AGB stars.

Remarkably, the simulations show that radial pulsations are not imposed on these models but emerge spontaneously from the convective motions themselves, a result first demonstrated for AGB stars by Freytag and colleagues in 2017 and confirmed in later work by Ahmad and coauthors in 2023. The extracted pulsation periods agree with those observed in long-period variable stars. This interplay between convection and pulsation leaves an observable fingerprint in stellar spectra. Spectral lines form over a range of atmospheric depths, and each line carries the imprint of the local velocity field through its shift, width and asymmetry. The tomographic method, originally conceived to study shock waves in pulsating Cepheids and Mira variables, exploits this by sorting spectral lines according to their formation depth and cross-correlating them with observed spectra. In Mira stars, the technique beautifully reproduces the classic Schwarzschild scenario, in which a shock front marching upward through the atmosphere produces a sequence from red-shifted absorption to line doubling and finally blue-shifted emission.

Red supergiants, it turns out, play by different rules. Applied to years of high-resolution spectra of the prototypical red supergiants μ Cephei and Betelgeuse, the tomographic method revealed no such clean shock scenario. Instead, the atmospheric layers exhibit a phase shift between velocity and temperature variations, tracing a hysteresis loop in the temperature-velocity plane. Crucially, synthetic spectra computed from three-dimensional CO5BOLD simulations show the same hysteresis behavior with timescales matching the observed photometric variability. The interpretation is elegant: the loop is the signature of convective turnover, in which hot rising material reaches the upper atmosphere, cools, and falls back, modulated by acoustic waves generated by the turbulent convective flow below. The method even provided insight into Betelgeuse’s famous Great Dimming of early 2020, when the star faded to magnitude 1.6. Tomography identified two successive shocks, in February 2018 and January 2019, that expanded part of the atmosphere and, through increased molecular opacity in the cooler gas, obscured the star, an explanation consistent with, and complementary to, the dust-veil picture from direct imaging.

The Athena++ simulations have yielded equally consequential results on the theoretical side. Goldberg and colleagues in 2022 performed the first three-dimensional calibration of mixing length theory in the red supergiant regime, recovering surprisingly high convective efficiencies with a mixing length parameter of roughly 3 to 4, and showing that radiation pressure provides about one-third of the support against gravity in the envelope. The simulations also revealed an inverse correlation between density and radial velocity in the outer layers, uncharacteristic of ordinary convection, and an atmosphere far more extended than one-dimensional models predict. In these luminous layers, where the opacity locally exceeds the Eddington opacity, the usual correlation between dense sinking material and convection flips entirely: cold, opaque, dense regions move outward. This has direct implications for how red supergiants die. When such a star explodes as a Type II-P supernova, its shock must traverse this wildly inhomogeneous envelope, whose surface undulates over radial ranges of 10 to 20 percent of the stellar radius. The shock therefore breaks out at different radii and different times around the star, smearing the ultraviolet flash of shock breakout over roughly 3 to 6 hours instead of the sub-hour duration predicted by spherical models, and reducing peak luminosities by factors of 2 to 10. This means astronomers cannot use breakout rise times to measure progenitor radii via light-travel-time effects, but they can instead use breakout durations to probe the scale of convective inhomogeneities themselves.

The convergence of three-dimensional radiation-hydrodynamics modeling, interferometric imaging, ALMA observations of molecular envelopes, spectropolarimetric monitoring of surface magnetism, and tomographic spectroscopy is transforming our understanding of the late lives of stars. Convection, once a fudge factor in one-dimensional codes, is now a quantitatively constrained, observable, three-dimensional process that shapes winds, dust formation, pulsation, magnetism and the very first light of supernovae. As future ultraviolet missions such as ULTRASAT catch hundreds of shock breakouts at high cadence, and as ever more powerful simulations push toward even greater realism, the giant convective cells churning on the surfaces of dying stars will continue to yield their secrets, one shock wave, one spectral line, and one hysteresis loop at a time.

Subject of Research: Three-dimensional radiation-hydrodynamics simulations of convection in the atmospheres of asymptotic giant branch and red supergiant stars

Article Title: Signatures of convection in the atmospheres of cool evolved stars

Article References: Signatures of convection in the atmospheres of cool evolved stars. (n.d.). https://doi.org/10.1007/s41115-024-00020-w

Image Credits: AI Generated

DOI: 10.1007/s41115-024-00020-w

Keywords: stellar convection, red supergiants, AGB stars, radiation hydrodynamics, CO5BOLD, Athena++, tomography, Betelgeuse, shock breakout, supernovae, mixing length theory, stellar mass loss