The CHARA Array’s Near-Infrared Interferometry Reveals Complex Early Stages of Nova Explosions

Introduction: High-Resolution Imaging of Nova Eruptions

The Center for High Angular Resolution Astronomy (CHARA Array) at Georgia State University has employed near-infrared interferometry—an advanced technique combining light from multiple telescopes—to capture detailed, high-resolution images of the early post-explosion phases of two nova events detected in 2021. These observations mark the first direct visualizations of the dynamic material ejection and interaction processes in nova explosions, addressing long-standing challenges in studying transient astronomical phenomena.

Background: The Nature of Novae

A nova arises in a binary star system when a white dwarf accretes hydrogen-rich gas from a companion star, triggering a thermonuclear runaway reaction on its surface. This process causes a sudden brightening, but the faint, rapidly expanding ejecta post-explosion have historically been difficult to observe directly, with astronomers relying on indirect methods to infer early-stage dynamics.

Explosive Observational Findings

Case Study 1: V1674 Herculis—A Rapidly Evolving Nova

V1674 Herculis, a nova in the constellation Hercules, exhibited unprecedented rapidity: it reached peak brightness within 16 hours of discovery and faded dramatically over days. Near-infrared interferometric images, captured just days post-discovery, revealed a non-spherical explosion with two distinct ejecta flows (northwest and southeast) and an elliptical structure radiating perpendicular to these streams—direct evidence of interacting ejecta.

Spectroscopic analysis confirmed velocity variations in hydrogen’s Balmer series: pre-peak absorption lines were ~3,800 km/s, while post-peak components surged to ~5,500 km/s. Critically, the appearance of this high-velocity ejecta coincided with high-energy gamma-ray detections by NASA’s Fermi Gamma-ray Space Telescope, implicating a collision-induced shock wave as the source of gamma-ray emission.

Case Study 2: V1405 Cassiopeiae—Probing Extended Ejecta Dynamics

V1405 Cassiopeiae, located in Cassiopeia, provided even more striking insights. During its peak phase (53 days post-discovery), initial observations showed a bright central source (~0.99 milliarcseconds in diameter, corresponding to a radius of ~0.85 au) with minimal surrounding ejecta. A discrepancy emerged: if the hydrogen-rich outer layer had expanded uniformly, its radius would have reached 23–46 au after 53 days—a far larger scale than the observed 0.85 au. This implied most of the outer layer was not fully ejected, suggesting the binary system’s common envelope phase persisted until the visible-light peak.

By the third observation, the central source contributed only ~50% of total radiation, with a broad emission component (~2,100 km/s) from expanded material. This transition coincided with new shock waves and high-energy emissions, indicating ongoing mass redistribution driven by the binary system’s orbital dynamics.

Novae as Cosmic Laboratories

These findings underscore novae as complex, multi-scale phenomena, far beyond simple spherical explosions. Observations by NASA’s Fermi telescope over 15 years have detected gigaelectronvolt (GeV) gamma rays from >20 novae, positioning them as natural laboratories for studying shock waves and particle acceleration.

For V1405, the delayed ejection of outer layers suggests orbital motion in its binary system may drive material expansion, while the common envelope phase (persisting for weeks in slowly evolving novae) offers rare insights into stellar binary interactions—an event occurring in >10% of stellar systems but poorly understood.

Conclusion: Unveiling the Complexity of Novae

Novae, once thought of as singular, brief explosions, are revealed to be dynamic systems governed by multiple ejecta interactions, shock wave physics, and binary orbital mechanics. The CHARA Array’s interferometric imaging has opened a new window into these transient events, offering unprecedented clarity on the early, hidden processes that shape some of the universe’s most dramatic stellar outbursts.

This research originated from WIRED Japan and has been translated from Japanese.