Epsilon Aurigae (commonly called Almaaz or Maaz) is a bright star in the constellation Auriga notable for its unusual eclipses. Visible to the unaided eye at most times, it is the primary member of an eclipsing binary system whose photometric dips last unusually long compared with typical stellar eclipses.

Basic characteristics

The visible primary is a luminous yellow supergiant of spectral type roughly F (often listed as F0Ia). It is orbited by a compact, massive object enshrouded in a large, opaque disk of dust and gas. The system exhibits an eclipse roughly once every 27 years; each eclipse persists for about two years, producing a gradual dimming and recovery rather than a short, steep transit.

What causes the eclipse

The long, deep dimmings are caused not by a simple stellar companion but by a thick circumstellar disk surrounding the unseen secondary. That disk absorbs visible light while emitting strongly at infrared wavelengths, producing an infrared excess when observed outside eclipse. Interferometric imaging and multiwavelength monitoring during recent eclipses have resolved the disk’s silhouette and tracked its motion across the face of the supergiant.

History of study and changing models

Observers have noted Epsilon Aurigae’s strange behavior since the 19th century. For many decades astronomers debated geometries and masses: early models ranged from a swollen single star to exotic compact objects. A now-disfavored suggestion at one time invoked a black hole; today the consensus favors a B-type star or binary pair embedded within the dusty disk. High-resolution interferometry during the 2009–2011 eclipse provided key images that overturned older, simpler models.

Importance and open questions

Epsilon Aurigae remains important because it provides a nearby laboratory for studying circumbinary disks, disk evolution, and interactions between massive stars and dusty environments. Open questions include the disk’s internal structure, the exact nature and mass of the central object inside the disk, and how the system evolved to its current configuration. Professional and amateur observers continue to monitor brightness, spectra, and infrared emission between eclipses.

Further reading and resources