The nature of the Epsilon Aurigae system is unclear. It has long been known to consist of at least two components which undergo periodic
eclipses with an unusual flat-bottomed dimming every 27 years. Early explanations with exceptionally large diffuse stars, black holes, and odd doughnut-shaped discs are no longer accepted. There are now two main explanations that can account for the known observed characteristics: a high mass model where the primary is a
yellow supergiant of around ; and a low mass model where the primary is about and a less luminous evolved star. Variations on the high mass model have always been popular, since the primary star is to all appearances a large supergiant star. Spectroscopically it is early F or late A with
luminosity class Ia or Iab. Distance estimates consistently lead to luminosities expected for a
bright supergiant, although there is a huge variation in published values for the distance. The Hipparcos parallax measurement has a margin of error as large as the value itself and so the derived distance is likely to be anything from 355 to 4,167
parsecs. The
Gaia Data Release 2 parallax is somewhat more precise, leading to a distance of , towards the low end of estimates by other methods. The
Gaia Data Release 3 suggest a higher distance of 1,062 parsecs, or 3,460 light-years. The main problem with the high mass model is the nature of the secondary, which is required by the known
mass function to have a mass comparable to the primary, at odds with observations where it appears as a
B-type main-sequence star. The secondary may be a close binary involving two lower-mass main sequence stars, or a more complex system. The low mass model, popularized by the Citizen Sky project, proposes that the primary is an evolved
asymptotic giant branch star of . This relies on distance and luminosity estimates lower than most observations. The star would be an unusually large and bright giant star for the given mass, possibly as the result of very high mass loss. To match the observed eclipse and orbital data, the secondary is a fairly normal B main sequence star of about embedded in a thick disc seen nearly edge on. The orbit itself is now fairly well determined, inclined at over 87 degrees to Earth. The primary and secondary are around 35 AU apart (in the high mass model), which is further than the planet
Neptune from the
Sun. In the low mass model, the separation is only 18 AU.
Visible component The visible component, Epsilon Aurigae A, is a
yellow supergiant star belonging to the
spectral class F0. This F-type star is 37,875 times more luminous than the Sun, but reliable sources vary considerably in their estimates of both quantities. Its
angular diameter was measured at , the physical size will depend on the distance. Assuming distances ranging from 600 to 1500
parsecs, the radius ranges from using the angular diameter. If the star were in the position of the Sun, it would envelop Mercury (at the smallest radius) to Mars (at the larger radius). F-type stars like Epsilon Aurigae tend to glow white and display strong ionized calcium absorption lines and weak hydrogen absorption lines; being a class above the Sun (which is a G-type star), F-type stars are typically hotter than sunlike stars. Other F-type stars include
Procyon's primary star, the brightest star in the constellation
Canis Minor. The supergiant pulsates, showing small variations in its brightness and spectral lines. The pulsations have been given periods of 67 and 123 days, with an amplitude of about 0.05 magnitudes. The profiles of many spectral lines show variations that would be expected from a pulsating supergiant, but whether they have the same period as the brightness variations is unclear. There may be a small variation in the
effective temperature of the
photosphere as the star pulsates.
Eclipsing component The eclipsing component emits a comparatively insignificant amount of light, and cannot be directly seen in visible light. A heated region, however, has been discovered in the center of the object. It is widely thought to be a dusty disc surrounding a class B
main sequence star. Modelling the
spectral energy distribution for ε Aurigae as a whole produces the best fit with a B5V star at the centre of the disc. Such a star would have a mass around . The observed orbit, assuming a fairly normal F-type supergiant for the primary star, requires a secondary with a mass over . The low mass model accepts the secondary and so also requires a low-mass primary. The high-mass model accepts a normal mass supergiant primary and argues for a pair of B-type stars, or an unusual single higher-mass star. The disc around the secondary star is 3.8 AU wide, 0.475 AU thick, and blocks about 70% of the light passing through it, allowing some light from the primary star to be seen even during the eclipses. It radiates like a 550 K
black body. The 2009–2011 eclipse was well observed and
CHARA array was able to directly image the shape of the disk in a silhouette. The secondary eclipse is predicted to occur at 2025 December 20–2028 March 29. ==Observation==