Big News in Epsilon Aurigae Mystery

Astronomers today announced a significant advance in solving the long mystery of Epsilon Aurigae, an enigmatic star that, every 27.1 years, loses half its light for almost two years. The star has mystified astronomers for nearly two centuries despite the fact that it’s easily visible to the naked eye and has been intensively observed by professional and amateur astronomers for decades.

Epsilon Aurigae model
In this artist's concept, Epsilon Aurigae (the supergiant star at right) is starting to be eclipsed by the dust disk circling a single, much dimmer B star. A new model explains the decades-old paradoxes of this system by assuming that its stars are relatively old, not young.
NASA / JPL-Caltech
At the American Astronomical Society meeting in Washington, DC, Donald Hoard of Caltech described recent infrared observations from NASA’s Spitzer Space Telescope and a new model that apparently, for the first time, fully ties together the mountains of available data. “What our result has provided is a big-picture solution,” said Hoard. But he was quick to add, “There are still a lot of details that need to be worked out.”

Prior to the most recent dimming, which began in August 2009, astronomers had built up a picture of the system in which the visible star, a type-F supergiant, is much more massive than the Sun. That's what its extreme luminosity (130,000 times the Sun's brightness) would suggest. Every 27.1 years, a huge dusty disk seen almost edge-on slides across the face of the star, producing the long-lasting partial eclipse. But big questions remained about the nature of the bright star, the eclipsing disk, and especially the unseen massive object or objects that must occupy the disk's center (Sky & Telescope, May 2009, page 58).

The Spitzer observations, combined with many observations at visible and ultraviolet wavelengths, provide a more complete picture of the system. The Spitzer work conclusively reveals the presence of a disk about 8 astronomical units in diameter, as expected, and also shows that it consists of relatively large particles mostly the size of sand grains, not the usual microscopically fine space dust. Far-ultraviolet observations also indicate the presence of a smaller, very hot star at the center of the disk, probably spectral type B (about three times hotter than the Sun).

The problem with this model has been that the disk's central object seems to have about the same mass as the F supergiant, and if it's a normal star with such a high mass, it should shine about equally bright. But we hardly see it at all — even though the center of the disk seems to be clear.

Epsilon Aurigae is at the center of the sky area in this moonlit Himalayan scene by Babak A. Tafreshi. The peak of Mount Everest is just to the lower right of bright Capella.
Babak A. Tafreshi
Hoard and his colleagues have proposed a model they say fits all the observations. The key part is that the bright F supergiant is much less massive than previously thought. It could still shine so powerfully if it is very far evolved and nearing the end of its life. In this scenario it started off with around 10 solar masses (as opposed to the 15 or 20 usually assumed for it) and has since blown off much of even that. The companion B star is then allowed to have only about 6 solar masses, and therefore shines much dimmer. In this scenario, the dark disk is not the sign of a newborn star still gathering material. The disk instead is made of material that the B star gravitationally captured from the dying primary star's wind.

The disk currently contains much less than an Earth mass, but it probably began with much more. Its original gas and microscopic dust grains have been blown out of the system, leaving only the larger grains behind. The system itself is probably about 10 million years old. Over the next thousands of years, the dying F star will puff off most of its remaining mass to form a planetary nebula.

“All of these intertwined parameters just sort of work out,” says Hoard, who adds that he previously favored another model. “I’m a convert to this model, but I am very comfortable with it.”

Arne Henden, director of the American Association of Variable Star Observers, emphasizes that the mystery of Epsilon Aurigae has not yet been solved. “We’re nearing the middle of the eclipse, and lots of interesting things will happen over the next year. There are still things about this system we don’t understand.”

Hoard and Henden both point out that the AAVSO has organized a global network of “citizen scientists” to monitor Espilon Aurigae’s changing brightness and spectrum. The massive amount of information from high-quality amateur observations (photometry and spectroscopy), as well as continued professional observations, should finally solve the mystery of Epsilon Aurigae. “If there is any time we will understand this system, it’s with this current round of observations,” says Henden.

11 thoughts on “Big News in Epsilon Aurigae Mystery

  1. Henrik Lovén

    If this model is indeed corret, it raises the following questions in my mind:

    a) By which process can the disk block out close to 100% of the secondary’s light (across all wavelengths?), yet only some 40% of the primary’s even though the latter has to pass through twice the thickness of disk to reach us?

    b) How can the disk mass as little as “much less than an Earth mass” given a), the composition proposed and the fact that it must occupy a volume of space of (at least?) 25 cubic AUs?

    c) Can the brightening at mid-eclipse and the spectral changes be explained as the sum of € Aurigae A + B (taking into account which part(-s) of the spectrum the light of B permeates)? If so, a ball-park figure for the absorption of the disk can be arrived at, which takes us back to a)

    Thank you!

  2. Jeff Hopkins

    The article by Alan McRoberts has some errors. The image shown is wrong and not at all proportional. THe F star from that perspective would be much smaller.
    The “real” Campaign to observe this star system is the International Epsilon Aurigae Campaign 2009, see
    We had a similar Campaign during the 1982 eclipse. This Campaign was started in 2006 and has thousands of data contributed, both photometric and spectroscopic. We have published 15 Newsletters with #16 due out shortly. THese are available free on the web site. The latest V Band photometric data plot can be see at,
    We have no connections with the AAVSO or Citizen Sky which are mainly just visual observational data and are of little value for this project.
    I encourage anyone interest in this star system to visit our web page and join the Campaign.

  3. Arne Henden

    There is no such thing as a “real” or official campaign on epsilon Aurigae; any researcher can run a campaign on this star as he/she wishes. I think Jeff is mistaken if he feels that the AAVSO, or Citizen Sky in particular, is mainly just visual observations, and is uninformed if he feels that visual data is of little value. This is not the place to argue such points; I strongly suggest that readers look at all avenues when thinking about getting involved in a campaign and make their own decisions as to where they want to contribute. Readers should primarily note that the Spitzer observations are the important item here, and the “light” that they shed on the nature of this system!

  4. Robin Leadbeater

    The continued presence of light from the F star during eclipse whereas the B star remains mostly hidden is most likely explained by the relative sizes of the two stars. although low mass, the post AGB star is very much larger than the width of the disc. The disc blocks (almost) all the light from the F star but only from the part of the star that it occults. The much smaller B star can be completely hidden within the width of the ecliipsing disc.

  5. illexsquid

    I don’t have enough knowledge to make an informed comment on this, but I wonder if anyone can provide insight. It seems to me that a dust disk of the sort presumed for epsilon Aurigae’s companion closely resembles a scaled-up version of the planetary rings we see in our solar system. Would not the particles quickly (short time compared to history of the system) evolve to a narrow range of orbital inclinations, like rings? I know this field is well-researched if not yet well-understood. In any event, the “puffy” nebulous appearance of the artist’s rendition seems unlikely.

  6. Tom Whiting

    Which brings up another question, as in astronomy, there
    are very few unique items. Could there not be more stars
    like Epsilon Aurigae which maybe aren’t edge on to our
    line of sight? Time to start on a new hunt for other
    similar double stars like epsilon.

  7. Enrico the Gr eat

    Kudos, all very good points, and a fascinating article, looking forward to the follow up articles on this fascinating system.
    Jusdt imagine seeing this system from, say an asteroid in a far flung orbit around the system’s center of mass!

  8. Enrico the Gr eat

    Kudos, all very good points, and a fascinating article, looking forward to the follow up articles on this fascinating system.
    Jusdt imagine seeing this system from, say an asteroid in a far flung orbit around the system’s center of mass!

  9. Henrik Lovén

    In the proposed model, the total age for the system is given as 10 million years. The ring itself is supposedly formed by “particles mostly the size of sand grains” that were captured from the stellar wind of the dying primary.

    What’s wrong with this? The time factor! How long would it take for the stellar wind ejecta to coalesce into “sand grains” and create a “Saturnian” disc 8 au in diameter and appx 20 solar diameters in width?

    Since the main star would not start to eject huge amounts of matter until it had evolved to the red supergiant stage, not more than something on the order of a million of years would be available for the disc to form and acquire its present shape and composition. This seems far too short a period of time to me.

  10. Andrew

    So the star has the disk because it captured the other star’s winds but doesn’t have the smaller grains because they were blown away. Either I’m understanding something wrong or there is something wrong. But as Hoard says: “There are still a lot of details that need to be worked out.

    And Henrik, for your question “b,” I would like to voice my opinion: the dust is more spread out than the Earth’s mass is here. However, that raises the question of if it could still block light with that kind of density.

  11. Stephen Anderle

    what if the eclipsing object were a neutron star or black hole with a dust disk around it or even a black hole with no disk. would not this explain the brightening during the middle of the eclipse? the black hole would initially bend the light away from our site, then in the mid eclipse focus the other , epsilon aurigae,s light mostly to us giving a brightening effect in the middle. now if it also had a disk far enough away from it that would explain the slow rate of dimming initially, and the variation between first dimming and the slow brightening after the middle. no matter falls on the black hole so it doesnt show up, but as it passes across the primary star the increase, quick dimming , then increased brightness then fading to the eclipse normal, all seem to show something with an intense magnifying effect.