A binary star is a system of two stars that orbit around their center of mass.  In an eclipsing binary, the orbit is aligned so that one star passes in front of the other along our line of sight.  This causes a periodic dimming in light as the stars pass in front of each other.  The CHARA Array can study eclipsing binaries by spatially resolving the component stars.


Epsilon Aurigae

Astronomers used the CHARA Array to image the eclipse of the star Epsilon Aurigae by its mysterious, less luminous companion star. Epsilon Aurigae has been known since 1821 as an eclipsing double star system, but astronomers have struggled for many decades trying to decipher the clues to what was causing these eclipses, which happen every 27 years. The images of the eclipse in 2009-2010 largely settle the matter: the eclipse is caused by a disk of material, probably similar to the state of our solar system 4.5 billion years ago as the planets began to form around our own infant sun.


Images of Epsilon Aurigae being eclipsed by a companion star enshrouded by an opaque disk.  Image credit: Kloppenborg et al. 2010, Nature, 464, 870


The images of Epsilon Aurigae show the intrusion of an apparently wedge-shaped structure across the face of a huge star, nearly 150 times the size of our sun. The images of the star and wedge-shaped structure show the direct motion over the course of the eclipse, yielding a measurement of the relative masses of the components. The primary star itself is thought to be in a very interesting phase of its own evolution, turning out to be less massive than the eclipsing disk and the star hidden at the center of that disk.

Because astronomers hadn't observed much light from the faint companion, the prevailing opinion labeled it a smaller star orbited edge-on by a thick disk of dust. The theory held that the disk's orbit must be in precisely the same plane as the dark object's orbit around the brighter star, and all of this had to be occurring in the same plane as Earth's vantage point. This would be an unlikely alignment, but it explained observations. The CHARA images show that this is indeed the case. A geometrically-thin, dark, dense, but partially-translucent cloud can be seen passing in front of Epsilon Aurigae.


Silhouette of the disk created by aligning the individual images of Epsilon Aurigae over time. Image credit: Kloppenborg et al. 2015, ApJS, 220, 14



Kloppenborg et al. 2010, Nature, 464, 870

Kloppenborg et al. 2015, ApJS, 220, 14


Beta Lyrae

Beta Lyrae is an interacting and eclipsing binary where material is gravitationally pulled away from one star and accretes on to the surface of the companion.  The system has an orbital period of 13 days.  Using the CHARA Array, researchers obtained images of beta Lyrae showing the mass donor and the thick disk surrounding the mass gainer.  The donor is brighter and appears elongated; this is the first direct detection of photospheric tidal distortion due to Roche lobe filling.  The disk component is fainter and more elongated than the donor.


Zhao et al. 2008, ApJ, 684, 95



Algol is a hierarchical triple system.  The inner pair has an orbital period of 2.87 days.  Mass is being transferred from the sub-giant secondary to the main-sequence primary in the inner binary.  Astronomers used the CHARA Array to map the orbits of the inner and outer pairs and to measure the angular sizes and mass ratios of the three components.  The inner and outer orbits are nearly perpendicular.



Baron et al. 2012, ApJ, 752, 20