Epsilon Aurigae: The Largest Star?

Five decades ago, while I was in junior and senior high school, I became intensely interested in astronomy. I read all that I could find on the subject and, quite naturally, some of my reading involved trivia, such as extremes in size and distance. Some of the most amazing things about creation are the extremes. This is particularly true of astronomy. At that time, many astronomy sources identified Epsilon Aurigae as the largest star.

On a shelf, I have some books from my high school years. One, the third edition (1969) of Pictorial Astronomy1 says that Epsilon Aurigae is the largest star, giving its diameter as 2,700 times the diameter of the sun. Another source on that shelf, Astronomy Made Simple, carrying a 1958 printing date and a 1955 copyright, says that the largest star, Epsilon Aurigae, is 3,000 times the sun’s size. This tersely-written book was part of the Made Simple Self-Teaching Library. Apparently, 2–3 generations ago, this was the equivalent of the __________ for Dummies series.

Shortly after a new family moved into the neighborhood, an incident happened that I’ve never forgotten.

Shortly after a new family moved into the neighborhood, an incident happened that I’ve never forgotten. I was one of the last boys in the neighborhood to meet the two boys in the new neighbor family. Apparently, other boys in the neighborhood had already told them about me, because when Mike, the younger brother, met me, he said that he had heard that I knew a lot about astronomy. I guess that he thought that he knew a lot about astronomy too, because right away he tested my knowledge. Among his questions, he asked, “What’s the largest star?”

He seemed impressed when I immediately answered, “Epsilon Aurigae.” He knew the answer too, and must have thought that only someone very knowledgeable of astronomy would know that bit of trivia.

Technically, it wasn’t correct to call this star Epsilon Aurigae, because Epsilon Aurigae is a binary star. That is, Epsilon Aurigae consists of two stars orbiting one another under their mutual gravity. Astronomers usually refer to the components of a binary star using the capital letters A and B, with A usually referring to either the brighter or more massive star. It was Epsilon Aurigae B that was thought to be the monster. More specifically, Epsilon Aurigae is an eclipsing binary star, where we on earth lie close to the orbital plane of the two stars so that the stars periodically pass in front of one another, blocking each other’s light. These eclipses gain our attention because they cause an eclipsing binary star to periodically become fainter. We can learn a lot about eclipsing binary stars by studying their eclipses. For instance, how long the eclipses last reveals how large the components of the eclipsing binary star are, as in the case of Epsilon Aurigae.

When I was passing Mike’s impromptu quiz, little did I know that within a few years I would embark on a lifetime of studying eclipsing binary stars. In graduate school, I wrote a master’s thesis on eclipsing binary stars under a leading figure in the field. Even though my doctoral dissertation covered a different topic, I remain active in eclipsing binary research, even today.

The orbital period of Epsilon Aurigae is 27 years. Therefore, it is 27 years between eclipses, with each eclipse lasting about two years. Early in graduate school, I realized that in a few years (early 1980s) Epsilon Aurigae would go through another eclipse. This timing seemed to suggest that I should contribute to our understanding of this system.

After some background research on the system, I learned that wouldn’t be easy. The straightforward solution of the system resulted in Epsilon Aurigae B being very large. Normally a very large star is also very bright. But astronomers weren’t seeing much, if any, light from the large star. That made no sense. As a solution, astronomers posited that, instead of a large star, a very large dust cloud was orbiting Epsilon Aurigae. To some astronomers, that made even less sense, and some made other bizarre proposals. I soon realized that my limited resources and expertise probably weren’t up to the task.

In 1983 and again in 2010, astronomers observed the Epsilon Aurigae eclipse using many different instruments. The large dust cloud theory gained traction. Over the years, astronomers began to realize that instead of a very large, under-luminous supergiant star, Epsilon Aurigae B was a relatively hot, normal star surrounded by a large, opaque dust cloud rendering that star invisible to us. The very long eclipses were caused by the dust cloud, not the much smaller star hidden inside. Meanwhile, attention had turned to the star that we could see—Epsilon Aurigae A. All was not right with it. Was it a single massive supergiant, or was it something else? If it were a single star, it might be very massive, but if it were something else, it might have much less mass.

Epsilon Aurigae was recently in the news when the Gaia mission provided the first direct measurement of Epsilon Aurigae’s distance.

One possible something else was that the “normal star” might itself be a binary star in very close orbit to one another. In such close binary stars, mass may exchange between the components of the two stars. I’ve observed mass transfer like this in eclipsing binary stars. Mass transfer can alter the development of the stars involved in ways very different from single stars. This has been the preferred solution to the question of the nature of Epsilon Aurigae A. One problem was that, due to uncertainty in the distance of Epsilon Aurigae, the masses of the stars in the proposed close binary star were not known.

But Epsilon Aurigae was recently in the news when the Gaia mission provided the first direct measurement of Epsilon Aurigae’s distance. Now the masses have been pared down to 2.2 solar masses for Epsilon Aurigae B and 5.9 masses for Epsilon Aurigae A.

So it seems that at long last astronomers may have come to grips with just what is going on in the Epsilon Aurigae system—without my help.

Epsilon Aurigae is no longer the largest star. Currently, the title of the largest star goes to UY Scuti, about 1,700 times larger than the sun. If UY Scuti were placed as the center of the solar system, its surface would extend nearly to Saturn’s orbit. Mike, we need to talk.