Mystery of eclipsing star near solution?

Press, 13 April 1984, Page 22

Mystery of eclipsing star near solution?

By

JOY ASCHENBACH,

National

Geographic News Service

Every 27 years it happens. Some mysterious object comes between the supergiant star, Epsilon Aurigae, and Earth. It cannot be seen — the invisible body leaves no trace of itself — but you know it is out there because the star’s brightness slowly dims. The eclipse,of Epsilon Aurigae — 250 times bigger than the sun and 50,000 times as bright — is one of the classical mysteries of modern astronomy. Among the longest of the known eclipses, lasting nearly two years and occurring only every 27.1 years, it has bedeviled astronomers throughout the twentieth century. Is the star’s dark companion a swarm of meteorites, gas clouds, a hot star in a cold shell, a planetforming disc, a black hole? At varying times since the eclipsing pattern was first recognized in 1903, scientists have speculated about all of these. Today, the most likely possibility appears to be a developing planetary system. Epsilon Aurigae is again in eclipse and is not expected to return to full brightness until May. “I think chances are better than ever this time that we will discover what’s going on,” says Dr Robert E. Stencel. “We have all the advantages in terms of new technology. And it’s our last chance to clear up this mystery before the century ends. The next eclipse won’t start until 2009.” Dr Stencel, of the astrophysics division of the National Aeronautics and Space Administration, is a coordinator of the worldwide eclipse-monitoring campaign. The long time between eclipses, along with the invisibility of the object, makes Epsilon Aurigae “difficult for mere mortals to study. By the time just two eclipses have gone by, your career as an astronomer is about over,” Dr Stencel adds. What scientists know so far is that this eclipse of a binary star system is like no other. Usually there are two stars that simply

move in front of each other. This eclipsing body seems to be a very cool, spinning, dense disc of gas and dust that is extremely large, taking almost two years to move past the star. During the more than a year that it is completely across the star — blocking out about half its surface area — it cuts the star’s brightness by half. The blinking or dimming was first noticed by a German amateur astronomer in 1821, although the precise 27-year pattern was not recognised until early this century. The 1982-84 eclipse is the seventh known. Normally, the primary star in the Epsilon Aurigae system is among the brightest in our galaxy. It is a massive, Fl supergiant, visible from Earth even though it is about 2000 light years away. (One light year, the distance light travels in a year, is 5.8 trillion miles.) Part of the constellation Auriga, the star can be seen with the unaided eye in the northern sky, not far from Gemini and Taurus. A careful observer will detect its slow brightening from June. The price of being so big and bright is burning out quicker. The star probably has a life span of only a few hundred million years, compared with the sun’s eight billion to 10 billion years, says Dr Stencel The fact that it is relatively short-lived could mean that its companion has not yet formed, he notes, or that it somehow is preventing its companion from forming. At least 55 professional and amateur astronomers in about a dozen countries have telescopes pointed toward Epsilon Aurigae to try to detect its secret companion. Besides ground-based instruments taking photometric, spectroscopic, polarimetric, and infrared measurements, satellites also are making ultraviolet and infrared observations.

The eclipse of Epsilon Aurigae has astronomers baffled about what comes between the star and Earth only every 27 years. This drawing, based on observations of the current eclipse by James C. Kemp, shows a disc at totality as it obscured half of the star. Dr Kemp of the University of Oregon believes the disc may be a developing planetary system.

“It has to be a protoplanetary system. I don’t know how it could be anything else,” says the astronomer, James C. Kemp, of the University of Oregon, who bases his statement on preliminary results of his monitoring of the polarised light. Professor Kemp and his assistants have been watching Epsilon Aurigae since late July, 1982, through telescopes at the Pine Mountain Observatory, perched 6300 feet up in the mountains of Oregon’s central desert region. “The pattern of polarised light indicates that the eclipsing body is a highly organised dense disc strongly condensed and probably rapidly spinning,” he says. “Presumably there is a star buried in the middle that holds it together. Given what we know of the formation of planets, this disc seems to have the right properties. “It may be in the process of condensing, shrinking, and cooling into cooler gaseous blobs that may further condense into planets — at the same time becoming flatter

and flatter.” His studies indicate that the disc is about 100 million miles thick, with a billion-mile diameter. However, preliminary infrared measurements by Dana E. Backman, of the University of Hawaii, show the disc to be somewhat thicker. Epsilon Aurigae’s mysterious companion was first thought to be disc-shaped and dense by the astronomer, Su-Shu Huang, of Northwestern University, in 1965. An orbiting infrared telescope, which last year provided the first direct evidence of other solar systems, is expected to give clues about Epsilon Aurigae when further data are analysed. The Infrared Astronomy Satellite found twopotential planetary systems, around the stars Vega and Fomalhaut. Barring some astronomical surprises, Dr Stencel believes scientists should be able to say the same thing about Epsilon Aurigae. “I hope those studying the eclipse of 2009 to 2011 will be affirming what we have discovered,” he says.