For centuries, humans have looked up at a bright star called Epsilon Aurigae and watched as it seemed to disappear into the night sky, slowly fading before coming back to life again. Today, as another dimming of the system is underway, mysteries about the star persist. Though astronomers know that Epsilon Aurigae is eclipsed by a dark companion object, the nature of both the star and object has remained unclear. Epsilon Aurigae is a star in the constellation Auriga. It is also known as Almaaz, Haldus, or Al Anz. Epsilon Aurigae is an eclipsing binary system. About every 27 years, Epsilon Aurigae's brightness drops from an apparent visual magnitude of +2.9 to +3.8. This dimming lasts about 700 days. The system lies approximately 2,000 light years from Earth.
For centuries, humans have looked up at a bright star called Epsilon Aurigae and watched as it seemed to disappear into the night sky, slowly fading before coming back to life again. Today, as another dimming of the system is underway, mysteries about the star persist. Though astronomers know that Epsilon Aurigae is eclipsed by a dark companion object, the nature of both the star and object has remained unclear.
Epsilon Aurigae is a star in the constellation Auriga. It is also known as Almaaz, Haldus, or Al Anz. Epsilon Aurigae is an eclipsing binary system. About every 27 years, Epsilon Aurigae's brightness drops from an apparent visual magnitude of +2.9 to +3.8. This dimming lasts about 700 days. The system lies approximately 2,000 light years from Earth.
!ADVERTISEMENT!Epsilon Aurigae was first suspected to be a variable star when German astronomer Johann Fritsch observed it in 1821. Later observations. Hans Ludendorff, however, was the first to study the star in great detail. His work revealed that the system was an eclipsing binary variable star, or a star that dims when its partner obscures its light.
The star is easily found because of its brightness and apparent proximity to the star Capella. It is the apex of the isosceles triangle formed as the 'nose' of the constellation Auriga. The star is bright enough to be seen from most urban locations with moderate amounts of light pollution.
New observations from the NASA's Spitzer Space Telescope, in combination with archived ultraviolet, visible and other infrared data, point to one of two competing theories, and a likely solution to this age old puzzle.
One theory holds that the bright star is a massive super giant, periodically eclipsed by two tight-knit stars inside a swirling, dusty disk.
The second theory holds that the bright star is in fact a dying star with a lot less mass, periodically eclipsed by just a single star inside a disk of debris. The Spitzer data strongly support this theory as being correct.
"We've really shifted the balance of the two competing theories," said Donald Hoard of NASA's Spitzer Science Center at the California Institute of Technology in Pasadena. "Now we can get busy working out all the details." Hoard presented the results at the 215th meeting of the American Astronomical Society in Washington.
Last August, Epsilon Aurigae began its roughly two-year dimming, an event that happens like clockwork every 27.1 years and results in the star fading in brightness by one-half.
Astronomers study these eclipsing binary events to learn more about the evolution of stars and ultimately how it may apply to our own sun. Because one star passes in front of another, additional information can be gleaned about the nature of the stars by how the two objects interact. Certain aspects of the event, for example the duration of the eclipse, and the presence of wiggles in the brightness of the system during the eclipse, have not fit nicely into earlier models.
The main uncertainty is the nature of the brighter star of the two objects. Its spectral features indicate that it's a large star, called an F super giant, with 20 times the mass, and up to 300 times the diameter, of our sun. Many different and sometimes odd scenarios have been proposed for the other object such as a black hole, a pair of dimmer B stars with an orbiting disk of dark debris, a single large dying star, and massive planets.
Hoard became interested in the problem from a technological standpoint. He wanted to see if the Spitzer telescope could be coaxed to observe it using a clever trick. "We pointed the star at the corner of four of Spitzer's pixels, instead of directly at one, to effectively reduce its sensitivity." The observation used exposures lasting only one hundredth of a second.
The resulting information, in combination with past Spitzer observations, represents the most complete infrared data set for the star to date. They confirm the presence of the companion star's disk, without a doubt, and establish the particle sizes as being relatively large like gravel rather than like fine dust.
The radius of the disk could be also determined as about four times the distance between Earth and the sun. This enabled the team to create a model that explained all the features of the system. If they assumed the F star was actually a much less massive, dying star, and they also assumed that the eclipsing object was a single B star embedded in the dusty disk, everything snapped together.
"It was amazing how everything fell into place so neatly," said Steve Howell of the National Optical Astronomy Observatory in Tucson, Ariz. "All the features of this system are interlinked, so if you tinker with one, you have to change another. It's been hard to get everything to fall together perfectly until now."
According to the astronomers, there are still many more details to figure out. The ongoing observations of the current eclipse should provide the final clues needed to put this mystery of the night sky to rest.
For more information about Spitzer, visit http://www.spitzer.caltech.edu/spitzer and http://www.nasa.gov/spitzer.
