The Mysterious Epsilon Aurigae

The Mysterious Epsilon Aurigae
by
Jeff Hopkins
Hopkins Phoenix Observatory
February 2005

Variable stars are stars that we see vary in brightness over time. There are many causes for these variations. One class of variable stars is known as eclipsing binaries. Our Sun is a unique star in that it is a lone star. Many if not most stars have companions. These star systems have two, three and sometimes more stars all revolving in a very complex manner around a common center of gravity. These binary stars should not be confused with double stars. While some widely spaced binaries are indeed seen as double stars, many if not most doubles are only apparent and the stars are not part of the same star system.

Binary Star Systems
As binary stars revolve around their system's center of mass, some of these star systems have orbital planes that lie in line with the Earth. These systems allow us to make some unique and interesting observations. Because closely spaced binary stars cannot be resolved from Earth, even with largest telescopes, measuring the brightness of the star system as one star passes in front of another yields important data about the star system. When one star passes in front of another star, the combined light from the system is decrease as one star is blocking some or all of the light from the other star. In addition during the ingress and egress of the eclipse, one star's light passes through the outer shells or atmosphere of the eclipsing star yielding more information about the star. Making precise measurements using filters can yield information about the two star's spectral characteristics and which star is occulting which.

Short Period Eclipsing Binary Star Systems
Many thousands of eclipsing binary systems have been discovered. The periods of eclipse vary from hours to over a quarter of a century. The shortest period eclipsing binaries have stars that actually touch each other and complete an orbit in hours. One very famous short period eclipsing binary is Algol in Persus. During an evening of an eclipse with just unaided eyes, you can watch the star system dim from 2.1 to nearly 3.3 magnitude in just a few hours. In ancient times this star system caused awe and concern in the world. Many feared it and called it the "Wink of the Demon." On the other end of the scale is a mysterious star system known as epsilon Aurigae.

Long Period Eclipsing Binary Star Systems
Epsilon Aurigae (Almaaz) is a 3rd magnitude star system that under goes a total eclipse every 9885 days or 27.1 years dimming to nearly 3.8 magnitude and making it the longest known eclipsing binary system. Epsilon Aurigae is located 3 degrees from Capella (alpha Aurigae) in a small triangular group of stars known as "The Kids." Epsilon Aurigae is the northmost star in the group. Eta and zeta Aurigae are the other two stars in the base of the triangle. During the winter months epsilon Aurigae is nearly overhead around 8 to 9 PM in mid-northern hemisphere latitudes. The distance to the star system is not known very accurately. It is estimated to be between 600 and 1900 light years.

Auriga Star Chart

Holding the record as being the longest period eclipsing binary system is not the most interesting part of epsilon Aurigae. Normally the larger the orbit of an eclipsing star (the longer the period of the eclipse), the shorter the time the eclipsing body spends in line with the Earth and thus the shorter the eclipse duration. The eclipsing binary star system 32 Cygni varies from 4.0 to 4.2 magnitudes every 1148 days or 3.1 years. The eclipse lasts 11 days. Carrying that out to the epsilon Aurigae star system one would expect an eclipse to last less than 11 days, possibly even just hours.

Something Very Big
Surprise! The epsilon Aurigae star system's eclipse lasts nearly two years or 670 days. This means the eclipsing object is gigantic. The orbit of the eclipsing object is estimated at over 27 astronomical units. By contrast the orbit of Uranus around our Sun is about 20 astronomical units. The main star of the epsilon Aurigae star system is a giant F star with a mass of about 15 times our Sun and a diameter of 200 times. That means if that star were the Sun, Venus's orbit would be 27 million miles beneath the surface. The eclipsing object has an estimated mass of nearly 14 Suns and a radius of around 2,000 solar radii or some 2,000,000,000 miles in diameter making it the largest known single object in the universe. To further deepen the mystery in the middle of the eclipse the star system's brightness increases significantly for a short time. Many theories have been put forth as to what the eclipsing body is. A black hole or gigantic star are just a few. Most popular is the idea of a large cloud of dust or gas, but if that is true something massive must be embedded to hold the cloud together. Try as they might, astronomers have not been able to detect a star in the cloud. The only star that can be detected in the system is the main F star. One interesting theory is a binary pair of stars in the middle of the cloud going around and sweeping out a hole producing kind of a super large donut. The light going through this hole would produce the mid-eclipse brightening. While this seems to make sense, again, no other stars except the main F giant can be detected, photometrically or with spectroscopy.

Epsilon Aurigae Star System Model
Carroll et al. 1991 Ap.J. 367: 278

Epsilon Aurigae Campaign
The last eclipse was 1982 -1984. During that eclipse professional astronomers decided to try to crack the mystery . A world-wide campaign was set up to observe the system during the eclipse. Observations were made at major observatories around the world, amateur observatories and with space-based telescopes. A wealth of information was collected. At the end of the campaign in 1985 a workshop was held at the American Astronomical Society meeting in Tucson, Arizona. Several dozen astronomers presented papers. Despite all the information presented, the mystery still remains. What is the precise nature of the star system? What is the eclipsing object? See http://www.hposoft.com/Astro/PEP/EAurWS85.html

The next eclipse will begin in the late spring of 2009. Another campaign is planned. See http://www.du.edu/%7Erstencel/epsaur.htm

Most astronomers don't get the observing fever for the system until close to the start of the eclipse, however. As such few are interested in observing the system in detail between eclipses. The Hopkins Phoenix Observatory (HPO) has been actively observing the system photometrically in the ultraviolet, blue and visual (UBV) bands for many years since the last eclipse. While HPO made no observations during the 1990's several dozen observations were made after the eclipse in the 1980's. At HPO an intense observing program of the star system started in the fall of 2003. Plans are to continue this program through the next eclipse with hopes of adding longer wavelength observations, in particular the J and H bands. Anyone interested in providing photometric observations, single channel UBV photon counting, analog BVRI photodiode (e.g., the Optec SSP-3) or BVRI CCD photometry, please contact Jeff Hopkins at phxjeff@hposoft.com. The pre-eclipse data should provide a good baseline as to what the main star is doing. So far it turns out the star system is not quiet out-of-eclipse, but has significant variations and an apparent periodic increase in brightness. There may be a significant burst in brightness that occurs about every 625 days. The next burst should be in the late summer of 2005. There are also smaller periodic variations. Much more data are required to better understand and determine precise periods for these variations. If all goes well, the Hopkins Phoenix Observatory and any others interested in helping will provide a detailed baseline of data over multiple bands for the next eclipse.

HPO Astronomy Hopkins Phoenix Observatory Not so Amateur Astronomy! Astronomical Photoelectric Photometry Epsilon Aurigae
		Epsilon Auriage 1982-1984 Eclipse HPO UBV Photon Counting

The Mysterious Epsilon Aurigae by Jeff Hopkins Hopkins Phoenix Observatory February 2005 Variable stars are stars that we see vary in brightness over time. There are many causes for these variations. One class of variable stars is known as eclipsing binaries. Our Sun is a unique star in that it is a lone star. Many if not most stars have companions. These star systems have two, three and sometimes more stars all revolving in a very complex manner around a common center of gravity. These binary stars should not be confused with double stars. While some widely spaced binaries are indeed seen as double stars, many if not most doubles are only apparent and the stars are not part of the same star system. Binary Star Systems As binary stars revolve around their system's center of mass, some of these star systems have orbital planes that lie in line with the Earth. These systems allow us to make some unique and interesting observations. Because closely spaced binary stars cannot be resolved from Earth, even with largest telescopes, measuring the brightness of the star system as one star passes in front of another yields important data about the star system. When one star passes in front of another star, the combined light from the system is decrease as one star is blocking some or all of the light from the other star. In addition during the ingress and egress of the eclipse, one star's light passes through the outer shells or atmosphere of the eclipsing star yielding more information about the star. Making precise measurements using filters can yield information about the two star's spectral characteristics and which star is occulting which. Short Period Eclipsing Binary Star Systems Many thousands of eclipsing binary systems have been discovered. The periods of eclipse vary from hours to over a quarter of a century. The shortest period eclipsing binaries have stars that actually touch each other and complete an orbit in hours. One very famous short period eclipsing binary is Algol in Persus. During an evening of an eclipse with just unaided eyes, you can watch the star system dim from 2.1 to nearly 3.3 magnitude in just a few hours. In ancient times this star system caused awe and concern in the world. Many feared it and called it the "Wink of the Demon." On the other end of the scale is a mysterious star system known as epsilon Aurigae. Long Period Eclipsing Binary Star Systems Epsilon Aurigae (Almaaz) is a 3rd magnitude star system that under goes a total eclipse every 9885 days or 27.1 years dimming to nearly 3.8 magnitude and making it the longest known eclipsing binary system. Epsilon Aurigae is located 3 degrees from Capella (alpha Aurigae) in a small triangular group of stars known as "The Kids." Epsilon Aurigae is the northmost star in the group. Eta and zeta Aurigae are the other two stars in the base of the triangle. During the winter months epsilon Aurigae is nearly overhead around 8 to 9 PM in mid-northern hemisphere latitudes. The distance to the star system is not known very accurately. It is estimated to be between 600 and 1900 light years. Auriga Star Chart Holding the record as being the longest period eclipsing binary system is not the most interesting part of epsilon Aurigae. Normally the larger the orbit of an eclipsing star (the longer the period of the eclipse), the shorter the time the eclipsing body spends in line with the Earth and thus the shorter the eclipse duration. The eclipsing binary star system 32 Cygni varies from 4.0 to 4.2 magnitudes every 1148 days or 3.1 years. The eclipse lasts 11 days. Carrying that out to the epsilon Aurigae star system one would expect an eclipse to last less than 11 days, possibly even just hours. Something Very Big Surprise! The epsilon Aurigae star system's eclipse lasts nearly two years or 670 days. This means the eclipsing object is gigantic. The orbit of the eclipsing object is estimated at over 27 astronomical units. By contrast the orbit of Uranus around our Sun is about 20 astronomical units. The main star of the epsilon Aurigae star system is a giant F star with a mass of about 15 times our Sun and a diameter of 200 times. That means if that star were the Sun, Venus's orbit would be 27 million miles beneath the surface. The eclipsing object has an estimated mass of nearly 14 Suns and a radius of around 2,000 solar radii or some 2,000,000,000 miles in diameter making it the largest known single object in the universe. To further deepen the mystery in the middle of the eclipse the star system's brightness increases significantly for a short time. Many theories have been put forth as to what the eclipsing body is. A black hole or gigantic star are just a few. Most popular is the idea of a large cloud of dust or gas, but if that is true something massive must be embedded to hold the cloud together. Try as they might, astronomers have not been able to detect a star in the cloud. The only star that can be detected in the system is the main F star. One interesting theory is a binary pair of stars in the middle of the cloud going around and sweeping out a hole producing kind of a super large donut. The light going through this hole would produce the mid-eclipse brightening. While this seems to make sense, again, no other stars except the main F giant can be detected, photometrically or with spectroscopy. Epsilon Aurigae Star System Model Carroll et al. 1991 Ap.J. 367: 278 Epsilon Aurigae Campaign The last eclipse was 1982 -1984. During that eclipse professional astronomers decided to try to crack the mystery . A world-wide campaign was set up to observe the system during the eclipse. Observations were made at major observatories around the world, amateur observatories and with space-based telescopes. A wealth of information was collected. At the end of the campaign in 1985 a workshop was held at the American Astronomical Society meeting in Tucson, Arizona. Several dozen astronomers presented papers. Despite all the information presented, the mystery still remains. What is the precise nature of the star system? What is the eclipsing object? See http://www.hposoft.com/Astro/PEP/EAurWS85.html The next eclipse will begin in the late spring of 2009. Another campaign is planned. See http://www.du.edu/%7Erstencel/epsaur.htm Most astronomers don't get the observing fever for the system until close to the start of the eclipse, however. As such few are interested in observing the system in detail between eclipses. The Hopkins Phoenix Observatory (HPO) has been actively observing the system photometrically in the ultraviolet, blue and visual (UBV) bands for many years since the last eclipse. While HPO made no observations during the 1990's several dozen observations were made after the eclipse in the 1980's. At HPO an intense observing program of the star system started in the fall of 2003. Plans are to continue this program through the next eclipse with hopes of adding longer wavelength observations, in particular the J and H bands. Anyone interested in providing photometric observations, single channel UBV photon counting, analog BVRI photodiode (e.g., the Optec SSP-3) or BVRI CCD photometry, please contact Jeff Hopkins at phxjeff@hposoft.com. The pre-eclipse data should provide a good baseline as to what the main star is doing. So far it turns out the star system is not quiet out-of-eclipse, but has significant variations and an apparent periodic increase in brightness. There may be a significant burst in brightness that occurs about every 625 days. The next burst should be in the late summer of 2005. There are also smaller periodic variations. Much more data are required to better understand and determine precise periods for these variations. If all goes well, the Hopkins Phoenix Observatory and any others interested in helping will provide a detailed baseline of data over multiple bands for the next eclipse.

	Present Page Version as of 13 March 2005 phxjeff@hposoft.com www.hposoft.com