The "Star" of Our Project

The "Star" of Our Project

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The “Star” Of Our Project is epsilon Aurigae, a mysterious, bright, eclipsing binary variable star. Below is a list of questions (and answers!) that you may have about this star starting with the basics and progressing to a little more advanced concepts.

How do you say epsilon Aurigae?

ep’ si lon  Au ry’ gee

Listen to the pronunciation here.

Aurigae, the Charioteer

Where is epsilon Aurigae?

Epsilon Aurigae (eps Aur) is a bright star located in the constellation Auriga, the charioteer.

What type of star is epsilon Aurigae?

Epsilon Aurigae is an eclipsing binary variable star.  Variable stars change in brightness over time. Collecting data on these changes can help us understand the star.  Eclipsing binary variable stars are systems consisting of two stars orbiting around their common center of mass in a plane along our line of sight.  So imagine 2 stars circling around each other on an invisible Frisbee. Now imagine holding the invisible Frisbee up and looking at the 2 stars on it from the edge of the Frisbee.  From time to time one star will get in the way and block your view of the other star – this is an eclipse.  When one star eclipses the other it blocks the light shining from that eclipsed star, so the total light shining from the two stars is less during the eclipse.  Now imagine that the invisible Frisbee is all the way across the park, still side-on.  You may not be able to tell that there are 2 separate stars circling around on it, but you can still see that during the eclipse the overall brightness fades.

What is a light curve and what does epsilon Aurigae’s light curve look like?

A light curve is a graph of brightness versus time for a variable star. So as time moves along the graph from left to right the data points will move up and down based on whether the star is getting brighter or fainter.  The shape of the light curve for an eclipsing binary star system depends on a few things: 1) the difference in brightness between the two circling stars, 2) the difference in size between the two stars, and 3) their orbital inclination as seen from Earth.  (This last one is basically a measurement of how tipped the imaginary Frisbee is.  Is it exactly edge on or is it angled a little?) Here is a light curve showing how epsilon Aurigae faded and regained its brightness during its last eclipse.

What can we learn about eclipsing binaries by studying their light curves?
  • p = Period (How long it takes for the two stars to make one full orbit.)
  • i = Orbital Inclination (How tipped the imaginary Frisbee is - see question above.)
  • M1, M2 = Masses of the Stars (How much matter makes up each of the 2 stars.)
  • L1, L2 = Luminosities of the Stars (How bright each of the 2 stars is.)
  • R1, R2 = Radii of the Stars (The distance from the center of the star to the edge - a measure of the size of each star.)
What is the period of epsilon Aurigae?
27.1 years
How long does the eclipse last?
Between 640 and 730 days.
When was epsilon Aurigae discovered?
Johann Fritsch was the first to note the variability of epsilon Aurigae in early 1821, when the star was likely in the midst of a deep eclipse.  The German astronomers Argelander and Heis both began "regular" observing once every few years around 1842-1843, and the data from both men showed that the star became significantly fainter around 1847.  Observers later in the 19th Century recorded another dimming event in 1874-1875, and another in 1901-1902.
So what’s the mystery?
Although they didn't know it at the time, what these 19th century astronomers had observed was an extremely long-period eclipsing binary, and one that was interacting as well. In 1928, Harlow Shapley correctly concluded that the two stars were about equal in mass. Based on this information they should be about equal in brightness as well.  But the spectrum of the system showed no light from the companion at all.  The visibly bright first star (called the primary) was being eclipsed by a massive, invisible second star (called the secondary.)
What could the mysterious invisible secondary be?

A 1937 paper by three of the greats of observational astronomy, Gerard Kuiper, Otto Struve, and Bengt Strömgren, suggested the system was an eclipsing binary composed of an F2 star and an extremely cool and tenuous star that they described as "semitransparent".  According to this model, the F star was being eclipsed by this ‘transparent shell star’, and its light was scattered by the extremely thin atmosphere of the eclipsing star.

A 1965 paper by Su-Shu Huang introduced the suggestion of an edge-on thick disk as the eclipsing body. In 1971, Robert Wilson introduced a tilted, thin disk with a central opening, suggesting that this model could most easily describe all of the observed effects of the eclipses, particularly the mid-eclipse re-brightening.

There is a slight brightening during mid-eclipse, suggesting the disk has a hole in it which the F star shines through.  The central brightening was stronger in 1954-56 than in earlier eclipses. It is possible that the hole is growing.  The time of minimum light lengthened by about 64 days while the overall duration of the eclipse had decreased by 44 days!

What happened during the most recent eclipse? Has the trend continued?

During the 1982-84 eclipse the central brightening was the brightest ever.  The duration of minimum was the longest, and the fading and brightening happened fastest.  The F star’s companion is changing on timescales of decades.  From 1901 to 1983 the time of minimum has increased from 313 to 445 days.  The overall eclipse duration has declined from 727 to 640 days.

What about between eclipses?
Precise measurements out of eclipse revealed a quasi-periodic low amplitude variation of 96 days from 1984-87.  During the 2003-2004 observing season this variation had sped up to 71 days.  In 2007-2008 the period became 65 days.
What is the nature of the object or objects at the center of the disk?
It could be two B type stars in a tight orbit. This would account for the mass with less luminosity than 1 larger star.  A pair of stars would act as a gravitational eggbeater, keeping the center of the disk clear.
Is there a giant planet involved?
One or more proto-hot-Jupiters would affect the distribution of matter in the disk.  A hot Jupiter spiraling inward to meet its death might account for the low amplitude variations and their decreasing periodicity.
What is the scale of this system?

The primary is 300 times the diameter of our Sun! The secondary orbits almost at the distance of Neptune from the Sun. Both components are 14-15 solar masses.


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