Visual Observing of Epsilon Aurigae: The Art of Variable Star Observing By Chris Stephan

1. Visual Observing of Epsilon Aurigae: The Art of Variable Star Observing By Chris Stephan Science teacher in Highlands County, Florida

2. Variable star observing is a most enjoyable and fulfilling way to record and submit real data to professional astronomers. Our “Star” of this conference, Epsilon Aurigae is a type of variable star. Let’s learn how you can observe this star and teach your students how to observe it. It’s fun and is no where as difficult as you may think.

3. What is a “variable star”? Good question! A variable star is a star the changes, or varies in brightness over a certain period of time. They fall into two broad categories. Intrinsic stars vary due to something happening internally. Extrinsic stars vary due to something externally. Epsilon Aurigae is an eclipsing binary star, and these are extrinsic.

4. Let’s look at a few examples of intrinsic stars. This is Mira, in Cetus the Whale. Mira varies from 2nd to 10th magnitude in 332 days.

5. Courtesy Gordon Myers Chi Cygni from maximum to minimum light. Chi ranges from 3rd to 14th magnitude in 408 days.

6. Courtesy Gordon Myers Supernova SN2007LE. Clockwise from upper left. 09/02/2007, 11/04/2007, and 12/19/2007. This was an exploding star.

7. Extrinsic stars vary in brightness due to something outside of the star. Eclipsing binary stars are extrinsic. Eclipsing binary stars are systems made up of “two stars orbiting around a common center of mass in a plane along our line of sight”. One star eclipses the other, causing a change in light that we can see from earth.

8. This illustration courtesy of NASA shows what happens to the light during an eclipse. In the case of Epsilon Aurigae, the primary star is 300 times as big as our sun, and the distance between the stars is the distance between the Sun and Neptune.

9. How do variable stars get their names? The name of the variable star is usually made of one or two capital letters or a Greek letter, like “Epsilon”, followed by the constellation name. When letters are used up, names of variable stars begin with “V”, such as V477 Cygni, or V702 Scorpii. This system of naming variables began in the mid-1800’s.

10. OW Geminorum, let’s use it for some practice. How do you use this AAVSO chart? What do the numbers mean? Which direction is which? Hint- this star needs a telescope.

11. Some things to know. Variable stars have names like OW Geminorum, R Leonis, Epsilon Aurigae. Apparent magnitude is how we see the star from earth. Absolute magnitude is the actual brightness of the star. Magnitudes on the star charts are marked by numbers such as 80, 98, 108. The decimals are omitted so you do not confuse them with stars. So, 80 is really 8.0, 98 is 9.8, and 108 is 10.8. Smaller numbers are brighter, larger numbers are dimmer. So, the 8.0 is brighter than the 9.8 which is brighter than the 10.8. Larger dots are brighter stars. Directions are marked on the chart. The OW Gem chart has N at top, and E to the left. Knowing the directions are important for orienting the chart.

12. When you find your star, you estimate its brightness. Let’s say that OW is a little dimmer than the 90, but quite a bit brighter than the 99, you might give it a 92. Make sure you record the date and time of the observation to the nearest minute. Include the comparison stars in your report. During this conference you will be taught on how and where to submit your data.

13. OW Geminorum- sample eclipsing binary star Courtesy AAVSO

14. Now for the “Star” of our show! Epsilon Aurigae Why is it called “Epsilon Aurigae”?

15. The brightest stars in each constellation go by Greek letters. The brightest star in Aurigae, the Charioteer is Capella, or Alpha Aurigae. Epsilon is the 5th letter of the Greek alphabet, so the 5th brightest star is called Epsilon Aurigae. By the way, Aurigae is the Genitive form of the name. Simply, it is Auriga. The abbreviation is “Aur”, so our “Star” is Eps Aur.

16. Auriga the Charioteer. Capella is the very bright 0 magnitude star. Note the winter Milky Way.

17. Auriga is on the right, and Gemini is to the left. Where is our “Star” of the show in this photo and last?

18. Can you match the chart to the photo? North is up on both photos.

19. Epsilon is in the tick marks. The magnitude range is 2.9-3.8 magnitude. I suggest using the 26, 32, 38, and 43 comparison stars. Remember, decimals are omitted and 26 is the brightest, while 43 is the dimmest. If Epsilon appears just slightly dimmer than 32, but quite a bit brighter than 38, you might estimate it at 33 or 34. By the way, note the nice triangle made of Epsilon, the 32 and the 38 stars. Thispattern is called “The Kids”.

20. Here are two models of Epsilon Aurigae. The top one shows the opaque disk around the primary star, with the center of the disk somewhat transparent. The transparent “hole” allows some light to get through, thus the slight brightening at mid-eclipse. Both courtesy of AAVSO

21. Courtesy G. Samolyk and AAVSO This light curve from the last eclipse of 1982-84 shows the drop in magnitude, the flat minimum, the space where the slight brightening would be, and the rise back to maximum light.

22. You have all received the “10 Star Tutorial”. This manual is a great resource for information on Epsilon Aurigae. I highly recommend it as a guide to observing our “Star” for the next three years. By all means use the charts for some of the other practice stars in the Tutorial. These are real variable stars and your observations can be submitted to the AAVSO. The stars R Lyrae and Mu Cephei are great naked eye intrinsic variables. These are summer and autumn stars, so observing these can get you ready for Epsilon Aurigae. Beta Persei, Algol, is a great naked eye eclipser that can be observed in four to five hours, making observations every 15 minutes.

23. You can do it! You can be a Citizen Scientist and contribute your observations to astronomers and help them unwrap the “mystery” about Epsilon Aurigae. Are you up to it? Good luck and good observing to you. The famous American astronomer Harlow Shapley said this: “Great observations never die, but theories fade away”. I quote this several times a year to my science students. Courtesy AAVSO

24. Chris Stephan Part-time Education Specialist for Florida Atlantic University’s Center for Environmental Studies at the Riverwoods Field Lab in Highlands County, Florida.