Visual Observing of Epsilon Aurigae:

The Art of Variable Star Observing

By Chris StephanScience teacher in

Highlands County, Florida

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.

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.

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.

Chi Cygni from maximum to minimum light. Chi ranges from 3rd to 14th magnitude in 408

days.

Courtesy Gordon Myers

Supernova SN2007LE. Clockwise

from upper left. 09/02/2007,

11/04/2007, and 12/19/2007. This was

an exploding star.

Courtesy Gordon Myers

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.

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.

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.

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.

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.

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.

OW Geminorum- sample eclipsing binary star

Courtesy AAVSO

Now for the “Star” of our show!

Epsilon Aurigae

Why is it called “Epsilon Aurigae”?

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.

Auriga the Charioteer.

Capella is the very bright 0

magnitude star. Note the winter

Milky Way.

Auriga is on the right, and Gemini is to the left. Where is our “Star” of the

show in this photo and last?

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

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. This pattern is called “The

Kids”.

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

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.

Courtesy G. Samolyk and AAVSO

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.

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

Chris Stephan

Part-time Education Specialist for Florida Atlantic University’s

Center for Environmental Studies at the Riverwoods

Field Lab in Highlands County, Florida.