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