The Toronto Centre Variable Star Challenge

Geoff Gaherty




Variable stars are stars which vary in brightness over time, for either physical reasons (e.g. one star of a close binary eclipsing another) or due to astrophysical processes occurring within the star itself. This is an area where amateurs with very simple equipment can contribute observations which are used by professional astronomers around the world.

There are currently fewer than two dozen variable star observers in Canada. Recently Richard Huziak of the Saskatoon Centre issued a challenge to RASC members to increase that number to 100. To do its part, the Toronto Centre is challenging its members to join in this fun activity. So far seven members have agreed to try to observe variable stars, but, as the largest centre in Canada, I feel we should be able to double that!

Why? It’s fun, easy, satisfying, and exciting, because you rarely know in advance what you’re going to see. Although most variable stars change rather slowly, some may be observed to vary over a few hours or even minutes.
When? Any time it’s clear. A full Moon has only minor effects on the ability to observe variable stars.
Where? Anywhere, even from a light-polluted urban back yard like mine!
What? All you need is a telescope (or even binoculars for the brighter stars) and charts for your stars. Any telescope can be used, and the charts are available free on-line from: http://charts.aavso.org/
How? There are four steps:
     1. Locate the star using the charts.
     2. Compare its brightness to the comparison stars on the charts.
     3. Record the date, time, brightness, and the comparison stars and chart used.
     4. Report the observation to the American Association of Variable Star Observers using their on-line WebObs system.

The key to all this is the AAVSO web site ( http://www.aavso.org/ ), which contains everything you need to become an active participant. There are charts for thousands of variable stars available there. I initially found this intimidating until I narrowed down to a list of five stars which are reasonably bright and will be visible over the next few months:
 
Star R.A. Dec. SA Near Charts Range
T Cassiopeia 00h23m14s +55°47.5’ 1 Lambda BD 7.3-12.6
R Andromeda 00h24m02s +38°34.6’ 4 Rho ABD 6.1-15.3
Omicron Ceti 02h19m21s -02°58.4’ 10 Delta AB 3.6-9.1
R Trianguli 02h37m01s +34°16.1’ 4 15 ABD 6.0-11.5
T Arietis 02h42m45s +17°05.5’ 4 Pi BD 8.3-10.9

All five stars are marked with open circles in the Sky Atlas 2000.0 on the charts indicated in column "SA" above. AAVSO charts are designated by letters: "A" for naked eye and binocular charts, "B" for low power telescope views, and "C" through "F" for increasingly high power, showing fainter and fainter stars. "A" charts usually have north at top; the others, for use at the telescope, have south at top, and are also available in "R" versions which are mirror images for people using star diagonals. The AAVSO thinks of everything! Choose a couple of stars from the list to start with (John Bohdanowicz and I have been working on the first two), and download the charts from the AAVSO site. You might also download the AAVSO observing manual at: http://www.aavso.org/cdata/manual/

Locating the star is no different from locating a deep sky object, except that you never know ahead of time how bright it may be. Initially this takes a lot of time, but once you’re familiar with the field, it becomes much easier. Find two or three comparison stars on the chart, which are close to the variable in brightness. The numbers on the charts are the brightness of the star to one decimal place, with the decimal omitted, so that it won’t be mistaken for a star. Examples might be "68" and "72", for stars of magnitude 6.8 and 7.2. If the variable falls exactly in between the two, you’d estimate it at 7.0; if it’s closer to the "68" star, make it 6.9. Record the star name, date and time in UT, magnitude estimate, brightnesses of comparison stars, and date and scale of the chart you used, for example:

R AND, 2002-10-06 01:18 UT, 9.7, 95, 97, 98, SD1999

This can then be entered directly into the AAVSO’s WebObs page, and 15 minutes later your observations will be on-line and available to anyone in the world! Before you can enter your observations, you need to register for WebObs and get your three-letter observer code; mine is GHT. All of this is free; you don’t have to join the AAVSO to access the whole site and become one of their observers.

That’s really all there is to it, except that you may find it such fun that you become seriously addicted, like I have!

2002-11-14