AAVSO: What Are Variable Star Charts

One of the most daunting tasks new observers encounter is actually locating variable stars in the sky. Although finding the region of the sky where the variable resides may be straightforward, the actual identification of the variable is often a learned skill involving patience, persistence, and a good "map" to observe by. To aid observers with the identification of the variable, AAVSO finding charts are available. These charts display the field of the variable, along with other pertinent information that may be useful when observing. Also shown on the charts are stars of known constant magnitude, referred to as comparison stars, which are used to make brightness estimates of the variable. (See the sample chart below.) With over 4000 charts, the AAVSO is the major source for charts of variable stars. All of these charts are currently available online and may be downloaded for at no charge, or may also be purchased for a fee through Headquarters.

Types of Charts

Note: When making variable star estimates for the AAVSO, we require observers to use AAVSO charts in order to avoid the conflict that can arise when magnitudes for the same comparison star are derived from different sets of charts. This could result in two different degrees of variation being recorded for the same star.

The AAVSO provides different types of charts tailored to meet the needs, experience, and programs of our observers:

Constellation Finder Charts

Constellation finder charts present wide-field plots that encompass an entire constellation with the location of bright stars and selected variables charted. Originally produced for the Hands-On Astrophysics educational project, these charts may also be of use to the beginner trying to find their way around the sky.

Special Charts

Special-purpose charts such as those used for observing eclipsing binary or RR Lyrae stars or for observers with photoelectric photometers or CCD cameras are also available. These charts are available by querying the chart database.

Chart Scales

The charts range in scale from 5 arcminutes per millimeter ('a' scale charts) to 2.5 arcseconds ('g' scale charts). The scales needed for your observing program will depend on the the equipment you are using. The table below summarizes this information:

A Sample AAVSO Chart

The information below is taken from the AAVSO Manual for Visual Observing of Variable Stars.

Click image to view the sample chart.

The figure to the right shows a typical AAVSO star chart with its features labeled. The heading of each chart contains quite a bit of information including the designation of the variable, a letter identifying the scale of the chart, and the name of the star. Below the variable's designation are: the range of variation in magnitude; period of variation; class of variable; and spectral type of the star. The position of the variable for the epochs 1900 (or sometimes for 1950) and 2000 are listed below the star's name. The chart itself corresponds to the earlier of the two epochs given (with the exception of some "b" charts). The coordinates for right ascension are in hours, minutes, and seconds, and those for declination are in degrees and minutes. The latest revision date for the chart is shown in the upper right-hand corner of the chart as well as the scale of the chart in seconds or minutes of arc per millimeter. Many older style charts give this information in a different format or may be incomplete. The stars on an AAVSO chart are shown as black dots on a white background. The sizes of the dots indicate relative brightness. Through a telescope, of course, the stars will appear as points.

The position of the variable is generally in the center of the field and is indicated by this symbol:

On some of the older charts, the variable may be indicated by a simple open circle, sometimes with a dot in the middle. In most cases, when more than one variable in the AAVSO program occurs on the chart, an additional heading is provided for each.

Surrounding the variable or variables are comparison stars. Comparison stars are stars of known constant brightness which are used to estimate the brightness of a variable. Such stars are recognizable by the fact that they have magnitudes associated with them. These magnitudes are determined to the nearest tenth of a magnitude, the decimal point being omitted to avoid possible confusion with star disks. For example, "8.6" would appear on the chart as "86". The numbers are placed to the right of the disk spot of the star wherever convenient, otherwise a short line connects disk and number.

Comparison star sequences (magnitudes) have been determined visually and photovisually at Harvard and elsewhere, except for stars measured by photoelectric photometry (underlined) and CCD (overlined). The sequence source is given at the bottom of the chart.

AAVSO Variable Star Atlas

The AAVSO Variable Star Atlas (Second Edition, 1990) may be used as approximate finder charts for locating a variable star, estimating the brightness of a comet, planet, minor planet, satellite or other human-made object in space, etc. It is a perfect companion to the AAVSO constellation finder charts and detailed individual-star charts because of its scale and content.

The Atlas is an extremely useful observing resource, particularly for anyone needing to estimate the magnitude of objects such as brighter variable stars and comets, because it shows magnitudes of many comparison stars - stars of known brightness that may be used to make magnitude estimates. The Atlas covers the entire sky down to visual magnitude 9.5, and includes the following additional objects:

all variable stars of 0.5-magnitude amplitude or more (as of 1990);
all variable stars in the AAVSO and Royal Astronomical Society of New Zealand (RASNZ) observing programs (as of 1990);
comparison star magnitudes for these variables (where space permits);
all Messier objects;
all galaxies in the Shapley-Ames Catalogue (Harvard Annals, vol. 88, no.2);
selected open and globular clusters;
selected diffuse and planetary nebulae.

Visit the publications ordering page to order an Atlas.

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