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Guide to Alpha Herculis

A slowly changing variable star that is observable with the naked eye from reasonably dark observing sites.

With Alpha Herculis being a quite red star, magnitude estimates can differ by a few tenths of a magnitude between observers, so don't worry if you saw Alpha Her slightly brighter or slightly fainter than shown in the above light curve. Over the course of a year, each observer should see a similar pattern of rises and falls in brightness.

Alpha Herculis is a red supergiant star. It doesn't show huge changes in brightness but, with care, its brightness changes can be followed visually. The brightness changes are due to pulsations in the star's cool outer layers.

Extreme brightness range 3.0 - 4.0
More typical amplitude about 0.4 mag
Period of variation Some sources suggest 100 days ; others suggest around 300 days
Frequency of observation Worth checking 2 or 3 times per month
Observe using Naked eye from reasonably dark observing sites. May need to use 40mm or 50mm binoculars if light pollution is a problem.
Visibility Can be observed all year round, but is not visible in the evening sky from December to April

The finder chart which follows show the location of Alpha Herculis.

You can follow the brightness changes of Alpha Herculis by comparing it with the lettered comparison stars.