Guide to Alpha Herculis

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

Four year light curve for Alpha Herculis

With Alpha Herculis being a quite red star, magnitude estimates can differ by a few tenths of a magnitude between observers. Hence, rather than plot this light curve with magnitude values on the vertical scale (and risk confusing observers whose magnitude values don’t exactly match those shown) , the scale has been left blank.  Although some observers will routinely see Alpha Her brighter than will other observers, the hope is that each observer will 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 a few times each 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.