半规则变星是光谱类型为中期和晚期的红巨星或超巨星，显示出可以观察到的周期性和光度变化，但有着各种各样的不规则性或中断的变化。变光周期从20天至超过2000天不等，同时光度曲线在每个周期内或多或少的都有一些变化。光度的变化范围从数百至数星等 (通常在可见光的范围是1-2星等)。

半规则变星可以分为几种类型：

光度变化的振幅范围在0.1至4等，而周期从30天至1100天。武仙座 SX和大熊座 SV是这种类型的例子。业余的观测者可以在M 13观察到一打左右光度在11.95至12.25等的红色变星，而周期在43天(V24)至97天(V43)之间。

Semiregular variable stars are giants or supergiants of intermediate and late spectral type showing considerable periodicity in their light changes, accompanied or sometimes interrupted by various irregularities. Periods lie in the range from 20 to more than 2000 days, while the shapes of the light curves may be rather different and variable with each cycle. The amplitudes may be from several hundredths to several magnitudes (usually 1-2 magnitudes in the V filter).