There is a class of stars that are not in this equilibrium state, and all the above parameters change to varying degrees, which may take the form of periodic pulsation, irregular generation or huge destructive explosion. Such stars are called unstable stars.

1. Pulsating variable star

The bright and dark change period of pulsating variable stars is called light change period. There are three main types of fluctuating variables: long-period fluctuating variables; Cepheid variable and RR variable in Lyra.

Cepheid variable star is particularly important. Its typical star is Cepheus delta star, which is called Cepheid No.1 in Chinese. Cepheid No.1 has a light period of 5 days, 8 hours, 46 minutes and 38 seconds, which is as accurate as a clock, and its brightness changes by 1.9 times.

2. Non-radial pulsation and special variable stars

3. Nova

If a star suddenly appears in a constellation without stars, or even reaches a high brightness, lasting for several months or a year, then gradually darkens and finally disappears, it is called a "new star" in the west and a "guest star" in ancient China.