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Stellar activity is the changing behavior of a star, i.e., its transient phenomena. In contrast is its general, long-term behavior such as its average luminosity and spectrum. Our advantage in observing solar activity, the stellar activity of the Sun, gives us our best detailed information about some types of stellar activity, such as sunspots, solar flares, and coronal mass ejections, i.e., the causes of space weather, much of which is attributed to the variation of the Sun's magnetic field induced by the Sun's convection. Activity on other stars is presumed to include these, but the activity of other stars can be different in severity and frequency and can include severe activity not seen in the Sun, such as stellar pulsation.
Stellar activity is of interest to planetary science given its consequences during planet formation and the early life of planets: some kinds could affect atmospheric escape, an important factor regarding a planet's subsequent habitability. Stellar activity is also of interest because of the challenge it presents to the transit method used to detect and observe extra-solar planets and binary stars. It can also affect the radial velocity method. Generally, the less activity, the easier it is to detect any planets the star has.