Excerpt:
Alhough there were occasional reports of new or guest stars in all cultures through the ages, medeval western astronomers looked at the "fixed" stars as eternal and invariable entities. This view was blown up in a dramatic show by Tycho's supernova ("Stella Nova", "New Star") in 1572. Soon after, some more variable stars were discovered, including the first periodic one, Mira (the periodicity of which was only discovered considerably later in 1638, by Holwarda; up to this time the four known variables had all been classified as "Stellae Novae", although none of them was actually a nova).