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).

Semiregular variables are classified in several subtypes:

A variable star is a star whose brightness as seen from Earth (its apparent magnitude) fluctuates.

This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either:

Many, possibly most, stars have at least some variation in luminosity: the energy output of our Sun, for example, varies by about 0.1% over an 11-year solar cycle.

Discovery

An ancient Egyptian calendar of lucky and unlucky days composed some 3200 years ago may be the oldest preserved historical document of the discovery of a variable star, the eclipsing binary Algol.

Of the modern astronomers, the first variable star was identified in 1638 when Johannes Holwarda noticed that Omicron Ceti (later named Mira) pulsated in a cycle taking 11 months; the star had previously been described as a nova by David Fabricius in 1596. This discovery, combined with supernovae observed in 1572 and 1604, proved that the starry sky was not eternally invariable as Aristotle and other ancient philosophers had taught. In this way, the discovery of variable stars contributed to the astronomical revolution of the sixteenth and early seventeenth centuries.

Variable Star is a 2006 novel written by Spider Robinson based on the surviving seven pages of an eight-page 1955 novel outline by the late Robert A. Heinlein. The book is set in a divergent offshoot of Heinlein's Future History and contains many references to works by Heinlein and other authors. It describes the coming of age of a young musician who signs on to the crew of a starship as a way of escaping from a failed romance. Robinson posted a note on his website in 2009 noting that his agent had sold a trilogy of sequels based on the novel and its characters.

From Heinlein to Robinson

Robinson states in an appendix to the book that he was working from an outline that lacked an ending. He was told by his publisher that they wanted him to write in his own style, not Heinlein's, and the abundance of profanity and puns makes it clear that this is not a Heinlein novel. The outline is almost exactly contemporaneous with Heinlein's juvenile novel Time for the Stars, and shares many of its details, such as the use of faster-than-light telepathic communication between twins. Although Heinlein apparently wrote the outline for Variable Star to be used, like Time for the Stars, as part of his Scribner's juvenile series, Robinson's realization deals with a variety of topics, including drugs and sexuality, that would have been completely unacceptable for a juvenile novel in 1955. Heinlein's original title for Variable Star was The Stars are a Clock.