A star that blinked every 11 months helped overturn Aristotle
In 1638 Johannes Holwarda noticed that Omicron Ceti brightened and faded on an 11-month cycle. David Fabricius had taken it for a nova in 1596. Together with the supernovae of 1572 and 1604, that rhythmic star, later named Mira, showed the heavens were not the changeless realm Aristotle had taught, and fed the astronomical revolution.
A variable star is one whose brightness, as seen from Earth, changes over time. Some are intrinsic, genuinely changing their output by swelling and shrinking, flaring or exploding. Others are extrinsic, merely appearing to change because a companion passes in front or the star rotates. Even the Sun varies, by about 0.1 percent over its 11-year cycle, while a supernova can briefly outshine a whole galaxy. Of the 58,200 catalogued by 2023, pulsating stars number just under 30,000 and eclipsing ones over 10,000.
People noticed long before telescopes. An Egyptian calendar of lucky and unlucky days from some 3,200 years ago may record the eclipsing binary Algol, though the claim is disputed. Aboriginal Australians wove changes in Betelgeuse and Antares into oral stories. Geminiano Montanari described Algol's variations in 1669, and John Goodricke correctly explained them in 1784, the same year Edward Pigott found the first Cepheid, Eta Aquilae. Ten variables were known by 1786; photography from the mid-19th century, including a Harvard survey from 1885, sent the count soaring.
The breakthrough came in 1912, when Henrietta Swan Leavitt found that a Cepheid's period reveals its true brightness, turning these stars into cosmic measuring sticks. In 1924 Edwin Hubble spotted one in the Andromeda Nebula and proved it lay far beyond the Milky Way, settling the Great Debate over spiral nebulae. In the 1930s Arthur Eddington showed how pulsation works: as a star's outer layer expands and cools, its gas becomes more transparent and releases energy, so the star contracts, heats, turns opaque again, traps radiation and swells once more.
Naming follows a quirky scheme devised by Friedrich Argelander, starting with R in each constellation because Bayer never used it, then RR, SS and onward, skipping J, for 334 combinations before switching to numbers. Amateurs still contribute: the British Astronomical Association's Variable Star Section, founded in 1890, pools observers' brightness estimates for scientists.
Source: Variable star
