Field guide · Stars
Variable starVariable star
A star whose brightness changes — because it pulses, flares, or is eclipsed by a companion. Some change enough to watch by eye.
- Pulsating
- Mira, Betelgeuse, the Cepheids
- Eclipsing
- Algol, every 2.87 days
- Watched by
- Amateurs worldwide, by eye
Stars were long taken as the one unchanging thing in the sky. Mira, “the wonderful”, broke that in 1596, when David Fabricius saw it fade from easily visible to invisible over a few months. It keeps doing so every 332 days.
Some variables pulse: they swell and shrink, like Mira or Betelgeuse. Some are eclipsing pairs: every 2.87 days Algol dims for a few hours as a fainter companion passes in front of it — John Goodricke, a deaf teenager in York, worked out why in 1783.
One kind changed everything. In 1912 Henrietta Swan Leavitt found that Cepheid variables pulse at a rate that gives away their true brightness. Compare that with how bright one looks and you have its distance — the ruler Hubble used to measure the universe.
See it in the sky
Each one opens the sky map pointed at it, for tonight, from where you are.