Fixed stars

The fixed stars (from the Latin "stellae fixae") are celestial objects that do not seem to move in relation to the other stars of the night sky. Hence, a fixed star is any star except for the Sun. A nebula or other starlike object may also be called a fixed star. People in many cultures have imagined that the stars form pictures in the sky called constellations. In ancient Greek astronomy, the stars were believed to exist on a giant sphere that revolved around the Earth daily.

The phrase originated in classical antiquity, when astronomers and natural philosophers divided the lights in the sky into two groups. One group contained the "fixed stars", which appear to rise and set but keep the same relative arrangement over time. The other group contained the naked eye planets, which they called "wandering stars". (The Sun and Moon were sometimes called planets as well.) The planets seem to move and change their position over short periods of time (weeks or months). They always seem to move within the band of stars called the zodiac by Westerners. The planets can also be distinguished from fixed stars because stars tend to twinkle, while planets appear to shine with a steady light.

In 1718 Edmund Halley announced his discovery that the fixed stars actually have their own motion called proper motion. The star with the largest known proper motion is Barnard's star. Proper motion was not noticed by ancient cultures because it requires precise measurements over long periods of time to notice. In fact, the night sky today looks very much as it did thousands of years ago, so much so that some modern constellations were first named by the Babylonians. Nevertheless, the phrase fixed star is technically incorrect, so it is rarely used except in a historical context.

Fixed stars do have parallax, which is a change in apparent position caused by the orbital motion of the Earth. This effect was small enough to not have been noticed until modern times. It can be used to find the distance to nearby stars.

The star catalogue compiled by Claudius Ptolemy in the 2nd century CE lists 1,022 fixed stars visible from Alexandria. This became the standard number of stars in Western culture for thousands of years. The total number of stars visible to the naked eye is about 6,000; only about half are visible at a given time of night from a given point on the Earth. They are all stars in the Milky Way, and they are actually at different distances from us. Most of the millions of stars in the Milky Way can only be detected with the aid of telescopes, or their existence inferred, because they are too faint or are obscured by dust, gases, and other stars.

See also

* Apparent brightness
* Firmament