Fixed stars

The fixed stars (from Latin stellae fixa ) are celestial objects that seem to move with respect to the other stars of the night sky. A fixed star is therefore any star except the Sun . A nebula or other star-shaped object can also be called fixed star. In different cultures have imagined that the stars are pictures in the sky called constellations . In the ancient Greek astronomy, the stars were believed to coexist in a celestial sphere or firmament , which revolved around daily in the Earth.

Origin of name

The phrase originated in the Classical Period , when the astronomers and philosophers natural celestial lights divided into two groups. One contained the fixed stars , which seemed to rise and set without changing the layout over time. The other comprised the naked eye planets , which called wandering stars . (The Sun and the Moon also sometimes called them planets.) The planets seemed to move and change its position within a short period of time (weeks or months), and appeared to move among the bands of stars called the zodiac . They can also be distinguished from the fixed stars because they tend to flash, while those seem to glow with a steady light.

The star catalog summarized by Claudius Ptolemy in the second century CE includes 1,022 fixed stars visible from Alexandria . This became the standard number of stars of Western culture for thousands of years. The number of stars visible to the naked eye is about 6,000, only about half are visible at a specific time of night and at a specific point on Earth. All are in the Milky Way and are at different distances from Earth. Most can only be detected by telescopes or indirectly inferring their existence, because either they are very pale or covered by interstellar gas, dust or other nearby stars.

The fixed stars do not are fixed

The fixed stars have parallax , a change in apparent position caused by Earth's orbital motion, which was observed only in modern times. Can be used to measure distances between nearby stars. This movement is only apparent.

However, the fixed stars exhibit real movement, which can be understood within the movement of the galaxy to which the star belongs, in part due to its rotation, and partly to the proper motion of the star.

The actual motion of the star movement is divided into radial and himself , the second component along the line of sight. In 1718 Edmund Halley enunciated his discovery that the fixed stars actually had proper motion. cultures old had not inferred because it requires precise measurements over long periods. In fact, the night sky today looks much as it did thousands of years ago, while some of the modern constellations had been named by the Babylonians .

The proper motion can be determined by measuring the position of a star on a selected and limited set of very distant objects that do not include mutual movement, which by its distance, can be assumed to have very little movement itself. can also be done otherwise comparing photographs of a star at different times with respect to a large background of more distant objects. The star with the largest known proper motion is Barnard .

The phrase fixed star is technically incorrect, but still used in historical contexts, and the mechanical classic.

The fixed stars in classical mechanics

In times of Newton the fixed stars were presented as a reference system assumption regarding the absolute space . In other systems of reference at least with respect to the fixed stars or even travel on these stars, Newton's Laws were supposed to be valid. In contrast, in accelerated systems compared to the fixed stars, in particular those of rotation relative to these, these were not valid in its most simple. Adicionárseles were fictitious forces such as centrifugal and Coriolis effect .

As we now know, the fixed stars are not fixed . The concept of inertial frames is not already connected to the fixed stars or absolute space. By contrast, the identification of an inertial system is based on simplicity of physical laws in the system, in particular, the absence of fictitious forces.