Stellar Dynamics

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Stellar dynamics deals with the apparent and absolute motion of stars in different star clusters and other stellar systems in order to deduce their formation and further evolution.

This branch of astronomy has to contend with numerous difficulties, including the vast distances involved, the smallness of the effects to be measured, the effect of systematic errors, and the mutual influence of hundreds to millions of heavy masses.

Measurement methods

The underlying measurement techniques are of astrometric nature (especially high-precision meridian circles and passage instruments), astrophotography (today also CCD sensors) with corresponding evaluation devices (mono- and stereocomparators), and since the 1990s special astrometry satellites.

The primary measurement results are so-called proper motions of the stars in two components (right ascension, declination) on the celestial sphere; they are mostly in the range of some 0.01″ per year, for very close stars also 1-10″. Multiplied by the distance (see annual parallax), the linear motion results – typically some 10 km/s. The third component is the radial velocity (in the direction of view).

Motion analysis of star clusters and galaxies

In particular, stellar dynamics studies how star clusters and galaxies form, evolve, and decay through precise analysis of the space motions. It uses long-time series of measurements by means of astrometry, photographic plates and satellite scanning, whose changes yield the stellar motions. The theoretical model is based on Newton’s axioms and the general theory of relativity.

Special methods have been developed for the study of stellar associations and streams. Such groups of stars have formed together and move approximately parallel through our Galaxy, including the nearby Bear Group of about 50 stars that pass all around our solar system. However, such analyses have to be adjusted for two tricky influencing factors: the solar apex (movement in the direction of the constellation Hercules) and the locally different rotation around the Milky Way centre (approximately 200-250 km/s), which in turn depends on the exact mass distribution (see also astronomical velocimetry).

Computer simulations

With the growing use of mainframes and partly also supercomputers, very extensive simulations of the motion in stellar systems became possible, creating so-called “experimental” stellar dynamics. In this process, various models are calculated and compared with observational data. In this way, the mutual influence of the celestial bodies – the multi-body problem, which was unsolvable until recently – can also be treated computationally.

See also

  • Celestial Mechanics
  • Oort’s rotation formulas
  • Motion pile
  • Sunapex

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