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Double Stars

When observed through a telescope, many stars will actually turn out to be pairs of stars that are situated very close to one another. Such stars are known as physical binary stars. Binary stars rotating around each other are a common phenomenon. It is estimated that for every 100 stars, 30 are a part of a binary system, and 23 are a part of multiple systems. Some of the binary stars went through evolution as a pair and developed simultaneously, others became a pair as a result of one star capturing another during close approach. (This frequently occurs in ball clusters and the central regions of galaxies).

Kepler's third law describes the rotation of binary system components:

(m1+m2)T 2 = A3
(M+m)P 2 a3

Where m1 and m2 are masses of two stars rotating with a period T, and A is the major semi-axis of the stars' orbits. Masses Ì and m are masses of the Sun and the Earth, and P = 1 year, while à is the distance from the Earth to the Sun.

If components of a star system are situated close to each other, they may overshadow each other from observer on the Earth. In this case, we will witness an occurrence of fluctuations in the total brightness of a binary star.

This model demonstrates just such a phenomenon. The left-side window shows two stars (blue and yellowish) that revolve around a common center of gravity. You may select one of two systems from the list in the top right corner (AR Lacerta or Algol), or set your own parameters for the system of binary stars. Such parameters primarily comprise the masses of system components m1 and m2, and the distance between them (in astronomical units.) Such values, subject to change through appropriate input windows, fully determine the system's motion, including its period of revolution as indicated below.

Visual Angle parameter determines the angular value of the binary system plane slope in respect to the observer. For example, the angular value of AR Lacerta and Algol is equal to 0? in the "side view" mode. In this case, the stars sometimes overshadow each other from an observer on the Earth, and system brightness varies.

Brightness fluctuations over time are shown on the bottom-left diagram. A vertical line designates the system's current status (it appears once you press "Run" button). When we observe the binary star plane at full circumference (that is, when the angle of observation is equal to 90?), components do not obscure each other and no fluctuations take place (In fact, interference ceases to manifest at a much steeper angle. Try to experiment with the input window and determine the angle of observation at which interference stops.)

Use "Show Orbit" switch to make stellar rotation more visible. Pressing "Stop" button will bring the model to a halt. Press "Reset" to return the model to its initial state.

© OpenTeach Software, 2007