A special type of eclipses are eclipsing binary stars. And the good thing
is: they happen every day! The best known
eclipsing binary Algol (β Persei) . Every 2.87 days the magnitude of Algol
reduces from 2.1 to 3.4. What happens can be seen in the picture on the left. In a close double star system, both stars have orbits around a common centre of gravity. During phase 1 and 3 we receive the combined light of both stars. However, in case the Earth lies in the plane of these orbits, during phase 2 part of the light of the primary star will be blocked by the companion star. This is called the primary minimum. During phase 4 the primary star blocks the companion star, and this leads to the so-called secondary minimum. As in this example the primary star is more luminous, the magnitude drop during the primary miniumum is larger than during the secondary minimum. In a theoretical system with two identical stars the primary and secondary minimum should have the same magnitude drop. The period of the eclipsing binary stars is determined by their mutual distance and masses. If the stars are very close material will flow from one star to the other. This will change the eclipsing period. Therefore, a measured change of period, is a possible indication that mass transfer takes place. |
The Dutch Observatory in Ausserbinn (Switzerland)
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