Eclipsing Binary Stars

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)

During the 1980-s a Dutch observatory was in operation in Ausserbinn, Switserland (4623'05" N, 0808'45" E). It was equipped with a 40 cm Cassegrain telescope, and the Utrecht Photometrical System (UPS), that recorded in 4 wavelengths: 474, 672, 781 and 871 nm). The observatory with its slide-off roof is shown on the left.

In June and July 1981 I spent five weeks there performing photometrical observations of eclipsing binary stars, including U Coronae Borealis (HD 136175, SAO 64619, see photo of constellation below) with a period of 3.4522 days, and visual magnitude 7.8. The full light curve, based on many observations including those by me, is shown at the bottom of this page.

The detail chart below shows that close before a primary minimum of UCrB, in this case on 21 July 1981, the magnitude changes are noticeable on a scale of just minutes, time shown in UT.

Magnitude drop during primary minimun of UCrB on 21 July 1981

The constellation Corona Borealis (The Northern Crown). Picture Leica CL with 2.8/90mm Tele-Elmarit.

Figure taken from: R.H. van Gent (1989): Astron. Astrophys. Suppl. Ser. 77, pp. 471-485