The simplest case to consider is that of two stars that are both spherical that both appear uniformly bright over their surface (no limb darkening), and that have circular orbits. In this case we can easily distinguish between the two different types of eclipse, partial and total. When i is close enough to 90° for eclipses to occur, but not close enough for total eclipses to occur, each star does not quite manage to obscure the other, even at the minimum of the light curve, and the eclipses are only partial.

To see this, imagine the disc of one star, let us call it star A, passing across the face of the other, star B. The eclipse begins when the disc of A first appears to touch the disc of B and the light curve begins to dip. The light curve continues to decrease until at the minimum, when the maximum fraction of disc B is covered by disc A. Thereafter the light curve increases symmetrically until the two discs again appear to touch. Hence when the eclipse is partial, the shape of the dip hi the light curve is that of a decrease, followed by a rounded minimum and a symmetrical increase. We should also note that the eclipse immediately following, when star B eclipses star A, will be exactly similar in shape and that the maximum amount of star B that is eclipsed at one minimum is equal to the maximum amount of star A that is eclipsed at the next

When i is nearly 90°, the two eclipses are a total eclipse and a transit, suppose star A is smaller than star B and imagine the minimum caused by the eclipse of B by star A The eclipse again begins when the two discs appear to touch externally – this is called FIRST CONTACT and the light curve begins to dip at this point. A short time later the two discs appear to touch internally – second contact. After second contact star A proceeds to transit across the face of star B until the discs again appear to touch internally at third con¬tact. Because star B is uniformly bright, the amount of light cut out by A as it transits across B is constant and thus the light curve stays constant between second and third contact. The light curve in¬creases between third contact and fourth contact (when the discs appear to touch externally) in a symmetric manner to the transition from first to second contacts. The next eclipse, when star B obscures star A, will clearly be a total eclipse. First, second, third and fourth contacts can be defined similarly and since the orbit is circular each takes place exactly half a period later than their counterpoints in the previous eclipse. Between second and third contacts, star A is totally hidden by star B (total eclipse) and the light curve there¬fore again remains constant As before for partial eclipses, the amount of star B that is obscured by star A is exactly equal to the amount of star A obscured by star B half a period later. The depth of the eclipse in an ALGOL-TYPE SYSTEM is a measure of the surface brightness of the eclipsed star – that is of the temperature of the eclipsed star. The hotter star is obscured at primary minimum. We also note that outside eclipses the light curve should stay constant.

We have seen that for a simple eclipsing system such as that considered here we can expect to obtain information about i from the shape of the eclipses. We can find out, for example, whether they are partial or total eclipses, and can determine information on radii from the length of duration and actual shape of the eclipses.

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