The apparent brightness of a star remains constant at most time, except that at precise intervals of 173 hours, the star becomes dimmer for about 2 hours. The most likely explanation is that the star is __.

The most likely explanation is that the star is a member of an eclipsing binary star system.

Explanation:

An eclipsing binary star system takes place when two stars which are close to each other move in an orbit in which sometimes, one star can hide the light behind the other.

There are two types of eclipses depending on the orientation of the orbit and the size of the stars: the totals or the annular ones. It may also be the case that the eclipses are partial.

Algol is the best known eclipsing binary star, and the first of this kind to be discovered. Its interval between eclipses is 173 hours (2.9 days).

When this type of eclipses happens, the brightest star provides 92% of the light of the system, while the eclipsed only contributes less than 8%, and there is a third star, which is not eclipsed.

Therefore, if there is any star that has been bright most of the time, and during certain time intervals it becomes dimmer, that star is being overshadowed.