A distant object that appears as a point of light shows a large decrease in brightness every 3 days, and a smaller decrease in brighness exactly between the large decreases. Both decreases last just a few hours. What kind of object is it?

4- it is a binary star system seen edge-on, so that the two stars eclipse each other from out point of view

Explanation:

The complete question is

A distant object that appears as a point of light shows a large decrease in brightness every 3 days, and a smaller decrease in brightness exactly between the large decreases. Both decreases last just a few hours. What kind of object is it?

1- it is a star going through the planetary nebula stage of its life

2- it is pulsating star that is swelling and shrinking in size

3- it is a red supergiant star that has moved off the main sequence and is about to blow up as a supernova

4- it is a binary star system seen edge-on, so that the two stars eclipse each other from out point of view

The correct answer is binary star system, because in this case, a brighter star and dimmer star are tightly tugged in gravitational field and revolving round each other. So for the observer, brighter star comes forward during its revolution round the dimmer one, both starts eclipse each other for certain time and the cycle of brightness and dimness occurs.

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