what is a double star? how to calculate its time period?

Question

it is based on gravitation?

Answer 1

A double star is when two stars appear close to each other as seen from Earth.

There are two kinds of double star. When two stars are actually in orbit around each other, they are considered to be a true binary star system. When two stars only are apparently close, but in fact are separated by a great distance and are not gravitationally bound to each other, they are known as an optical double or optical binary.

Optical doubles are distinguished from binary stars by observing them for a long period of time, usually years. If the relative motion looks linear, it may be safely assumed that the motion is due to proper motion alone and that they are an optical double. In the case of a true binary, the position angle changes progressively and the distance between the two stars oscillates between a maximum and minimum.

The first recorded discovery of a true binary star system was by Giovanni Battista Riccioli in 1650, when he found Mizar (ζ Ursae Majoris) was a double star. Since that time, the search for double stars has been carried out thoroughly and every star down to the 10th stellar magnitude has been examined. At least 1 in 18 stars in the northern half of the sky which are as bright as 9.0 magnitude is a close double star visible with a 36-inch refracting telescope
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Answer 2

Most of stars you see at night have companions, a great many obviously double even through a modest telescope. The components of some double stars are nearly equal in mass and brightness. More commonly, one dominates the other, sometimes to the point where a little companion is not really visible at all, and detectable only with the most sophisticated techniques. At the lowest end, we have stars with low-mass brown dwarfs for companions. The stars of some doubles are so far apart that they take thousands of years to orbit; others are so close that they revolve around each other in only days or even hours. Gravitational theory allows us to measure the masses of the stars from the orbits' characters; indeed such measurements are the only way in which we can find stellar masses

Answer 3

duble stars r stars which circles rount eachother hunter star is also abouble its partner is a white dwarf star