The star in Milky Way is in motion. Because of their distance, they change positions and can only be measured by using large telescopes for a long time. Rarely, a foreground star passes a star at the close proximity as seen from the earth. Light from the gravitational field of the foreground star follows a straight path. 

The distortion of the light by foreground star is called as gravitational lensing. The background starlight is focused into a similar angle and the star appears brighter. 

The major effect is the change in the star’s significant position in the sky because the deviation shifts the center of light rather than distant stars. Both of these effects depend on the mass of lensing body. The mass measuring of stars is not the part of a binary star. 

Beforehand, the difficulty in this method was being predictable the actions of the stars with high precision. The unique data set of literary billions of stellar positions and motions are published as the Gaia data release 2. ESA Gaia has made this research possible. Jonas Kluter, who is doing a Ph.D. at Heidelberg University, to search for such close passages of stars has used these data. 

In the next 50 years, many close encounters will happen. Two passages are going on now and the closest separation will be reached in the next weeks with measurable effects. The two foreground stars are Luyten 143-23 and Ross 322. They move across the sky with the velocities about 16,00 to 1,400 milliarcseconds per year. 

In 2018 July the closest angular separations between background and foreground stars will occur. Due to the astronomic microlensing effect, 1.7 and 0.8 will shift the background stars milliarcseconds. One milliarcsecond corresponds to the angle under which a human being lying on the surface of the moon would be seen. It is very challenging work and with the use of the best telescopic on earth, the stellar position can be measured. 

Jonas Kluter and his colleagues Ulrich Bastian, Markus Demleitner, and Joachim Wambsganss are doing observational campaign by using the European Southern Observatory (ESO) in Chile telescopes and tracking the positional changes of the background stars and the masses of foreground stars Luyten 143-23 and Ross 322.

The gravitational lensing is the powerful actions in which scientists can measure the weight of the stars easily in a milky way.