Even in the regions of the Galactic Center , where the stellar densities are the highest, the chance that at any given time a star is aligned that precisely is only about 1 in one million. This means that at least one million stars must be carefully monitored in order to have a reasonable chance of detecting the brightening and dimming that is characteristic to microlensing. The possibility to take and analyze data from one million stars has been made possible only very recently with the advent of large modern electronic detectors coupled to fast analysis computers. Even so, the task is daunting. The source star did not vary for more than a year and then suddenly increased in apparent brightness by a factor of 7 before returning to its baseline brightness, which it has maintained ever since.
Time scales are in days.
Click on the figures for a zoom. At any given moment, several known microlensing events are on-going in the direction of the bulge of our own Galaxy. Click on figure for a zoom.