Re: What is a micro-lensing event and how long does it last?

Thank you for your question Tori. First, we need to be clear about what 
microlensing is, or more specific "gravitational microlensing". The 
distances of our universe are enormous, and it is populated by "blobs" of 
concentrated matter-energy (galaxies of stars and "dark matter"). As 
Einstein theorized in 1915, gravity is a result of concentrations of 
matter-energy (which are ultimately interchangeable)which bend or warp 
space-time. This was observed by the Eddington expedition of 1919 to 
observe the solar eclipse. Eddington observed the star positions in the 
vicinity of the line of sight to the sun were shifted just slightly in 
agreement with Einstein's relativity. This bending of light by gravity is 
analogous to the bending of light by optical materials. Only it requires 
a large gravitational body along the line-of-sight between the object of 
interest and the observer.

To answer your question about "how long you can observe the recent event 
in Sagittarius?", the answer is as long as the line-of-sight 
configuration is compatible. Typical event durations given galactic 
motion and the mass of the gravitational "focus element" are such that 
the duration of events are on the order of a days to nearly a year. I am 
not sure how long the event you reference will last. Its a function of 
the alignment of the earth, the intermediate object and the object of 
interest being imaged. I assume you are referring to the gravitational 
lensing of extra-solar planet, MOA-2007-BLG-400Lb discovered in September 
and announced in December I believe after scientific review of the data. 
If so, I believe the event duration was on the order of about 2 weeks. I 
do not know whether the event will repeat. Besides all the elements 
required to line up for such an event, this was a "strong magnification" 
microlense event which are less understood than the weak-microlensing 
events. However, I would assume that it is unlikely to be visible again 
next year, but much information needed to know for sure is missing. 
Nevertheless, if you intend to go look at it, you will still need a very 
large telescope. 

Below are some links for more information. Thank you again for your 
question.

 http://csep10.phys.utk.edu/astr162/lect/galaxies/gravlens.html
 
 http://www.physics.fsu.edu/Courses/spring98/AST3033/Micro/lensing.htm
 
 http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=925&mode=thread&order=0&thold=0