The Solar Neutrino Problem

When I was a student (many years ago), astronomers were very confused because their theories did not match with experimental data.  This became known as the solar neutrino problem.  Please answer the following questions (in your own words):

• What is a neutrino?  What produces them?  How easy are they to detect?
  
  When I was a student, we were taught that a neutrino was a massless particle that carried away energy in a nuclear reaction.  Also, neutrinos were extremely difficult to detect because they very rarely interacts with ordinary matter.  For example, during the time it took you to read this sentence, trillions of neutrinos passed harmlessly through your body.   The techniques for detecting neutrinos (in the 1970's) was incredibly primitive. 
   
• What was the solar neutrino problem?  In your answer explain why the focus was on neutrinos and not something else like light or cosmic rays?  That is, what were the properties of neutrinos that made them so special?
  
  In those days, astronomers were quite confident in their understanding about the conditions and the nuclear reaction rates (hydrogen to helium) occurring in the core of the sun.  One way to confirm their ideas was to see if the number of neutrinos coming from the sun matched the number predicted by theory.  Why concentrate on neutrinos?  Simple - any neutrinos produced in the core of the sun could easily travel through the outer layers of the sun and reach our detectors only 8½ minutes after they were produced.  Despite the paltry neutrino detectors at the time, it was clear that the number coming from the sun were only about ⅓ predicted by theory.  Something was horribly wrong!!!  This became known as the solar neutrino problem.
   
• Has the solar neutrino problem been solved?  If so, how?  If not, what is being done to fix it?
  
  Over the years, techniques to detect neutrinos have improved quite a bit.  The better the detectors got, the worse the solar neutrino problem became ... until it was discovered that a neutrino did, in fact, have a very, very small mass.  With this property, it was possible for neutrinos to change "flavors".  You see, there are actually three types of neutrinos and the kind coming from the core of the sun were known as "electron neutrinos"  (the other two flavors are muon and tau neutrinos).    During that 8½ minute journey from the core of the sun to our detectors, a neutrino can oscillate from one flavor to another.  By the time we receive them, we only detect ⅓ the number of "electron neutrinos" we expected from the sun.  The theory about the sun was fine, our understanding of neutrinos needed work.
   
• Give one other place in astronomy where the neutrino has played center stage in an event, solution, or debate.  Explain.
  
  There are many possible answers ... solution to dark matter, neutrinos from supernova 1987a, any story about neutrino detectors, etc.  Surprise me.