V838 Mon...

Gorgeous astrophotography of a mystery event.  In 2002, the star V838 Mon suddenly brightened to about a million times as bright as our sun (becoming, briefly, the brightest star in the entire Milky Way galaxy).  Just as suddenly, a month or so later, it rapidly dimmed.  The curve of its luminosity over time is unlike any other event astronomers have ever recorded, and there's a great debate and investigation into what the mechanism might have been.

For almost 10 years, that pulse of light (or flash) has been expanding outward from V838 Mon, illuminating an ever-expanding light-month thick “bubble” of interstellar gas and dust.  The Hubble telescope took the photo below in February 2004.  The accompanying description says the bubble at that point is about 6 light-years in diameter, but I think that must be mistaken – it should be 4 light-years in diameter at that point.  Via APOD, of course.