I was looking at the APOD for July 10th and marvelled how well-defined the Sun's surface is. I know about coronae and such, but still, it looks almost solid (though I know it's entirely gaseous). How well-defined is the surface? Does it go from transparent to opaque over a short distance? A bit of Googling has revealed that the Sun's core density is about 150g/cm^3, but what is the density at the surface?