The title is pretty self-explanatory. There are four moons in a straight line. The other bright spots are stars. Galileo first saw these in 1610. He thought that they were stars at first and named them the ‘Medicean Stars’ after a family that patronized him. Eventually, he noticed that they rearrange themselves around Jupiter regularly and deduced that they must be orbiting it. This was the first clear demonstration that there were bodies orbiting an object besides Earth. It thus dealt a blow to the church-sponsored view that Earth is the center of everything. It also got Galileo in a lot of trouble! Photographs like this are a little difficult, as Jupiter is far brighter than its moons, which is why there is such a variation in brightness in the picture.