From Sidney Harris, http://www.sciencecartoonsplus.com/originals2.html, see also http://astro.wsu.edu/worthey/astro/html/lec-cartoons.html

As implied by this cartoon, astronomers are not sure what starts the collapse of a cloud into stars, and therefore cannot really tell when they see the process start and when they are just seeing an interstellar cloud that is not going to do anything interesting!