Entwives en Hiver by kaiorton
I'm going to start off this first post on this topic with a simple axiomatic statement: Women and men are equally capable of being successful astronomers. There is no inherent difference in mental capacity, creativity, ability to learn, or any other factor that plays into the success of an astronomer.
Given this axiomatic starting point, it stands to reason that the fraction of women on the faculty at the top astronomy institutions should reflect the fraction of women earning PhDs. A quick glance through the "People" pages of the websites for Harvard, Princeton, Berkeley and Caltech astro departments (hereafter referred to as the "Big-4") shows that 31% of the graduate students (with gender-identifiable names) are women. Granted, there may be some errors on the various webpages, and I wasn't able to identify the gender of every student. But as a quick sanity check, I do have access to the student records over the past 10 years in Caltech astro. During this time (2001-2011) there have been 47 PhDs, 14 of them women, or 29.8%. So 30% is a pretty solid number for the fraction of women graduating with astronomy PhDs over the past decade (records going back to 1991 show about 27% over the past 20 years).
However, among the active (not retired), tenured or tenure-track faculty at these four institutions, only 12 out of 76 professors are women, or 15.7%. Thus, there is a factor of two decrease in the number of women between grad school and the tenure track. On the one hand, this is really good compared to physics. Take Caltech Physics as an example, where only 3 of the 49 faculty are women (6%). On the other hand if we stay focused just on astronomy, it is clear that there is a major leak in the pipeline.
Why should we care that the pipeline is leaky? Well, if women and men are equally capable of success and we are only drawing from the male side of the pool, then we are trading a non-negligible number of highly talented women for less-talented men. For any institution that prioritizes "excellence"---as all of the Big-4 claim they do---this is a major problem. This leak results in some number of junior faculty not getting tenure, while individuals who would have succeeded in those positions fall out of the hiring pool.