April 9, 2013 is National Equal Pay Day. In the U.S. women earn 14-23% less doing the same job as men. This gender wage gap affects women in STEM at all ranks including professors. Full time working women make .77 for every $1 their male counterpart makes. In 2011 the wage gap was the difference of $11,084 for a woman earning $37,118 and men earning 48,202. This wage gap has not changed much in the past 10 years more details can be found here: http://www.pay-equity.org/info-time.html.
Additional resources about what you can do about equal pay for equal work can be found here: http://www.whitehouse.gov/equal-pay