Tucson ( or occasionally locally ) is a city in and the county seat of Pima County, Arizona, United States, and home to the University of Arizona. The 2010 United States Census put the population at 520,116, while the 2013 estimated population of the entire Tucson metropolitan statistical area (MSA) was 996,544. The Tucson MSA forms part of the larger Tucson-Nogales combined statistical area (CSA), with a total population of 980,263 as of the 2010 Census. Tucson is the second-largest populated city in Arizona behind Phoenix, which both anchor the Arizona Sun Corridor. The city is located southeast of Phoenix and north of the U.S.-Mexico border. Tucson is the 33rd largest city and the 59th largest metropolitan area in the United States. Roughly 150 Tucson companies are involved in the design and manufacture of optics and optoelectronics systems, earning Tucson the nickname Optics Valley.