It has been estimated that in the 10th century Córdoba was the most populous city in the world, and under the rule of Caliph Al Hakam II it had also become a centre for education under its Islamic rulers. Al Hakam II opened many libraries on top of the many medical schools and universities which existed at this time. Such universities contributed towards developments in mathematics and astronomy. During these centuries Córdoba had become the intellectual centre of Europe and was also noted for its predominantly Muslim society that was tolerant toward its Christian and Jewish minorities. Today it is a moderately-sized modern city; its population in 2011 was about 330,000. The historic centre was named a UNESCO World Heritage Site.