Houston is the most populous city in the U.S and the fourth-most populous city in the United States. This city is well-known as the southeast anchor of the greater megaregion known as the Texas Triangle. Houston is home to many cultural institutions and exhibits, which attract more than 7 million visitors a year to a Museum District. It also has an active visual and performing arts scene in the Theater District. It is a global city with strengths not only in culture but also in medicine and research. Since the word “Houston” was the first spoken word from the moon, this city has become famous for its space and science contributions, also it is home to NASA's Johnson Space Center.