Thirtynine

Thirty-nine is the natural number that comes after thirty-eight and before forty. In mathematics, 39 is an odd composite number with prime factors 3 and 13, making it a semiprime. Its proper divisors are 1, 3, and 13, summing to 17, which is less than 39, so 39 is a deficient number. In Roman numerals it is written XXXIX.

In base notation, 39 has several common representations: it is 100111 in binary, 047 in octal, and 0x27 in hexadecimal. It can also be expressed in other bases, such as 1100 in base 3.

In science, the atomic number of yttrium is 39, and yttrium (symbol Y) is used in various materials applications, including electronics and phosphors. Potassium-39 is the most abundant and stable isotope of potassium, comprising the majority of natural potassium.

In astronomy, Messier 39 (M39) is an open star cluster in the constellation Cygnus, visible with binoculars under suitable conditions. In geography, the international calling code for Italy is +39, used when dialing Italian telephone numbers from abroad.

In culture, the number 39 appears in several notable works. Queen released a song titled “39” on their 1975 album Night at the Opera, known for its spaceflight-themed storytelling. The 39 Clues is a popular young-adult book series that blends mystery and adventure across multiple authors.

Historically, years designated as 39 BC or AD 39 mark events in Roman and other chronologies, illustrating how the number appears across different eras.