73 is the natural number following 72 and preceding 74.

In mathematics

Seventy-three is the 21st prime number. The previous is seventy-one, with which it comprises a twin prime. It is also a permutable prime with thirty-seven. 73 is a star number.

Every positive integer is the sum of at most 73 sixth powers (see Waring's problem).

In science

• The atomic number of tantalium

In astronomy,

Messier object M73, a magnitude 9.0 apparent open cluster in the constellation Aquarius

The New General Catalogue object NGC 73 , a barred spiral galaxy in the constellation Cetus

The Saros number of the solar eclipse series which began on -716 July 16 and ended on 582 September 3. The duration of Saros series 73 was 1298.1 years, and it contained 73 solar eclipses.

The Saros number of the lunar eclipse series which began on -378 May 16 and ended on 902 June 23. The duration of Saros series 73 was 1280.1 years, and it contained 72 lunar eclipses.

In other fields

Seventy-three is also:

• The total number of books in the Holy Bible in the Catholic version if the Book of Lamentations is counted as a book separate from the Book of Jeremiah.
• The designation of USA Interstate 73, a freeway in North Carolina.
• The registry of the U.S. Navy's nuclear aircraft carrier USS George Washington (CVN-73), named after U.S. President George Washington.
• In baseball, the single-season home run record set by Barry Bonds in 2001.
• The year AD 73, 73 BC, or 1973.
• Ham radio operators often use the number 73 as a way of saying aloha. This is because in morse code it forms a palindrome --... ...-- which has a very distinctive sound.