I Zwicky 18: Young Galaxy . ... Virginia ), ESA , NASA Explanation: The Milky Way is an ordinary 12 billion year old spiral galaxy, and even our middle-aged Sun is pushing 4.5 billion years. But all the stars in dwarf galaxy I Zwicky 18 are much younger. In fact, based on Hubble Space Telescope image data, that galaxy's first stars formed only about 500 million years ago, making it the youngest known galaxy. ... The tiny galaxy itself is a mere 3,000 light-years across . ...