I Zwicky 18 is a blue compact dwarf galaxy located about 59 million light years away in the constellation Ursa Major. The galaxy was first identified by Swiss astronomer Fritz Zwicky in a 1930s photographic survey of galaxies. Age Studies at the Palomar Observatory some 40 years ago led astronomers to believe that the galaxy erupted with star formation billions of years after its galactic neighbours. Galaxies resembling I Zwicky 18's youthful appearance are typically found only in the early universe. Early observations with the Hubble Space Telescope suggested an age of 500 million years old for I Zwicky 18. From a theoretical point of view, astronomers have hypothesised the existence of young dwarf galaxies in a cold dark matter-dominated Universe. Dwarfs as young as 500 million years would be, however, much fainter than I Zwicky 18. I Zwicky 18 is thus unlikely to have formed all its stars recently in a cold dark matter Universe. Later observations with the Hubble Space Telescope found faint and old stars contained within the galaxy, suggesting its star formation started at least one billion years ago and possibly as much as ten billion years ago. The galaxy, therefore, may have formed at the same time as most other galaxies. Contents Spectroscopic observations with ground-based telescopes have shown that I Zwicky 18 is almost exclusively composed of hydrogen and helium, the main ingredients created in the Big Bang. The galaxy's primordial makeup suggests that its rate of star formation has been much lower than that of other galaxies of similar age. The galaxy has been studied with most of NASA's telescopes, including the Spitzer Space Telescope, the Chandra X-ray Observatory, and the Far Ultraviolet Spectroscopic Explorer (FUSE). However, it remains a mystery why I Zwicky 18 formed so few stars in the past, and why it is forming so many new stars right now. In 2015 a study found a very large region of ionised helium in this small galaxy, which tends to be more frequent in very distant galaxies with low presence of metals. The ionization of helium implies the presence of objects emitting a radiation intense enough to knock electrons off the helium atoms. Scientists therefore conclude that I Zwicky 18 must contain Population III stars, which are almost absent in all other galaxies in the local universe. Distance Astronomers at the Space Telescope Science Institute in Baltimore, Maryland and the European