Space Agency discerned the distance by observing Cepheid variable stars within I Zwicky 18. These massive flashing stars pulse in a regular rhythm. The timing of their pulsations is directly related to their brightness. The team determined the observed brightness of three Cepheids and compared it with the actual brightness predicted by theoretical models. These models were calculated specifically for I Zwicky 18's deficiency in heavy elements, indicating the galaxy's stars formed before these elements were abundant in the universe. The Cepheid distance also was validated through the observed brightness of the brightest red stars older than one billion years. The Hubble data suggest that I Zwicky 18 is 59 million light-years from Earth. The galaxy's larger-than-expected distance may explain why astronomers have had difficulty detecting older, fainter stars within the galaxy. The faint, old stars in I Zwicky 18 are almost at the limit of Hubble's resolution and sensitivity. Classification I Zwicky 18 is classified as a dwarf irregular galaxy and also a blue compact galaxy and a starburst galaxy. I Zwicky 18 is much smaller than our Milky Way and a mere 3,000 light-years across. I Zwicky 18 may still be creating Population III stars—spectroscopy shows that its stars are composed almost entirely of hydrogen and helium, with heavier elements almost completely absent. The concentrated bluish-white knots embedded in the heart of the galaxy are two major starburst regions where stars are forming at a furious rate. The wispy blue filaments surrounding the central starburst regions are bubbles of gas that have been blown away by stellar winds and supernovae explosions from a previous generation of hot, young stars. This gas is now heated by intense ultraviolet radiation unleashed by a new generation of hot, young stars. See also Peekaboo Galaxy References External links Astronomers claim first 'dark galaxy' find Zoom in I Zwicky 18. Irregular galaxies Dwarf irregular galaxies 1-18 27182 UGCA objects Ursa Major (constellation)