I Zwicky 18 is a dwarf irregular galaxy located about 59 million light years away, and is much smaller than our Milky Way. The galaxy in the Ursa Major constellation (the orange circle next to the North Pole star Polaris in the sky map below), was first identified by Swiss astronomer Fritz Zwicky in a 1930s photographic survey of galaxies. It has a smaller companion Im galaxy, visible at the upper left. The companion may be interacting with I Zwicky 18 by 
gravitationally tugging on the galaxy. The interaction may have triggered the galaxy’s recent star formation. Initially, it was believed to be a young galaxy, until the Hubble Space Telescope found faint, older stars contained within it. The faint, old stars in the galaxy are almost at the limit of Hubble’s resolution and sensitivity. The galaxy, therefore, may have formed at the same time as most other galaxies.
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, with heavier elements almost completely absent. Thus, the galaxy’s primordial makeup suggests that its rate of star formation was much lower than that of other galaxies of similar ages. 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.
Astronomers at the Space Telescope Science Institute in Baltimore, Maryland and the European Space Agency discerned the distance to our Solar system by observing Cepheid variable stars (red circles) 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 these Cepheids and compared it with the actual brightness predicted by theoretical models. 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. The reddish extended objects surrounding I Zwicky 18 and its companion are ancient, fully formed galaxies of different shapes that are much farther away.
Credits: European Space Agency, Harvard University, Hubble, Wikipedia.
