This movie shows the evolution of the stars in Leo IV, a dim dwarf galaxy in the halo of our Milky Way galaxy.
The Hubble Space Telescope measured the ages of these faint stars in Leo IV and two other similar galaxies, dubbed ultra-faint dwarf galaxies. The Hubble observations revealed that the stars in all three galaxies are more than 13 billion years old, almost as old as the 13.7-billion-year-old universe.
The movie begins with Hubble's image of the region in which the galaxy resides. The video then morphs into a view of the colors of the galaxy's stars. The stars are next shown on the Hertzsprung-Russell (H-R) diagram, which traces the types and evolution of "main-sequence" stars by showing the relationship between a star's color, based on its temperature, (horizontal scale) and its intrinsic brightness (vertical scale). Main-sequence stars are in the stable, middle phase of their development. In this diagram, the galaxy's main-sequence stars are the same mass as our Sun or smaller.
The graph then reveals how the stellar distribution would look 1 billion years after the galaxy formed. Massive and hot, blue stars fill in the upper part of the main sequence. The animation compresses billions of years into just a few seconds, showing that over time the more massive stars evolve faster. They leave the main sequence to become red giant stars (upper right). Their departure leaves behind the cooler, lower-mass stars that evolve more slowly over many billions of years.
Based on the analysis of Leo IV and the two other ultra-faint dwarf galaxies in the survey, astronomers suggest that a global event, called reionization, shut down star formation in them simultaneously more than 13 billion years ago. Reionization is a transitional phase in the early universe when the first stars burned off a fog of cold hydrogen.
Leo IV is one of at least a dozen ultra-faint dwarf galaxies near our Milky Way.