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Rhea is a heavily-cratered, airless world, while Titan’s nitrogen-rich atmosphere is even thicker than Earth’s. This natural color image was taken in visible light by the Cassini spacecraft on Nov. 19, 2009, at a distance of approximately 713,300 miles (1,148,000 kilometers) from Rhea. After a nearly 20-year mission that overflowed with discoveries, the Cassini spacecraft ended its mission on Sept. 15, 2017. Credit: NASA/JPL-Caltech/Space Science Institute
Astronomers took this image as they were observing an extraordinary exploding star – a supernova – near the galaxy’s central yellow core! The star rapidly evolved from a supernova containing very little hydrogen to one that is hydrogen-rich — in just one year. This rarely observed metamorphosis was luminous at high energies and provides unique insight into the poorly understood final phases of massive stars. By studying similar galaxies we hold a scientific mirror up to our own, allowing us to build a better understanding of our galactic environment, which we cannot always observe, and of galactic behavior and evolution as a whole. Credit: ESA/Hubble & NASA/D. Milisavljevic (Perdue University)