When stars like the Sun grow old, after burning through their initial supply of hydrogen fuel, nuclear fusion grinds to a halt, their cores shrink and their outer atmospheres balloon outward, a process that turns the a main sequence star into a red giant. Increased pressure in the deep interior can cause hydrogen to begin fusing in a shell around the core, generating intense radiation that illuminates expanding shells of gas that were blown away earlier. Objects such as this one, NGC 2022 in the constellation of Orion, are known as planetary nebulae because their compact appearances made them look a bit like planets in early telescopes. In this view from the Hubble Space Telescope, the compact remnant of the original star is visible at the center of surrounding shells of gas that once formed its outer layers. When fusion completely stops, only a slowly cooling, Earth-size white dwarf will be left to mark the spot where a main sequence star once shined.
This Hubble Space Telescope image shows globular cluster NGC 1783 in the Southern Hemisphere constellation of Dorado. NGC 1783 lies within the Large Magellanic Cloud, a satellite galaxy of our Milky Way, some 160,000 light-years from Earth. NGC 1783 is thought to be less than 1.5 billion years old — very young for a globular cluster.
This Hubble image is of the peculiar galaxy NGC 1487, lying about 30 million light-years away. We are witnessing the possible merger of several dwarf galaxies into a new single galaxy. Its appearance is dominated by large areas of bright blue stars, illuminating the patches of gas that gave them life. This burst of star formation may well have been triggered by the merger.