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 NASA/ESA Hubble Space Telescope image shows NGC 4789A, a dwarf irregular galaxy in the constellation of Coma Berenices. It certainly lives up to its name — the stars that call this galaxy home are smeared out across the sky in an apparently disorderly and irregular jumble, giving NGC 4789A a far more subtle and abstract appearance than its glitzy spiral and elliptical cousins.
While NGC 278 may look serene, it is anything but. The galaxy is currently undergoing an immense burst of star formation as revealed in this NASA/ESA Hubble Space Telescope image. However, NGC 278’s star formation is somewhat unusual: why is it only taking place within an inner ring some 6,500 light-years across and not extend to the galaxy’s outer edges?