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.
In this NASA/ESA Hubble Space Telescope image we see the striking face-on spiral galaxy NGC 6814, whose luminous nucleus and spectacular sweeping arms are rippled with an intricate pattern of dark dust. The galaxy was discovered by William Herschel in 1788. NGC 6814 lies 74.4 million light-years away in the constellation of Aquila.
A composite image from the Hubble Space Telescope shows a dramatic range of color in galaxy NGC 3344, a weakly barred spiral half the size of the Milky Way some 20 million light years away in the constellation Leo Minor. Hot young stars, seen in blue, populate the spiral arms along with glowing red clouds of gas and dust that provide the raw materials for new suns.