This false color image of galaxy NGC 1921 was taken in infrared light by the Spitzer Space Telescope. The outer red ring is filled with new stars that are igniting and heating up surrounding dust which glows in infrared light. The stars in the center of the galaxy produce shorter-wavelength infrared light which is color-coded blue. The old stars in the center have long ago gobbled up the available gas supply, the fuel for making new stars.
NGC 1921 is roughly 12 billion years old. It is known as a barred galaxy because a central bar of stars (which appears as a blue S in this view) dominates its center. When barred galaxies are young and gas-rich, the stellar bars draw gas toward the center, feeding star formation there. As that star-making fuel runs out, the central regions calm down, and star-formation activity moves to the outskirts of a galaxy. There, spiral density waves and resonances induced by the central bar help gas coalesce into stars. The red outer ring is such s resonance location, where gas is being trapped and new stars ignited.
Image Credit: NASA