Caldwell 90 is a planetary nebula formed in the late stages of the life of a Sun-like star. Initially, the star’s energy was derived by fusing hydrogen into helium. When the supply of hydrogen ran low, it produced less energy, so the force of gravity caused it to contract. Eventually, that contraction increased the pressure in the star’s core and triggered fusion of the heavier element carbon. That process caused the star to expand into a red giant. Finally, the red giant’s outer layers were eject to form the nebula and the star collapsed again into a small, dense star whose intense radiation continues to push the nebula boundaries outward.
The Sun will probably go through a similar process in 4 or 5 billion years.
Image Credit: NASA / ESA