NGC 2022 is a planetary nebula in the constellation Orion. In early telescopes (and in today’s medium-sized amateur telescopes) such nebulae look like small grayish patches of light. Since they don’t look like stars, but a bit like the gas giant planets, early astronomers tagged them as “planetary nebula,” and the name has stuck.
When stars like the Sun grow old, they expand into red giants. They then begin to lose their outer layers into space, forming a shell of gas. As the evolving star’s core shrinks and grows hotter, it emits ultraviolet light that causes the expelled gases to glow.
Image Credit: NASA / ESA