关于这张图
This colourful bubble is a planetary nebula called NGC 6818, also known as the Little Gem Nebula. It is located in the constellation of Sagittarius (The Archer), roughly 6000 light-years away from us. The rich glow of the cloud is just over half a light-year across — humongous compared to its tiny central star — but still a little gem on a cosmic scale. When stars like the Sun enter retirement, they shed their outer layers into space to create glowing clouds of gas called planetary nebulae. This ejection of mass is uneven, and planetary nebulae can have very complex shapes. NGC 6818 shows knot…
