Whereas a supernova goes off in the Milky Way every few centuries, a new planetary nebula forms every year, as hundreds of older ones fade into obscurity.

• 银河系中，虽然每隔几世纪有一颗超新星爆发，但每年都有一朵新的行星状星云形成，并有上百朵老旧星云消褪隐遁。