M57 · Ring Nebula
| Chinese name | 环状星云 |
|---|---|
| Type | Planetary nebula |
| Constellation | Lyr |
| RA | 18h54m |
| Dec | +33° |
| Apparent magnitude | 8.8ᵐ |
| Hemisphere | Northern |
| Best season | Summer |
| Difficulty | Moderate |
| Focal length | 长焦 1500mm+ |
About
The Ring Nebula lies about 2,300 light-years away in Lyra and is the most famous planetary nebula, shaped like a perfect colored smoke ring. It is the shell of gas cast off by a dying Sun-like star: the star's outer layers were expelled, and the hot remnant core (a white dwarf) lights the shell with ultraviolet radiation. The shell grades from blue-green (doubly ionized oxygen, O III) inward to orange-red (nitrogen and hydrogen) outward, with the cooling white dwarf at its center. Though small (about 1 arcminute), its high surface brightness makes the ring visible even in small telescopes. Under long focal lengths its color and symmetry are captivating, and deep exposures reveal the faint outer hydrogen halo. It is a classic summer planetary-nebula target.