哈勃影像: 环状星云M57
M57: The Ring Nebula from Hubble
Image Credit: NASA, ESA, Hubble Legacy Archive; Processing: Judy Schmidt
Explanation: It was noticed hundreds of years ago by stargazers who could not understand its unusual shape. It looked like a ring on the sky. Except for the rings of Saturn, the Ring Nebula (M57) may be the most famous celestial circle. We now know what it is, and that its iconic shape is due to our lucky perspective. The recent mapping of the expanding nebula’s 3-D structure, based in part on this clear Hubble image,indicates that the nebula is a relatively dense, donut-like ring wrapped around the middle of an (American) football-shaped cloud of glowing gas. Our view from planet Earth looks down the long axis of the football, face-on to the ring. Of course, in this well-studied example of a planetary nebula, the glowing material does not come from planets. Instead, the gaseous shroud represents outer layers expelled from the dying, once sun-like star, now a tiny pinprick of light seen at the nebula’s center. Intense ultraviolet light from the hot central star ionizes atoms in the gas. The Ring Nebula is about one light-year across and 2,500 light-years away.
Tomorrow’s picture: the beasts at the center of our galaxy
哈勃影像: 环状星云M57
影像提供: NASA, ESA, Hubble Legacy Archive; 影像处理: Judy Schmidt
说明: 早在数百年前观星者就已注意到它，但他们无法理解其不寻常的形状。除了土星环之外，环状星云（M57）可能是最著名的环状天体。不过它经典的形状，我们如今了解其实是源自地球的特殊视角。最近根据这张极清晰的哈勃影像，为这不停扩张的星云所建构的三维结构显示，这团星云的结构是一个相对致密的甜甜圈，围拱着中央形似美式足球的明亮云气。从我们所在的地球看出去，视线正好顺着足球的长轴，直视星云环的正面。当然，这团已被大量研究的行星状星云之发光物质，并非来自行星。而是来自类太阳恒星在死亡的过程中所抛出的外壳，而星云中心的针尖状光点，则是这颗恒星的孑遗。这颗炽热的中心星，发出强烈的紫外光电离云气的原子并激发它们发光。距离我们约2,500光年的环状星云，宽度约有1光年。
明日的图片: the beasts at the center of our galaxy
