虹膜星云
This beautiful, blushing nebula is unique amongst its counterparts. While many of the nebulae visible in the night sky are emission nebulae — clouds of dust and gas that are hot enough to emit their own radiation and light — Caldwell 4, otherwise known as the Iris Nebula or NGC 7023, is a reflection nebula. This means that its color comes from the scattered light of its central star, which lies nestled in the abundant star fields of the constellation Cepheus. Located some 1,400 light-years away from Earth, the Iris Nebula’s glowing gaseous petals stretch roughly 6 light-years across.
This nebula is of particular interest to scientists because of its colors. Reflection nebulae glow because they are made up of extremely tiny particles of solid matter, up to 10 or even 100 times smaller than dust particles on Earth. These particles diffuse the light around them, giving the nebula a second-hand glow that’s typically bluish (like our sky). While the Iris Nebula appears predominantly blue, it includes large filaments of deep red, indicating the presence of an unknown chemical compound likely based on hydrocarbons. Studying nebulae like this one helps astronomers learn more about the ingredients that combine to make stars.
This close-up image, showing one rosy-colored region within Caldwell 4, is a composite of four exposures captured by Hubble’s Advanced Camera for Surveys in visible and near-infrared filters. Astronomers also studied the nebula with Hubble’s Near Infrared Camera and Multi-Object Spectrometer to determine which chemical elements are present in Caldwell 4.
Image Credit: NASA/ESA
这个美丽的红色星云在同类星云中是独一无二的。夜空中可见的许多星云都是由尘埃和气体云组成的发射星云，它们的热量足以发出自己的辐射和光，而Caldwell 4，又名虹膜星云或NGC 7023，是一个反射星云。这意味着它的颜色来自于它的中央恒星的散射光，它坐落在仙王座丰富的星群中。虹膜星云距离地球1400光年，其发光的气态花瓣横跨约6光年。
这个星云因其颜色而引起了科学家们的特别兴趣。反射星云之所以会发光，是因为它们是由极其微小的固体物质颗粒组成，比地球上的尘埃颗粒小10倍甚至100倍。这些粒子会散射周围的光，给星云带来了典型的蓝色辉光(就像我们的天空一样)。虽然彩虹星云看起来主要是蓝色的，但它包含了巨大的深红色丝状物，这表明可能存在基于碳氢化合物的未知化合物。研究像这样的星云有助于天文学家了解更多组成恒星的成分。
这张特写照片显示了Caldwell 4内的一个玫瑰色区域，是由哈勃望远镜的高级照相机用可见光和近红外滤光片拍摄的四次曝光合成的。天文学家还使用哈勃望远镜的近红外照相机和多目标光谱仪研究了星云，以确定Caldwell 4中存在哪些化学元素。
影像来源：NASA/ESA