Caption: First place in the 2021 IAU OAE Astrophotography Contest, category Comets. This image uses the chronophotography technique to capture the evolution of the comet C/2020 F3 (Neowise) over time, as it became visible in the northern skies in July 2020. Orbits of comets are extremely elliptical, which means that during part of their orbit they get close to the Sun. As a comet approaches the Sun, it gets heated, releases gas and dust creating an envelope or coma around the nucleus. The solar wind and photons (particles of electromagnetic radiation) interact with the coma producing the cometary tail, which can be seen clearly in this image. The tail of a comet always points away from the Sun, and extends as much as tens of millions of kilometres. This tail has two parts: the relatively straight bluish gas (ion or plasma) tail, which is made up of charged particles interacting with the magnetic fields of the solar wind; and the whitish dust tail compose of very small dust particles that are pushed by the radiation pressure from the Sun into a curve due to their slower speeds. Two regions in the Solar System are often associated with being “storehouses” of comets: the Kuiper Belt and the Oort Cloud. Comets with periods up to about two hundred years come from the Kuiper belt, a reservoir of cometary nuclei material with a disk-like shape located beyond Neptune. Longer period comets come from the Oort cloud, another huge reservoir of icy objects, with a spherical shape surrounding the Solar System. The outer limit of the Oort Cloud is not known as yet, but it could be as much as 10 thousand times the Sun-Earth distance, or even more. Due to gravitational disturbances, some of these cometary nuclei might be ejected towards the inner regions of the Solar System, sometimes approaching the Earth, offering some of the most spectacular views of a celestial body. The image also shows some prominent constellations and asterisms like the Big and Little Dippers, and also the North (Pole) Star – Polaris.
Credit: Tomáš Slovinský and Petr Horálek/IAU OAE

Caption: 2021 年国际天文学联合会 OAE 天体摄影比赛彗星类第二名。 彗星的结构非常有趣，主要由四个部分组成：由岩石、尘埃和冰冻气体组成的彗核，大小通常为几公里，但也观测到过更大的彗星；彗核周围的一小团气体（只有在彗星最接近太阳时才会出现），称为彗发；以及两条独特的彗尾（有时还有第三条尾）。彗发的绿色是由于其中的碳和氮与太阳的紫外辐射发生反应。我们通常观测到的尾迹是尘埃尾，由微米级的尘埃粒子组成；第二条尾迹由带电粒子组成--即离子尾或气体尾。只有当彗星在一定距离内接近太阳，来自恒星的热量和辐射强到足以使冰冻气体气化时，彗尾才会出现。尘埃尾是弯曲的，而气体尾则是笔直的，并且始终指向远离太阳的方向，因为这是由太阳风携带的--太阳发出的带电粒子流。由于彗星是由残留物质形成的，它们携带着太阳系形成早期的重要信息。这幅美丽的图片显示的是 2020 年 7 月从德国看到的彗星 C/2020 F3（Neowise），四个结构中的三个：彗尾、气体尾和尘埃尾清晰可见。
Credit: Dietmar Gutermuth/IAU OAE

Caption: 2021年国际天文学联合会教育办公室天文摄影大赛彗星类别季军作品。 这张优美而富有诗意的照片拍摄于2020年7月来自斯洛伐克，记录了彗星C/2020 F3（NEOWISE）的身影。彗星尾巴的方向为太阳的位置提供了线索。在过去，对于不了解这些天体本质的人来说，天空中彗星的出现常常会让他们感到忧虑甚至恐惧。然而，通过细致的观测以及物理学、化学和地质学知识，我们如今了解到，彗星是太阳系形成早期残留的天体。彗星最显著的特征是蓝色的离子（气体）尾和白色的尘埃尾，这些尾巴可以延伸数千万公里。这些独特的特征，即便肉眼也能轻松观测到，再加上科学的理解，不再会引起恐惧，而是帮助我们探索太阳系的历史，带来敬畏、喜悦和思索，正如这幅影像中所呈现的那样。
Credit: 罗伯特·巴尔萨/国际天文学联合会教育办公室

Caption: 彗星 C/1995 O1（海尔-波谱）的图像，拍摄于 1997 年 4 月 4 日，曝光时间为 10 分钟。显示的视场约为 6.5°x6.5°。明亮的彗尾上延伸出两条尾巴：一条白黄色的尘埃尾巴和一条始终指向远离太阳的方向蓝色的气体尾巴。
Credit: E.Kolmhofer, H. Raab; Johannes Kepler Observatory, Linz, Austria credit link

Caption: 67P/Churyumov-Gerasimenko 彗星的彗核是一个由冰冻物质和尘埃混合而成的 "脏雪球"。它的形状像两个大耳叶：一个体积为4.1 千米 × 3.3 千米 × 1.8 千米，另一个为 2.6 千米 × 2.3 千米 × 1.8 千米。这些耳叶由一座小桥连接在一起。当这样的彗星核接近太阳时，它的冰冻物质会被加热，变成气体。这些气体与内部的尘埃共同组成了彗星特有的彗发和彗尾。
Credit: 欧空局/Rosetta/NAVCAM credit link