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 年國際天文學聯合會 OAE 天體攝影比賽彗星類第二名。 彗星的結構非常有趣，主要由四個部分組成：由岩石、塵埃和冰凍氣體組成的彗核，大小通常為幾公里，但也觀測到過更大的彗星；彗核周圍的一小團氣體（只有在彗星最接近太陽時才會出現），稱為彗發；以及兩條獨特的彗尾（有時還有第三條尾）。彗發的綠色是由於其中的碳和氮與太陽的紫外輻射發生反應。我們通常觀測到的尾跡是塵埃尾，由微米級的塵埃粒子組成；第二條尾跡由帶電粒子組成--即離子尾或氣體尾。只有當彗星在一定距離內接近太陽，來自恆星的熱量和輻射強到足以使冰凍氣體氣化時，彗尾才會出現。塵埃尾是彎曲的，而氣體尾則是筆直的，並且始終指向遠離太陽的方向，因為這是由太陽風攜帶的--太陽發出的帶電粒子流。由於彗星是由殘留物質形成的，它們攜帶著太陽系形成早期的重要信息。這幅美麗的圖片顯示的是 2020 年 7 月從德國看到的彗星 C/2020 F3（Neowise），四個結構中的三個：彗尾、氣體尾和塵埃尾清晰可見。
Credit: Dietmar Gutermuth/IAU OAE

Caption: 2021年國際天文學聯合會教育辦公室天文攝影大賽彗星類別季軍作品。 這張優美而富有詩意的照片拍攝於2020年7月來自斯洛伐克，記錄了彗星C/2020 F3（NEOWISE）的身影。彗星尾巴的方向為太陽的位置提供了線索。在過去，對於不了解這些天體本質的人來說，天空中彗星的出現常常會讓他們感到憂慮甚至恐懼。然而，通過細緻的觀測以及物理學、化學和地質學知識，我們如今了解到，彗星是太陽系形成早期殘留的天體。彗星最顯著的特徵是藍色的離子（氣體）尾和白色的塵埃尾，這些尾巴可以延伸數千萬公里。這些獨特的特徵，即便肉眼也能輕鬆觀測到，再加上科學的理解，不再會引起恐懼，而是幫助我們探索太陽系的歷史，帶來敬畏、喜悅和思索，正如這幅影像中所呈現的那樣。
Credit: 羅伯特·巴爾薩/國際天文學聯合會教育辦公室

Caption: 彗星 C/1995 O1（海爾-波譜）的圖像，拍攝於 1997 年 4 月 4 日，曝光時間為 10 分鐘。顯示的視場約為 6.5°x6.5°。明亮的彗尾上延伸出兩條尾巴：一條白黃色的塵埃尾巴和一條始終指向遠離太陽的方向藍色的氣體尾巴。
Credit: E.Kolmhofer, H. Raab; Johannes Kepler Observatory, Linz, Austria credit link

Caption: 67P/Churyumov-Gerasimenko 彗星的彗核是一個由冰凍物質和塵埃混合而成的 "臟雪球"。它的形狀像兩個大耳葉：一個體積為4.1 千米 × 3.3 千米 × 1.8 千米，另一個為 2.6 千米 × 2.3 千米 × 1.8 千米。這些耳葉由一座小橋連接在一起。當這樣的彗星核接近太陽時，它的冰凍物質會被加熱，變成氣體。這些氣體與內部的塵埃共同組成了彗星特有的彗發和彗尾。
Credit: 歐空局/Rosetta/NAVCAM credit link