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小時候到過北京近郊的康西草原,晚上靠在私家車旁仰望,看到的景色大概如下:
A piercingly bright curtain of stars is the backdrop for this beautiful image taken by astronomer Håkon Dahle. The silhouetted figure in the foreground is Håkon himself surrounded by just a couple of the great dark domes that litter the mountain of ESO’s La Silla Observatory. Many professional astronomers are also keen photographers — and who could blame them? ESO sites in the Atacama Desert are among the best places on Earth for observing the stars, and for the same reason, are amazing places for photographing the night sky. Håkon took these photos while on a week-long observing run at the MPG/ESO 2.2 -telescope. During this time, the telescope was occasionally handed over to a different observing team, giving Håkon the opportunity to admire the starry night — as well as to capture it for the rest of us to see. The Milky Way is brighter in the Southern Hemisphere than in the North, because of the way our planet’s southern regions point towards the dense galactic centre. But even in the South, the Milky Way in the night sky is quite faint in the sky. For most of us, light pollution from our cities and even the Moon can outshine the faint glow of the galaxy, hiding it from view. One of the best aspects of La Silla Observatory is that it is far away from bright city lights, giving it some of the darkest night skies on Earth. The atmosphere is also very clear, so there is no haze to further muddy your vision. The skies at La Silla are so dark that it is possible to see a shadow cast by the light of the Milky Way alone. Håkon submitted this photograph to the Your ESO Pictures Flickr group. The Flickr group is regularly reviewed and the best photos are selected to be featured in our popular Picture of the Week series, or in our gallery.
圖一:美國維珍尼亞州星空。現在google 『康西草原星空』竟然找不到夜景。來源:Wikimedia Commons
中文有說法:「多如繁星」,這是幅栩栩如生的圖像。不過,其實我們肉眼看到的只是宇宙的一小部分。餘下的星體太暗太遠,更甚者根本不會發出可見光;地球的大氣層也會吸收或反射電磁波。這次介紹的 Spitzer太空望遠鏡,就是美國太空總署為了看到宇宙的每一方面發射升空的。它跟 Hubble(可見光)、Compton(伽瑪射線)和 Chandra望遠鏡(X 射線)一起號稱是太空總署的“Great Observatories” 。
Spitzer 是一台紅外線望遠鏡,在 2003 年到達軌道,並在 2009 年 5 月 15 日耗盡冷卻劑。為了科學任務能夠正常進行,望遠鏡內帶的液態氦將紅外線鏡頭冷卻到零下 268度,為的是將儀器本身熱量發出的紅外線減到最少。除此之外, Spitzer 任務設計最特別的,還是它那跟隨地球、圍繞太陽的軌道。與環地軌道相比,隨地軌道 (Earth-trailing orbit) 讓望遠鏡遠離發放大量紅外線的地球,取得更寬的視角。這種軌道第一次的應用就是在Spitzer上,而最近發現所謂「地球 2.0」 的 Kepler 望遠鏡,也是在隨地軌道上的。

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圖二:隨地軌道示意圖。技術所限,每年 Spitzer 都會飄離地球 0.1 AU (太陽-地球軌道的十分之一)。來源:California Institute of Technology

 

Spitzer的任務眾多,不過最引人注目的當然就是尋找太陽系外行星。例如,在 2004 年,它觀察到 G29-38白矮星周圍散發著「額外」的紅外線,其頻譜對應的溫度比白矮星的表面冷十倍。由於紅外線和白矮星光的強度在時間上同步變化,天文學家意識到前者是後者反射自某物體。透過光譜分析發現了行星類型的矽酸鹽(silicate)後,加州大學洛杉磯分校的 M. Jura 教授提出了一個簡單的解釋 [2]:包含豐富礦物的(小)行星撞上 G29-38,粉身碎骨並散落在周圍;碎石吸收了白矮星的電磁輻射後變熱,並釋放出紅外線。
你會問,這跟行星有什麼關係呢!?天文學家的解釋是,如果產生白矮星的超新星爆炸沒有把小行星驅到老遠,那麼更堅實、質量更大的行星不就應該還在白矮星附近嗎?無獨有偶,最近十年來天文學家發現了約四十個跟 G29-38 類似的行星系 [1]。它們是尋找太陽系外行星的好地方。
說到系外行星,那就不能不提 Spitzer 怎樣判斷它們的大氣層構造、溫度和甚至風速。原理上至少需要兩幅圖片(見圖三):1) 行星及其母星,2) 行星運行到母星背後被蓋住。將 2) 從 1) 裡面減走,就可以量度只來自行星的紅外線了。碰巧的是,生命的跡象 — 水和二氧化碳 — 會吸收頻率的紅外線。如果行星散發的電磁輻射缺少這些頻率的話,那麼行星上很可能就有這兩種化合物!
This diagram illustrates how astronomers using NASA's Spitzer Space Telescope can capture the elusive spectra of hot-Jupiter planets. Spectra are an object's light spread apart into its basic components, or wavelengths. By dissecting light in this way, scientists can sort through it and uncover clues about the composition of the object giving off the light. To obtain a spectrum for an object, one first needs to capture its light. Hot-Jupiter planets are so close to their stars that even the most powerful telescopes can't distinguish their light from the light of their much brighter stars. But, there are a few planetary systems that allow astronomers to measure the light from just the planet by using a clever technique. Such "transiting" systems are oriented in such a way that, from our vantage point, the planets' orbits are seen edge-on and cross directly in front of and behind their stars.  In this technique, known as the secondary eclipse method, changes in the total infrared light from a star system are measured as its planet transits behind the star, vanishing from our Earthly point of view. The dip in observed light can then be attributed to the planet alone.  To capture a spectrum of the planet, Spitzer must observe the system twice. It takes a spectrum of the star together with the planet (first panel), then, as the planet disappears from view, a spectrum of just the star (second panel). By subtracting the star's spectrum from the combined spectrum of the star plus the planet, it is able to get the spectrum for just the planet (third panel). This ground-breaking technique was used by Spitzer to obtain the first-ever spectra of two planets beyond our solar system, HD 209458b and HD 189733b. The results suggest that the hot planets are socked in with dry clouds high up in the planet's stratospheres. In addition, HD 209458b showed hints of silicates, indicating those high clouds might be made of very fine sand-like particles.
圖三:怎樣從 Spitzer的資料裡面抽出行星的光譜。來源:NASA

 

雖然 Spitzer 的液態氦冷卻劑早已耗盡,但在寒冷的外太空讓望遠鏡的溫度維持在約零下 243度(暖了二、三十度)。發射升空後的十二年,雖然靈敏度已大不如前,但 Spitzer 依然是天文學家探索星空的利器之一 [1]。小小的望遠鏡透過各種旁側敲擊的方法,讓人類一窺茫茫宇宙的奧秘,你不覺得這件事很美妙嗎?
轉載自作者博格 / Facebook Page
引用資料:
  1. Greicius, T. (2015, April 14). NASA’s Spitzer Spots Planet Deep Within Our Galaxy [Text]. Retrieved July 24, 2015, from http://www.nasa.gov/jpl/nasas-spitzer-spots-planet-deep-within-our-galaxy
  2. Jura, M. A., & Werner, M. W. (n.d.). Unlikely Suns Reveal Improbable Planets. Scientific American, (Jun 2009). Retrieved from http://www.scientificamerican.com/article/improbable-planets/

 

 

 

 

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