宇宙有多大？
可以觀測到的宇宙的半徑約為45.66*10^9 億光年，地球的半徑約 6.734*10^(-10)光年(6,371公里)，宇宙的半徑是地球半徑的6.78*10^19倍！
宇宙有多老？
宇宙約138億年(13..8*10^9)。人類祖先的人族出現在大約300萬年前(0.03*10^9），所以人族只佔宇宙歴史的0.21%！
以宇宙的尺度來衡量，不管是依照大小或者歷史的長短來看，地球和人類都是微不足道的存在。
但是，地球能繁衍生命，如果以其生存的條件來看，那還真是一個奇蹟。
地球有重力，可以把水和空氣留在地球表面，這使得地球變成氣候變化較溫和而且宜居的地方。
大氣層中的臭氧層，也會大幅度減弱有害的短波紫外線，保護生命。
地球的核心有熔岩鐵水，隨著轉動產生地磁，磁力線大致由南極地理附近南極地磁(N極)出發，沿地表上方往上往北至北極地理附近的北極地磁(S極)，然後穿過地心通到南極地磁。
地磁的磁力線在地球表面上空包覆著地球，也保護著地球。帶電的宇宙射線(=宇宙輻射)在抵達地球表面之前，就先被這個磁力線網給擋下來。由太陽釋放出來的俗稱太陽風，也是一種宇宙射線。
並不是每個星球都有熔岩鐵水，來使得在轉動的時候能產生地磁。一個沒有地磁和大氣層保護的地球，對生命是無法想像的。
地球的位置距離太陽，既不太遠也不太近，所以生命之源的水，既不會完全汽化，也不會完全冰封，這樣的條件使得生命在地球變得可能。
至於人類的繁衍，還真的得益6600萬年前Kpg事件的衛星撞地球，消滅了當時稱霸地表的恐龍，為人類清出了舞台。(更精確講是為人類祖先的靈長類)
那個衛星會在那個時間撞上今天墨西哥灣附近，以宇宙之大，機率可以說是微乎其微。今天地球的環境條件能承載生命，已經是一個奇蹟，而衛星撞地球又何嘗不是，更不用說在演化過程中一次又一次的突變人類在生命之樹上各種機率上的博弈。
我們今天為人，不要說抱歉，要感恩說謝謝。因為要順利走到今日，是一件又一件的意外，一次又一次的奇蹟所造成的。
天文學家用宇宙的尺度，已經預告人類終將滅絕。讓我們不要想太多，好好過手上掌握到的每一分和每一秒就好了。
在“The Universe, A Biography” 這本書中，有一段關於地球所在的位置，既不太熱也不太冷，所以宜居，有諸多探討，而那也是一個莫大的奇蹟。節錄一些於后，供大家參考：
On a cosmic scale the solar system is insignificant compared to galaxies and stars, and in cosmic history it is a late arrival, which is why it appears towards the end of this biography of the Universe. This book commenced with a Big Bang, but the story is only now reaching towards what appears to us to be its culmination: the emergence of humankind as a cosmic phenomenon. This perspective is self-centred and what appears to us to be the climax is, in a broader perspective, of limited impact and short duration (see Chapter 12).
以宇宙的尺度來看，比起諸多的星系和星球，太陽系是微不足道的。而就宇宙的歴史看來，太陽系的出現是晚近的事，這也是為什麼會擺在介紹在宇宙史中的後段討論。這本書開始於「大爆炸」，但是宇宙史只有到了太陽系出現的時候才開始進入到對我們而言達到高潮的時候，人類的出現成為宇宙的現象。這個觀點當然是人類以自我為中心來看的最高潮，但在更廣濶的宇宙角度來看，人類的影響極為有限而且持續的時間相對是很短很短的。
The potential for planets to exist and to host life on their surface is present in the interstellar medium of our Galaxy, and was realized in our solar system as the Sun came into existence in the solar nebula. The solar nebula formed a disc of gas and solid grains that rotated around the nascent Sun, travelling in more or less circular orbits (see Chapter 6). The gas originated in interstellar space and was mostly hydrogen and helium, made in the Big Bang, but it also contained simple molecular compounds, made of elements like carbon, oxygen and nitrogen, themselves made in stars. Molecules of few atoms were the more common because they were simpler to make in the vacuous spaces of the Galaxy than complicated molecules. They included hydrogen molecules (two hydrogen atoms, H2), water (two atoms of hydrogen and one of oxygen, H2O), ammonia (three atoms of hydrogen and one of nitrogen, NH,) and carbon mon- and di-oxide (one atom of carbon and one or two of oxygen respectively, CO and CO2); more complex molecules like methanol (one atom each of carbon and oxygen and four of hydrogen, CH3OH) and acetaldehyde (two atoms of carbon, one of oxygen and four of hydrogen, CH,CHO) also occurred.
The most complex molecules discovered so far in interstellar space have a dozen or more atoms, like benzene, C6H6, with six carbon and six hydrogen atoms arranged in a ring. Buckminsterfullerene molecules (informally known as ‘bucky balls’) have only one kind of atom but many of them: each has sixty carbon atoms arranged in a structure typical of a geodesic dome designed by the architect Buckminster Fuller.*
能在其表面滋養生命的星球的潛力早就存在我們的星系的星球之間，體現在太陽系的則是太陽星雲所產生的太陽。太陽星雲形成碟形的氣體和固體顆粒環著初生的太陽，以大約圓形的軌道繞行。氣體源自星際的太空，大部分是氫和氦，在宇宙大爆炸中產生的，但也包含一些分子聚合物，由碳、氧和氮等元素組成，由星球生成。只含有幾個原子的分子最常見，相對於較複雜的分子，它們在星系的各種太空中較容易生成。它們包括氫分子(兩個氫原子，H2)，水(兩個氫原子和一個氧原子，H2O)，阿摩尼亞(3個氫原子和一個氮原子，NH3)，和一氧化碳及二氧化碳(一個碳原子和一個或二個氧原子，CO 及 CO2)，再褑雜一點的分子如甲烷(一個碳原子，一個氧原子，四個氫原子，CHOH3)，及乙醛(二個碳原子，一個氧原子，4個氫原子，CH3CHO)也產生了。至今在星際太空發現的最複雜物質有超過12個原子的，譬如苯，有六個碳原子和六個氫原子，串成一個環。巴克明斯特富勒烯分子(俗名「大球」)只有一種原子，但每個分子有60個碳原子，連成中空球形拱頂的結構)
We could infer that these might have been about as complex as the molecules of the solar nebula, before it became warm and dense and cooked up anything more complicated. The chemical composition of the disc was similar to that of comets in the modern solar system; indeed, the solar nebula was the material from which comets originated. Interstellar space is cold, like comets, and these molecules, and others like them, were at the outset frozen onto the grains of the solar nebula as various forms of ice, then amalgamated together as comets 4.5 billion years ago. Until a comet for some reason ventures into the warmth of the Sun, which activates further chemical reactions, it will have remained little changed since then.
我可以推論太陽星雲，在它們變得溫度和密度更高之前，它的分子的複雜程度也是相當的。太陽系星雲碟的化學組成和今天太陽系的慧星是相似的，確實，太陽星雲的物質正是慧星所從出。星際的天空是寒冷的，慧星也是一樣，它們組成的分子，和其他類似的，一開始就是太陽星雲冷凝而成的顆粒，各種形式的冰，之後在45億年前合併為慧星。在慧星因為某種原因飛近太陽的暖區，啓動了進一步的化學反應之前，它則是一直保持不變的。
At the same time as these molecules were accumulating into the solar nebula, the Sun was forming at its centre, brightening and radiating more heat. This warmed those grains that were unshadowed in the zones nearest to the Sun. There, the ice melted and vaporized – or more correctly, in space the ice ‘sublimated’, turning directly from solid to gas without passing through the liquid form. (Frozen carbon dioxide commonly does this even on Earth, as when dry ice is used in a gothic theatrical production to simulate fog on the stage.) Solid grains were left behind as the sublimated vapour joined the other gases in the solar nebula.
當這些分子持續集中到太陽星雲中時，太陽在中心形成，變得亮而且輻射出更多的熱量。這溫暖了較接近太陽沒有遮蔽的地區。在那兒，冰融化而且汽化，或者更正確的是，在太空中冰是昇華，直接從固態變為氣態而跳過液態。(冰凍的二氧化碳在地表長溫就會如此，因此乾冰經常拿來營造舞台上起霧的戲劇效果)。昇華的氣體混入星雲中的其他氣體之中，剩下的就是乾硬的顆粒。
The boundary between the inner zone where most of the frozen ices are vaporized and the outer zone where they remain solid is called the snow line, by analogy with the contour line on a high mountain above which snow remains frozen all the year. Typically, the snow line in young stars like the Sun lies at a radius of two or three times the radius of the Earth’s orbit (the radius of the Earth’s orbit is defined as I astronomical unit, so 2-3 astronomical units, corresponding to about the orbit of Mars in our solar system). Inside the snow line, the solar nebula became dry, made of solid grains, and the ices vaporized. Molecules of the gases (mostly hydrogen and helium) warmed and dissipated away from the Sun. Outside the snow line, grains kept their icy cover and the gases stayed put.
在冰皆已汽化的內圈區域與冰還存在的外圈區域之間的界線，稱為「雪線」，這和高山中某個等高線以上終年有冰雪是類似的說法。像太陽這種年輕的星球，雪線一般位於地球繞太陽公轉的圓周半徑的二倍或三倍的距離(地球公轉的半徑定義為1倍的天文距離，那麼二倍或三倍的距離就是2-3個天文距離，大約是我們太陽系中火星的公轉半徑)。在雪線之內，星雲變乾而由固體顆粒組成，冰完全汽化了。氣體分子(大部分是氫和氦)加熱之後飛離太陽。在雪線之外的地區，固體顆粒被覆著冰層，氣體也被冰層所密封住。
The snow line differs somewhat in its location from compound to compound, depending on the temperature at which the solid form of the compound changes to vapour. ‘The’ snow line is taken as the boundary between water vapour and water ice. In general, however, there develops a progressive change of composition of the solar nebula as different compounds vaporize. This means that there are compositional differences in solar system bodies, depending on where they formed.
不同的物質有不同的雪線，要視什麼樣程度的溫度使得怎麼的物質由固體變成氣體。這𥚃的雪線，指的是水冰變成水汽的臨界點。一般而言，當太陽星雲中依被汽化掉的是什麼成份，那麼留存下來的物質的成份也會因此有所差別。意思是說，太陽系中的實體的成份差異，視它們形成的位置而定。
*：Paul Murdin,”The Universe, A Biography,” 2022, Thames & Hudson Ttd, London
2024/2/20 讓我們不要想太多，好好過手上掌握到的每一分和每一秒就好了 Damakey
