About once every century, a massive star somewhere in our galaxy runs out of fuel. This happens after millions of years of heat and pressure have fused the star’s hydrogen into heavier elements like helium, carbon, and nitrogen— all the way to iron. No longer able to produce sufficient energy to maintain its structure, it collapses under its own gravitational pressure and explodes in a supernova. The star shoots most of its innards into space, seeding the galaxy with heavy elements. But what this cataclysmic eruption leaves behind might be even more remarkable: a ball of matter so dense that atomic electrons collapse from their quantum orbits into the depths of atomic nuclei. The death of that star is the birth of a neutron star: one of the densest known objects in the universe, and a laboratory for the strange physics of supercondensed matter.
大約一個世紀會有一次, 我們的銀河某處, 有一個大質量的星體 把燃料用光了。 會有這種情況,是因為 數百萬年來,高熱和壓力 將該星體的氫氣融入到 更重的元素當中, 如氦、碳、氮,一直到鐵, 無法再產生出足夠的能量 來維持星體的結構, 它的引力壓力會讓它自己垮掉, 發生爆炸,成為超級新星。 這個星體會把它的 內容物都射入太空, 在銀河中播下重元素的種子。 但這種劇烈爆發 留下的東西或許更驚人: 一個球形物質,密度非常高, 以致於原子的電子 會從它們的量子軌道 落到原子核的深處。 那個星體的死亡, 成了一個中子星的誕生: 中子星是全宇宙 最高密度的已知物體之一, 它也是超級壓縮物質的 奇異物理實驗室。
But what is a neutron star? Think of a compact ball inside of which protons and electrons fuse into neutrons and form a frictionless liquid called a superfluid— surrounded by a crust. This material is incredibly dense – the equivalent of the mass of a fully-loaded container ship squeezed into a human hair, or the mass of Mount Everest in a space of a sugar cube. Deeper in the crust, the neutron superfluid forms different phases that physicists call “nuclear pasta,” as it’s squeezed from lasagna to spaghetti-like shapes.
但,中子星是什麼? 想像一個緊實的球體, 它裡面的質子和電子融入中子, 形成一種沒有摩擦力的液體, 叫做超流體, 被外殼包起來。 這種材料非常緊密, 等同於將滿載的貨櫃船的質量 擠壓到一根人類頭髮中, 或是將聖母峰的質量 擠壓到方糖的大小中。 在殼的深處, 中子超流體形成不同的相, 物理學家稱之為「核麵條」, 因為它被從扁麵平擠壓成 義大利麵條的形狀。
The massive precursors to neutron stars often spin. When they collapse, stars that are typically millions of kilometers wide compress down to neutron stars that are only about 25 kilometers across. But the original star’s angular momentum is preserved. So for the same reason that a figure skater’s spin accelerates when they bring in their arms, the neutron star spins much more rapidly than its parent. The fastest neutron star on record rotates over 700 times every second, which means that a point on its surface whirls through space at more than a fifth of the speed of light. Neutron stars also have the strongest magnetic field of any known object. This magnetic concentration forms vortexes that radiate beams from the magnetic poles. Since the poles aren’t always aligned with the rotational axis of the star, the beams spin like lighthouse beacons, which appear to blink when viewed from Earth. We call those pulsars. The detection of one of these tantalizing flashing signals by astrophysicist Jocelyn Bell in 1967 was in fact the way we indirectly discovered neutron stars in the first place. An aging neutron star’s furious rotation slows over a period of billions of years as it radiates away its energy in the form of electromagnetic and gravity waves.
中子星的大質量前身通常會旋轉。 當它們崩垮時,通常 有數百萬公里寬的星體 會被壓縮到只有 二十五公里寬的中子星。 但原始星體的角動量還被保存著。 和花式滑冰選手把手臂收進來 就能加速旋轉是相同的道理, 中子星的旋轉速度 會比它的母體更快。 紀錄中最快的中子星 每秒鐘能轉七百次, 這就表示,它表面上的點 在太空中的轉動速度 比光速的五分之一還要快。 中子星的磁場也是目前 所知物體中最強的。 這種磁力的集中,會形成旋渦, 從磁極放射出光束。 因為磁極未必一定對齊星體的轉軸, 光束就會像燈塔指引信號一樣旋轉, 從地球來看就像是閃爍。 我們稱之為脈衝星。 1967 年,天體物理學家約瑟琳貝爾 偵測到這些誘人的閃爍訊號, 事實上,這就是我們一開始 間接發現中子星的方式。 經過數十億年的時間, 年邁中子星的猛烈轉動會慢下來, 因為它以電磁波 和重力波的形式耗掉能量。
But not all neutron stars disappear so quietly. For example, we’ve observed binary systems where a neutron star co-orbits another star. A neutron star can feed on a lighter companion, gorging on its more loosely bound atmosphere before eventually collapsing cataclysmically into a black hole.
但並不是所有的中子星 都會如此安靜地消失。 比如,我們曾經觀察到雙星系統, 就是一個中子星 和另一個星體共用軌道。 中子星能以這個 比較輕的伙伴為食, 吞食它比較沒有緊密結合的大氣, 直到最後劇烈崩垮成為黑洞。
While many stars exist as binary systems, only a small percentage of those end up as neutron-star binaries, where two neutron stars circle each other in a waltz doomed to end as a merger. When they finally collide, they send gravity waves through space-time like ripples from a stone thrown into a calm lake.
許多星體以雙星系統的方式存在, 但當中只有一小部分 最後會變成中子星雙星, 也就是兩個雙子星 像跳華爾滋一樣繞著彼此, 最後注定會合併起來。 當它們終於相撞時, 會透過時空發出重力波, 就像把一顆石頭丟入 平靜的湖中會產生漣漪。
Einstein’s theory of General Relativity predicted this phenomenon over 100 years ago, but it wasn't directly verified until 2017, when gravitational-wave observatories LIGO and VIRGO observed a neutron star collision. Other telescopes picked up a burst of gamma rays and a flash of light, and, later, x-rays and radio signals, all from the same impact. That became the most studied event in the history of astronomy. It yielded a treasure trove of data that’s helped pin down the speed of gravity, bolster important theories in astrophysics, and provide evidence for the origin of heavy elements like gold and platinum.
愛因斯坦的廣義相對論 在至少一百年前 就預測了這個現象, 但它一直到 2017 年 才被直接驗證, 那年重力波天文台 LIGO 和 VIRGO 觀察到中子星相撞。 其他望遠鏡發現 伽瑪射線爆發,還有閃光, 後來還有 X 光和無線電訊號, 通通來自同一次撞擊。 那事件成為天文學史上 最被拿來研究的事件。 它衍生出了珍貴的資料, 協助確定了引力的速度, 支持重要的天體物理學理論, 並為金和鉑這類重元素的 來源提供了證據。
Neutron stars haven’t given up all their secrets yet. LIGO and VIRGO are being upgraded to detect more collisions. That’ll help us learn what else the spectacular demise of these dense, pulsating, spinning magnets can tell us about the universe.
中子星還有未知的秘密。 LIGO 和 VIRGO 已經被升級, 可以偵測到更多的撞擊。 那能夠協助我們了解,這些高密度 閃爍旋轉的磁鐵在壯麗地死亡時, 還能告訴我們什麼其他 關於這個宇宙的知識。