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The Andromeda and Milky Way collision, explained
In about 4 billion years, our home galaxy will merge with the nearest large spiral in the Local Group. Here's what will happen.
The Andromeda Galaxy is approaching the Milky Way at nearly a quarter million miles per hour. It is the closet major galaxy to our Milky Way and is the most distant thing you can see with the naked eye at 2.5 million light years away. Absolute chaos will begin as the two galaxies approach each other, eventually becoming Milkomeda. Everything will look like a massive pinball game, with huge amounts of rocks, dust, asteroids, planets, and stars being thrown in all directions.
Until 2012, it was not known whether the collision was definitely going to happen or not. Researchers, using Hubble to track the motions of stars in Andromeda with unprecedented accuracy, concluded the galaxies would collide in about 4 billion years. Such collisions are relatively common, considering galaxies long lifespans. According to simulations, the remnant will look like a giant elliptical galaxy, which is a blob-like galaxy without discernible spiral arms or an internal structure.
Andromeda galaxy contains about one trillion stars and the Milky Way contains about 300 billion. The stars involved are sufficiently far apart that it’s highly improbable that any of them will individually collide. For example, the nearest part of the Sun is Proxima Centauri, which is about 4.2 light years away. If the Sun were a ping pong ball, then Proxima Centauri would be a pea about 680 miles away. Although stars are more common near the centers of each galaxy, the average distance between stars is still 100 billion miles. That is like placing one ping-pong ball every two miles; thus, it’s extremely unlikely that any two stars from the merging galaxy would collide, but some stars might be ejected.
What happens to the black holes after the Andromeda and Milky Way collision?
The Milky Way and Andromeda each contain a central supermassive black hole. These black holes will eventually spiral into one another and converge near the center of the newly formed galaxy over a period that may take millions of years. When the supermassive black holes come within one light year of another, they will begin to strongly emit gravitational waves that will radiate further orbital energy until they merge completely. Gas taken up by the combined black holes could create an active galactic nucleus. If this happens it would release an inconceivable amount of energy.
OK, so what’s the future of our solar system?
Based on current calculations, scientists predict a 50% chance that in a merged galaxy the solar system will be swept out three times farther from the galactic core than its current position. Our galaxy is organized into spiral arms and the Sun is in a branch called the Orion Spur. If we were to be flung farther out, that means our solar system would land just at the fingertips of this arm. Scientists also predict sometime during the collision, there is a 12% chance that the solar system could be entirely ejected from the newly formed galaxy. For some reassurance, humanity would be long gone by then, of course. Such an event would actually have no adverse effect on the solar system and the chances of any sort of disturbance to the Sun or planets themselves may be remote, that is excluding planetary engineering.
By the time the two galaxies collide, the surface of the Earth will have already become far too hot for liquid water to exist, due to the gradually increasing luminosity of the Sun, ending all terrestrial life, but our planet will still be caught in the middle of this collision and its view of the universe will never be the same. If we could however see the night sky still, whether form the hellish perspective of Earth or from the far reaches of the outer solar system, Andromeda would be growing larger in the sky and eventually about 4 billion years from now, it could possibly stretch from horizon to horizon like a serene rainbow of dew.
注释：
pinball: n
表示" 弹球游戏"，means ""，如：He and a friend also made $50 a week by placing pinball machines in barber shops. 他还和一个朋友为理发店安装弹球游戏机从而每周赚得50美元。
blob: n
表示"一滴; 小斑点"，means "an indistinct shapeless form"，如：It will deform into an elongated blob. 它会变形而成为一个拉长了的滴状体。Blobs of paint were dotted around the canvas. 油画布上满是涂抹的颜料。
discernible: adj
表示" 可辨别的"，means "perceptible by the senses or intellect;"，如：His face was barely discernible in the gloom. 在黑暗中很难看得清他的脸。
converge: v
表示"会聚"，means "be adjacent or come together"，如：The roads converge just before the station. 四面八方的道路会聚在火车站前。
Galactic: adj
表示" 星系的；银河系的；"，means "of or relating to a galaxy (especially our galaxy the Milky Way);"，如：The Milky Way is a galactic system made up of many fixed stars. 银河系是由无数颗恒星组成的星际体系。
luminosity: n
表示" 光明；光度"，means "the quality of being luminous; emitting or reflecting light"，如：Luminosity glows through pale velum walls. 耀眼的光芒透射着苍白的幕墙。
中文简要说明：
我们的邻居仙女座星系（Andromeda Galaxy）正以每小时近40万公里的速度，逼近银河系。它是距离我们银河系最近的主要星系，也是肉眼可见的最远天体，距离我们约250万光年。两个星系接近时将带来绝对的混乱，最终合并成一个超大型星系。一切将彷佛是一场巨大的弹珠台游戏，岩石、尘埃、小行星、行星和恒星大量抛向各个方向，而我们的家——太阳系可能会被甩到距离现在银河系中心3倍远的地方。
据《天文学》（Astronomy）报导，2012年，天文学家研究人员使用哈伯太空望远镜，得出结论，我们两个星系将在约40亿年内相撞。考虑到星系的寿命十分漫长，这样的碰撞在宇宙中并不罕见。根据模拟，一但我们合并之后，看起来像一个巨大的椭圆星系（Elliptical galaxy），内部结构不在具有的螺旋臂，看起来像是一颗扎扎实实的实心星系。
仙女座约有1兆颗恒星，而银河系约有3000亿颗，恒星与恒星之间的距离足够远，因此恒星之间发生碰撞的机会不大。例如，离我们太阳最近的是半人马座α（Alpha Centauri, 南门二）距离约4.2光年。
如果太阳是一个乒乓球，那么半人马座α将是一颗距离它约1094公里的豌豆，实在很难碰到。尽管在每个星系的中心附近的恒星超级多、比次靠得很近，但平均恒星之间的距离仍然是1609亿公里。这就像每3.2公里放一个乒乓球；因此，星系合并的时候所产生的混乱，仍然不太可能让任何恒星发生碰撞，但一些恒星可能会被抛射出去。
至于仙女座星系和银河系碰撞后，黑洞会发生什么变化呢？银河系和仙女座星系中心都含有一个超大质量黑洞。这些黑洞最终将螺旋进入对方并在新形成的星系中心附近合并，这可能需要数百万年的时间。
当这两个超大质量黑洞距离不到一光年时，它们将开始发射猛烈的重力波，这将辐射进一步的轨道能量，直到完全合并。结合的黑洞吸收的气体可能会形成一个活跃的星系核，如果发生这种情况，它将释放难以想象的能量。
根据目前的计算，科学家预测，在合并的星系中，太阳系被从银河核心抛出3倍远位置的机率大概是50%。在银河系的的螺旋臂中，太阳位于猎户座支的分支中。如果我们被掷得更远，这意味着我们的太阳系将落在这个分支的尖端处。
科学家还预测，在碰撞期间的某个时候，太阳系有12%的机会被从合并新形成的星系中抛出。不过科学家也安慰，届时人类早已不存在了。这样的事件实际上对太阳系没有不利影响，对太阳或行星本身造成干扰的可能性可能很小。
因为，早在两个星系碰撞之前，地球表面早就已经变得太热以至于液态水无法存在，这是由于太阳的亮度越来越强，最终结束了所有陆地生命，但我们的星球仍然陷入这场碰撞的中心，它的宇宙景观将永远不再相同。
但如果我们仍然能够看到夜空，无论是从地球的还是从外太阳系的远处，仙女座星系将在天空中变得更大，最终大约在4亿年后，它可能像一道宁静的露水彩虹一样从地平线延伸到地平线。
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