How long would we have if the Sun went out?

Q & A: How long would we have if the Sun went out?

Category: Physics • Q & A • Solar System • Spaceflight • Stars
Posted on: July 12, 2010 5:58 PM, by Ethan Siegel

Yo ho! It's hot, the sun is not a place where we could live.
But here on earth there'd be no life without the light it gives.

We need its light. We need its heat. We need its energy.
Without the sun without a doubt there'd be no you and me. -They Might Be Giants

Ahh, the Sun. Beautiful and blinding to the naked eye, it's still the source of energy that gave rise to all of the life on Earth that we know.


The Sun emits energy all over the light spectrum, from long-wavelength radio waves (many meters long) to visible light to X-rays (just a small fraction of a nanometer). The surface of the Sun is an astounding 5,800 Kelvin, hot enough to melt (or sublime) absolutely everything we know, including tungsten, diamonds, and all known chemical compounds.

But not to worry, we're a good 93 million miles (150 million km) away from it, where the temperatures are much cooler.

I was recently asked a question that sounds simple, but is actually very deep:

How long would it take for us to notice if the Sun stopped shining?

Well, there are two ways to interpret this, and so I'll give you the answer to both interpretations.

1.) The Sun simply goes out. If you heat anything up to a temperature of 5,800 Kelvin, it's going to emit light, much like your standard household bulb does. What if, for example, you suddenly cooled the Sun down to a much lower temperature so that it didn't emit light anymore? How long would it take us, here on Earth, to notice that?

The only special thought you need to have is that the last of the Sun's rays would still be on their way towards us when you turned the Sun off. And light doesn't travel infinitely fast; it travels at the speed of light! So, depending where in its orbit the Earth is, it would take just under eight-and-a-half minutes (anywhere from 8:11 to 8:27, according to my calculations) for the lights to go out. And in this scenario, the Sun would go from its current colossal brightness to pretty much zero, instantaneously.

But what if I wanted a more physically realistic interpretation?

2.) The Sun's source of power -- nuclear fusion in the core -- suddenly stopped. This is a lot more interesting, and a lot more difficult to answer. Let's take a look at the science of what goes on inside the Sun.

The core of the Sun -- the innermost quarter of it -- isn't at 5,800 Kelvin. It's more like 14 million Kelvin. Inside, it converts about 620 million tonnes of Hydrogen into Helium every second, releasing the energy equivalent of 1.8 billion of the most powerful nuclear weapon ever detonated. Every second.

But that energy that gets made in the core? Yeah, you wish it looked like sunlight. Instead, it's ultra-high-energy gamma radiation, which will either give you green skin and huge muscles when you get angry, or will cause cancer and certain death, depending on whether you listen to Stan Lee or medical science.

But, fortunately, these gamma rays have hundreds of thousands of kilometers of Sun to go through before they head towards us. And this, perhaps unsurprisingly, is hard.

Each gamma-ray photon, once made, travels a maximum of a few millimeters before it gets absorbed and re-radiated. But when this happens, it gets re-radiated in a random direction and at a lower energy. It's still a photon, mind you, so it still moves at the speed of light, but it has to bounce around an incredible number of times, losing all that energy time and time again, until it finally reaches the surface of the Sun. Some take longer and reach lower energies, others get out more quickly while they still have higher energies, producing a nice, smooth spectrum.

How long does this process take? Between 10,000 and 170,000 years. That's not an uncertainty, mind you. The ones that "get lucky" and get out quickly take about 10,000 years to do it, and are responsible for the highest energy (high frequency and short-wavelength) emissions of light, such as X-ray and ultraviolet light. The ones that take longer have more collisions and lose more energy, as they spend more time bouncing around in the Sun.

So what does this mean, overall? If the Sun suddenly stopped fusing hydrogen into heavier elements, here's how things would change (neglecting the collapse of the interior of the star that would ensue):

• For the first 10,000 years, everything would appear the same as always; there would be no drop in any measurable quantity coming from the Sun.
• After 10,000 years, the X-rays, UV-rays, and bluer forms of light would start to disappear from the Sun's spectrum. To us, on Earth, it would appear that the Sun was gradually cooling.
• After 170,000 years of gradual cooling, the Sun would have changed colors from yellow to orange to red to deep, dark red, and would have gotten progressively dimmer. Over these 160,000 years, the Sun would drop from its current brightness to about one-thousandth of that brightness. In other words, the Sun would look as bright from Earth as it appears from Pluto today.
• Beyond 170,000 years, the Sun will fade out of the visible spectrum and will only emit infrared light; at this point, there's nothing left to prevent the Sun from collapsing under its own gravity.

So the answer, surprisingly, is that if the Sun "went out," we'd have about 10,000 years before we noticed anything, and even then, it would just be a gradual dimming and cooling for over 100,000 years while we figured out what to do about it!

Luckily, with the laws of physics in place, this isn't something we have to worry about, but that doesn't mean I won't cash in on the film rights...

What do you say, Hollywood?

Update: I have been informed that this movie was already made: Sunshine. And it looks like I nailed the movie poster, too. (Thanks, SteveM and Nemo.) What can I say, Hollywood? There's still a chance to get it right...

http://scienceblogs.com/startswithabang/2010/07/q_a_how_long_would_we_have_if.php

我来翻译一下吧，不过由于某些知识欠缺，有些地方不太明白 （绿色字体为不明白的地方），还请各位多指教~

问题：如果太阳死亡，我们人类还能活多久？
分类：物理·问答·太阳系·太空飞船·恒星
发表于：2010年7月12日
下午5:58，作者 Ethan Siegel


哟呵！太阳忒热了，它不是我们可以生存的地方。但是在地球上，生命的存活又离不开太阳的普照。
我们需要阳光。需要它的热量，需要它的能量。
毫无疑问，没有太阳，就没有你我。——他们可能是巨人


啊，太阳，美丽却又不可裸眼直视，但它仍然是我们所知的可以赐予地球上一切生命能量来源的星体。


太阳光包含从长波长的无线电波（有几米之长）到可见光再到x射线（长度仅为纳米级的几分之一）的所有光谱。太阳表面温度高达5800开尔文，这种高温足以熔化（或升华）我们所知的一切物质，包括钨，钻石以及所有已知的化合物。


但是别担心，我们与太阳相距至少9300万英里（15000万公里），到达地球的温度已经下降了许多。


我不久前曾经问过一个貌似简单但实则深奥的问题：如果太阳停止发光，那么多长时间后我们才能感受到？


嗯，这得分两方面来说明，我将把这两方面都解释给你听。


1）太阳早晚有一天会死去。如果你将某种物质加热至5800开尔文，那么它会开始发光，就像家用灯泡那样。如果你突然把太阳冷却至很低的温度以至于它不能再发光，将会发生什么呢？在地球上的我们会在多长时间后才能注意到呢？


你能感受的唯一的感觉（The only special thought you need to have）就是当太阳“熄灭”时，太阳发出的最后一束光线还在它来地球的路上。并且光线不会走的极快，它仍然以光速前进！所以，按照地球所在轨道位置的不同，光线将花费不少于8分半钟（据我推算，从8:11到8:27的任何地方【anywhere from 8:11 to 8:27, according to my calculations】）的时间熄灭。按此想法，太阳从它当前极高的亮度降到几乎接近0度是瞬间发生的事情。

但是如果以更加接近物理真实的方法来解释会怎样呢？


2）太阳能量的来源——太阳内部核聚变反应——突然停止。这样讲将有趣得多，但同时也更加难以回答。让我们首先以科学的角度回顾一下太阳内部在发生什么。


太阳内核——太阳最深处的部分——并非只有5800开尔文的高温。它至少高达1400万开尔文。在内部，每秒将有62000万吨氢原子转化成氦原子，并释放相当于18亿核弹爆炸所释放的能量。注意，是每一秒。


但是，这些能量都是在太阳内核产生的么？（But that energy that gets made in the core?）是的，你会想象它像太阳光那样。但其实它是超高能量的伽马射线。这种射线会让你在生气时变出绿色皮肤和强健的肌肉，或者引发癌症或者死亡，这要看你是相信斯坦·李（绿巨人的创作者——译者注）还是医学了。


但幸运地是，这些伽马射线在辐射到我们之前需要经过成千上万公里的距离才能到达太阳表面。所以它能伤害到我们的概率毫无疑问是很小的。


每一个伽马射线光子，一旦产生，它被再吸收和辐射之前将经历一个...（Each gamma-ray photon, once made, travels a maximum of a few millimeters before it gets absorbed and re-radiated.）但是当这种情况发生时，它会在任意方向上以很低的能量被再辐射出去。请注意，它仍然是一个光子，所以它仍然会以光速移动，但是它不得不这样被吸收再辐射来来回回数次，一次又一次的丢失所有能量，直到它最终到达太阳表面。一些光子会花费更多的时间并达到更低的能量级，另外一些光子由于它仍然有较高的能量，所以会很快的逃逸并产生美丽而平滑的光谱。


这个过程会持续多久呢？大约在10000到170000年之间。请注意，这并不是一个确定的时间。那些能够很快逃逸的“幸运的”光子会花费10000年的时间，并产生高能量（高频率小波长）的光，例如x射线和紫外线。那些持续时间较长的光子会产生激烈的碰撞并丢失较多的能量，因为它们会在太阳内部徘徊很长时间。


那么所有这些说明了什么呢？如果太阳突然之间停止将氢原子核聚变成重原子，那么事情将会变成这样（忽略恒星内部紧接着会产生的坍塌现象）：


1、在起初的10000年里，一切将如以往一样；来自太阳的任何可测量都不会有变化。


2、10000年以后，X射线，紫外线，以及蓝色谱线以外的光将会从太阳光谱中消失。对于地球上的我们来说，将会发现太阳逐渐冷却下来。


3、经过逐渐冷却的170000年以后，太阳的颜色将会从黄色变为橘色再变为红色，越变越深，变成深红色，并且逐渐暗淡。经过160000年的变化，太阳的亮度将会降到目前亮度的千分之一。换句话说，那时的太阳的亮度看起来就像现在从地球上看到的冥王星的亮度。


4、170000年以后，太阳将会淡出可见光谱，并只能发射红外线。此时，什么也无法阻挡太阳由于自身重力作用而产生的坍塌。


所以，答案是惊人的，那就是，如果太阳“熄灭”，我们将在大约10000年以后才发现，即使那样，它还需要超过100000年的时间去逐渐暗淡和冷却，而到那个时候，我们早已找到解决这个问题的答案了。


幸运地是，因为我们掌握了一些物理定律，所以我们不必对这感到担忧。但是这并不意味着我不会花重金于电影版权...
你说什么？好莱坞？


更新：我听说这样的电影已经出炉了：《太阳浩劫》。貌似我也把海报张贴出来了。（感谢SteveM和Nemo。）我能说什么呢，好莱坞？仍然还有机会得到版权...

终于编辑好了...
好像把字体调大后就不容易排版了...