我们如何偶然发现一颗白矮星 - 一具恒星尸体

05-11 9 3159 徐晃同学下盘并不稳
国外新鲜事



One of the great things about science is that, when you start to observe a new object in space, you can never be sure quite what you’ll find. We received a fantastic reminder of this during observations designed to check whether nearby stars had planetary companions. Our observations confirmed the discovery of a couple of planets, but also yielded an unexpected surprise.

科学的伟大之处在于,当你开始在太空中观察一个新物体时,你永远无法确定你会发现什么。在观测过程中,我们收到了一个奇妙的提醒,这是为了检查附近的恒星是否有行星伴星。我们的观测证实了一些行星的发现,但也产生了意想不到的惊喜。

Buried among our candidates was the corpse of a star – a white dwarf – a discovery we announced this month in The Astrophysical Journal.网站名称 http://www.ltaaa.com

在我们的候选恒星中埋藏着一具恒星的尸体——一颗白矮星——这是我们本月在《天体物理学杂志》上宣布的一项发现。

The search for stellar wobbles
Our story begins with a survey called the Anglo-Australian Planet Search (AAPS), which spent 17 years looking for alien worlds using the 3.9-metre Anglo-Australian Telescope at Siding Spring Observatory in New South Wales.
We often say a planet orbits a star (Earth orbits the Sun, for example), but the truth is slightly more complicated. Instead, the two orbit around their common centre of mass. As a result, a star that hosts a planet will wobble, rocking back and forth over time.

寻找恒星摆动
我们的故事始于一项名为“英澳行星搜索”(AAPS)的调查,这项调查花费了17年的时间,使用新南威尔士州赛丁泉天文台的3.9米英澳望远镜寻找外星世界。
我们常说行星绕着一颗恒星运行(例如,地球绕着太阳运行),但事实稍微复杂一些。相反,这两者围绕着它们共同的质心运行。结果,拥有行星的恒星将会摆动,随时间来回摇摆。

Radial velocity surveys search for planets by attempting to detect that telltale wobble. Over its lifetime, the AAPS discovered more than 40 planets in this manner. But it is almost certain that more planets remained undiscovered in the AAPS data. So we began searching for those hidden worlds.

径向速度测量通过探测这种明显的摆动来寻找行星。在它的生命周期中,AAPS以这种方式发现了40多颗行星。但几乎可以肯定的是,在AAPS的数据中还有更多的行星没有被发现。于是我们开始寻找那些隐藏的世界。

In several cases we found stars that exhibited distinct signs of a wobble, but for which less than a full orbit had been completed. Without observing a full orbit, we don’t know whether the companions causing the wobble are planets, or other stars. So how can we work out what we’ve found?

在几个例子中,我们发现了一些恒星,它们显示出明显的摆动迹象,但还没有完成完整的轨道运行。如果没有观测到完整的轨道,我们就不知道引起摆动的伴星是行星还是其他的恒星。那么我们如何计算出我们的发现呢?

Direct imaging – a new trick
We identified 21 stars around which there could be a planet, but to be sure, we needed more data. Unfortunately, the AAPS had ended, so we needed to do something innovative. For each of our stars, there were two possibilities: either the wobble is caused by a planet, or by something bigger (such as a brown dwarf or an unseen stellar companion).

直接成像:一个新的技巧
我们确定了21颗可能存在行星的恒星,但为了明确,我们需要更多的数据。不幸的是,AAPS已经结束了,所以我们需要做一些创新。对于我们的每一颗恒星,都有两种可能性:要么是行星造成的摆动,要么是更大的物体(比如褐矮星或一颗看不见的伴星)造成的。

Recent advances in astronomical imaging techniques mean we can now use the world’s largest telescopes to look at nearby stars and see objects very close to them – closer than has ever been possible before. We used the 8.1m Gemini-South telescope in Chile to obtain high-resolution images of our target stars, to see whether we could see any previously hidden companions.

最近天文成像技术的进步意味着,我们现在可以使用世界上最大的望远镜观察附近的恒星,并看到离它们很近的物体——比以往任何时候都要近。我们使用智利的8.1米双子座望远镜获取目标恒星的高分辨率图像,看看我们是否能看到任何以前隐藏的伴星。

Despite the power of the technique, any planets around our targets would remain invisible. But if the observed wobbles were caused by more massive objects, we should be able to see those objects and hence rule out the planetary hypothesis.

尽管这项技术很强大,但我们目标周围的任何行星都将保持隐形。不过如果观测到的摆动是由更大质量的物体引起的,我们应该能够看到这些物体,从而排除行星假说。

The peculiar case of HD 118473
In our 20 goals, everything is as we expected. In some cases, we found a previously undiscovered companion star, which we first thought was a planet. But for a planet, things get very strange. Based on the wobble data, we know that the companion star would have a minimum mass about 0.44 times the mass of the sun. It's too big to be a planet.

HD118473的特殊情况
在我们的20个目标中,一切都如我们所料。在某些情况下,我们发现了一个以前未被发现的伴星,首先我们认为这是一个行星。但是对于一颗行星来说,事情变得很奇怪。根据摆动数据,我们知道这颗伴星可能拥有的最小质量大约是太阳质量的0.44倍。这太大了,不可能是一个行星。

With that much mass, we would expect the companion to be a star, fainter and cooler than the Sun, but easily visible with Gemini-South. But when we looked at our images, no companion star was visible.

有了这么大的质量,我们预计伴星将是一颗恒星,比太阳更暗、更冷,但用双子座望远镜很容易看到。但是当我们看我们的图像时,并没有看见这颗伴星。

A macabre twist
The radial velocity data is clear – there is a massive companion orbiting HD118473, causing that star to wobble back and forth with a period of 5.67 years.
But it can’t be a planet (it’s far too massive), and it can’t be a star (we’d be able to see it). So what could it be? The answer comes down to the way stars live and die.

一个关乎“死亡”的转折
径向速度数据很清楚——HD118473轨道上有一个巨大的伴星,导致该恒星前后摆动的周期为5.67年。但它不可能是一颗行星(它太大了),也不可能是一颗恒星(我们能看到恒星)。那是什么呢? 答案归结于恒星的诞生与灭亡。

Vast as stars are, their supply of fuel is not unlimited. Eventually the fuel runs out and the end of the star’s life is imminent. The more massive the star, the more spectacular that end will be.
A star like the Sun will eventually swell to become a red giant, then will puff off its outer layers, creating a spectacular planetary nebula, and leaving behind a glowing ember – its core, bare and exposed to space.

尽管恒星很大,但它们的燃料供应并不是无限的。最终燃料耗尽,这颗恒星的生命即将结束。恒星的质量越大,其结局就会越壮观。
像太阳这样的恒星最终会膨胀成一颗红巨星,然后向外层膨胀,形成一个壮观的行星状星云,并留下一个发光的余烬——它的核心裸露在外,暴露在太空中。

That core is a white dwarf – around the size of Earth, but with the mass of a star. Tiny, compared with the star from which it came, the white dwarf gradually cools and fades to obscurity over billions of years.
More massive stars die violently – as supernovae that outshine whole galaxies. But they also leave behind corpses that are faint and hard to spot. Neutron stars – the size of a city, but with a mass greater than the Sun – and black holes – tiny and invisible, except when they’re devouring something.

核心会是一颗白矮星——与地球大小相仿,但质量和恒星差不多。与它的起源恒星相比,这颗白矮星很小,经过数十亿年的时间,它逐渐变冷变暗。
更多的大质量恒星以超新星的形式猛烈消亡,它的光芒超过了整个星系。但它们也留下了一些模糊的、难以辨认的尸体,会产生中子星——城市大小,但质量大于太阳;还有黑洞——很小,看不见,除非它们在吞噬什么东西。

Let's go back to the companion star hidden on HD118473, which has the mass of a star but is too weak to see. What would it be?

让我们回到HD118473上隐藏的那颗伴星——有着一颗恒星的质量,但太微弱而看不见。会是什么呢?

An unexpected ancient relic——By far the most likely answer is that the hidden companion is a white dwarf. In the distant past, HD118473 was a binary star with the two components shining bright as they orbited their common centre of mass.

一件意想不到的“古老遗迹”——到目前为止,最有可能的答案是这颗隐藏的伴星是一颗白矮星。在遥远的过去,HD118473是一对双子星,当两颗恒星围绕它们共同的质心旋转时,这两颗恒星的两部分就会发出明亮的光。

For a few billion years, nothing changed, until the more massive of the stars reached the end of its life. It swelled to become a red giant then shed its outer layers, leaving behind a white dwarf, too dim for us to detect.
The white dwarf’s companion continues through space as we speak, still whirling in a celestial waltz with what remains of its companion. A dim, hidden relic to deceive exoplanet hunters, and a reminder of how science always has another surprise waiting around the corner.

在几十亿年的时间里,一切都没有改变,直到更大质量的恒星走到生命的尽头。它膨胀成一颗红巨星,然后剥离了它的外层,留下了一颗白矮星,光线太暗,我们无法探测到。
就在我们说话的时候,白矮星的伴星还在太空中继续飞行,还在和它的伴星的残骸在空中旋转着华尔兹。一个昏暗的、隐藏的遗迹,用来欺骗系外行星猎人,并提醒人们科学总是有另一个惊喜在拐角处等待着。

(评论部分)

David Spicer
Thank you for a very interesting article. I would like to ask a question. I assume by the time that the star has become a white dwarf, it has converted all its hydrogen into helium, and most of its helium into heavier elements. For a white dwarf to be “around the size of the Earth but with the mass of the Sun”, these remaining elements must be very heavy. What might these elements be?

谢谢你这篇有趣的文章。我想问个问题。我假设当恒星变成白矮星的时候,它已经把所有的氢转化成了氦,大部分氦转化成了更重的元素。对于一颗“与地球大小相仿,但与太阳质量相当”的白矮星来说,这些剩余的元素一定非常重。这些元素是什么呢?

Jonti Horner(回复楼上)
南昆士兰大学天体物理学教授

That’s a great question : By the time a star becomes a white dwarf, it has exhausted the hydrogen within its core - converting that hydrogen to helium. Whether it can proceed further through the periodic table depends on the mass of the star - with more massive stars able to generate greater temperatures and pressures in their cores, which in turns allows heavier elements to fuse into those that are heavier still…

这是一个很好的问题:当一颗恒星变成一颗白矮星时,它的核心已经耗尽了氢——将氢转化为氦。它能否在元素周期表中继续下去取决于恒星的质量——质量越大的恒星能够在其核心产生更大的温度和压力,这反过来又允许更重的元素聚变成更重的元素……

I believe that, in the majority of cases, white dwarfs will be typically either primarily helium (i.e. the star was not massive enough to initiate helium burning), or, more often, carbon and oxygen (i.e. the star was massive enough to have a period of helium burning, but was not sufficiently massive to initiate fusion of the carbon and oxygen produced).

我相信,在大多数情况下,白矮星要么主要是氦(即恒星的质量不足以引发氦燃烧),要么更常见的是碳和氧(即恒星的质量足以进行一段时间的氦燃烧,但不足以引发碳和氧的聚变)。

The incredible density is the result of the strong gravitational pull of the mass left over - with radiation pressure to support it, the stellar remains that are ejected into the planetary nebula collapse under gravity. That collapse only stops when something is able to resist it. In the case of the Earth, the physical strength of the material from which our planet is made is enough to balance against gravity - so our planet won’t collapse further under its own gravity.

这种令人难以置信的密度是剩余物质质量强大引力的结果——在有辐射压的情况下,恒星残骸会在引力作用下被喷射到行星状星云的坍塌中。只有当某些东西能够抵抗它时,这种坍塌才会停止。以地球为例,构成我们星球的物质的物理强度足以与地心引力平衡,因此我们的星球不会在自身地心引力的作用下进一步坍缩。

For white dwarfs, the thing that arrests their collapse is known as ‘electron degeneracy pressure’. Essentially, the atoms of the white dwarf are packed together so tightly that their electrons butt up against one another. It all gets quite complex at that point, but you essentially have a ‘pressure’ related to the fact that you can’t force two electrons to occupy the same space - and that repulsion is what holds the white dwarf up. That’s what sets the size (about the size of the Earth) - you can’t get smaller without being able to overcome that pressure.

对于白矮星来说,阻止它们坍缩的东西被称为“电子简并压力”。从本质上讲,白矮星的原子紧密地聚集在一起,以至于它们的电子相互碰撞。这一切都变得相当复杂,但实际上会有一个“压力”,因为你不能强迫两个电子占据同一个空间,而这种排斥是支撑白矮星的原因。这就是决定大小(大约是地球的大小)的原因——如果不能克服这种压力,你就不能变得更小。

If you have something more massive than the ‘Chandrasekhar limit’ - about 1.44 times the mass of the Sun - then the attraction due to gravity is enough that the electron degeneracy pressure is overcome. At that point, things can collapse further, which is why you get ‘neutron stars’, around the size of a city.

如果你有比“钱德拉塞卡极限”更大的质量——大约是太阳质量的1.44倍——那么引力的吸引力足以克服电子简并压力。在那个时候,物质会进一步坍缩,这就是为什么你会得到一颗“中子星”,大约一个城市的大小。

Neutron stars are held up by a combination of neutron degeneracy pressure (like electron degeneracy pressure, but for neutrons - at these size scales the protons and electrons that made up the elements in the star before collapse have essentially been smushed together to make neutrons), and also the effect of repulsive nuclear forces.

中子星是由中子简并压力和排斥核力作用的组合所支撑的。(中子简并压力像电子简并压力,但对于中子——在这个尺度上,在坍缩之前组成恒星元素的质子和电子基本上被压碎在一起形成中子)

These are all overcome, too, if you get massive enough - somewhere between two and three times the mass of the Sun (if I remember right). At that point, not even nuclear repulsive forces can overcome gravity’s inexorable pull, and the collapse continues to create a black hole…Hope this helps clear things up a bit!

如果你的质量足够大——大约是太阳质量的2到3倍(如果我没记错的话),这些都会被克服。在那一点上,即使是核排斥力也无法克服地心引力不可阻挡的牵引力,而坍缩会继续产生黑洞……希望这能帮你把事情弄清楚一点!

Mike Trimboli  (回复楼上)
Electron degenerate pressure is a common phenomenon of quantum degenerate pressure. The Pauli exclusion principle does not allow two particles with identical half-integer spins to occupy the same quantum state at the same time, thus creating a pressure that resists compression. Why is it mentioned that the imperviousness of solid matter is caused by quantum degenerate pressure rather than the electrostatic repulsion previously thought?

电子简并压力是量子简并压力的一种常见现象的具体表现。泡利不相容原理不允许两个相同的半整数自旋的粒子同时占据相同的量子态,因此产生了一种抵抗压缩的压力。有人提到固体物质的不透水性是由于量子简并压力引起而不是以前认为的静电斥力,这是为什么呢?

Jonti Horner(回复楼上)
Another great question - and the most honest answer is that this is starting to push beyond my own personal area of expertise.
Whilst I wouldn’t normally recommend Wikipedia as a resource (since it is fluid, and can change at any time), it does currently have a reasonably detailed description of Electron degeneracy pressure that might help answer things!

另一个很棒的问题——最诚实的答案是,这开始超越我个人的专业领域。
虽然我通常不会推荐维基百科作为一种资源(因为它是不固定,可以随时更改),但它目前确实有一个相当详细的电子简并压力的描述,这可能有助于回答一些问题!

Sorry I can’t myself give a better answer - but whilst I did cover all this in my higher level courses as an undergrad (including the derivations, and the fine details), that was twenty years ago, and I haven’t to touch it since!

对不起,我自己不能给出一个更好的答案——但是,尽管我在本科阶段的高级课程中涵盖了所有这些内容(包括推导和详细的细节),但那是20年前的事了,从那以后我就再也没碰它了!
 
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