✨邻居小白✨The Little White Neighbour
每一颗星星都有自己的命运。
当它们的核心将所有的氢燃料聚变为氦燃料之后，
它们便会从主序阶段离开，进入生命的倒数。
Every star has its destiny,
while they move on from the main sequence phase after fusing all the hydrogen fuel into helium in their core, they enter the last stage of their life.
在只有一小部分大质量恒星才有资格终会成为神秘的黑洞，大多数的普通人只会有一个平静的结局——白矮星，退去行星状星云外层的恒星核心。
Only a small fraction of the massive ones end up becoming the fascinating and glorious black holes. The majority commoners just have a quiet ending – the white dwarfs. The now dead star’s core after losing its out layers as planetary nebulae.
图为一颗艺术绘画的白矮星，通常因为白矮星太暗，肉眼是看不见他们的
An illustration of a white dwarf, they are normally so faint that they are not visible to the naked eye
那今天就让我们注意力转向群星历史上的那些小人物吧。虽然微小普通，但他们无处不在。
Today let us turn our focus to some minor figures in the stellar history, though they seem ordinary they are all around us.
尽管他们不像黑洞那样迷人或复杂，但仍非常值得研究。首要原因就是，他们是我们太阳的未来，事实上也是所有质量低于8个太阳的恒星的未来；另一方面，他们能提供恒星的演化历史，恒星在宇宙中的数量和密度细节，甚至可能有宇宙中异常物质的线索。
Although not as fascinating and complex as black holes, they are still very worthwhile studying. For one reason, it is the future of our sun, in fact the future all stars below 8 Solar Mass; and for another, they tell the evolution of stars, give details on the stellar population and give insights on exotic matter.
图为不同质量恒星的命运，像太阳一般大小的恒星都将成为白矮星
Image showing the fate of stars with various masses, average stars like the Sun will all become white dwarfs
那群特殊的
The special ones
就像任何一个小团体一样，当中总有那么几个特别的存在。对于白矮星(WD)来说，它们就是极低质量白矮星(ELM WD)，质量均低于太阳质量的30%。
Just like any groups in the world, there is always a few special ones. For white dwarfs (WD), they are known as the extremely low mass white dwarfs (ELM WD), who’s masses are less than 30% of the Solar Mass.
根据目前的研究，一般恒星不可能在生命结束时留下那么微小的白矮星。唯一的可能性是，随着白矮星们年龄的增长而逐渐失去大量的重量。而这只有可能是它们在双星系统中，另外那个伴星拿走了那部分的质量。但不管怎么样，这个现象也是非常罕见的，现阶段只有大约100个已知先例。
Based on current studies, it is impossible for stars to leave these tiny WDs at end of their lifetimes. Therefore, the only possibility is for the WDs to loss significant amounts of weight as they age, which is possible if a binary companion from their in binary systems takes the masses; however, that is incredibly rare with only ~100 known cases.
虽然原母星的生命已经结束，但在某种程度上它也标志着白矮星生命的开始。作为古老的恒星核心，它们的温度非常高，具有高能紫外线辐射。温度如此之高，它们呈现出最热的火焰颜色——白色，也因此被称为白矮星。
Although the original parent star is dead, in a way it also marks the beginning of WD’s lifetime. Being old stellar cores, they are extremely hot with the high energy ultraviolet light radiations. So hot, they appear as the hottest flame color – white, and hence named white dwarfs.
图为NASA的哈勃望远镜拍到的星空，左边是右边图中放大的一小片区域，图中被圈出的均为白矮星。
An image of the sky observed by NASA's Hubble Telescope, on the left is an enlarged section of the image on the right, the circled stars are the white dwarfs.
然而，白矮星们并没能就这样从此过上了幸福的生活。随着时间的推移，它们会慢慢冷却，然后消失在浩瀚的宇宙中，成为黑矮星。但是，如果在它们身边有一个伴星（对ELM WD来说是极其可能的），它们可以在围绕彼此时发射引力波，甚至在它们接近后，因为质量太大而爆炸。
However, they do not stay like that and live happily ever after. Eventually, the WDs will cool and fade disappearing into the vast universe as black dwarfs. But, if the WD has a partner, which is most likely for the ELM WDs, they can emit gravitational waves as they orbit each other or even explode if they get too close and massive.
为了进一步研究这些过程，一个由Adela Kawka领导的澳大利亚团队开始了他们对偶然观测的2MASS J050051.85-093054.9（简称J0500−0930）的研究。
To look into these processes further, an Australian based team lead by Adela Kawka stared their research on a serendipitous observation, the 2MASS J050051.85–093054.9 (or J0500−0930 for short).
小邻居
The Little Neighbour
J0500−0930是一颗稀有的极低质量白矮星，它是盖亚卫星发现的离地球最近的白矮星，距离只有71秒差距，是第二近白矮星到地球的一半。
Confirmed as a rare ELM WD, J0500−0930 was discovered to be the closest of their kind to Earth by the Gaia satellite, only 71 parsecs away (about half the distance to the second closest).
同时，J0500−0930也恰巧在TESS卫星的观测区域之内，因此，有了大量的所需数据，对这个老朋友的研究无需太多新的探测。
It also happens that J0500−0930 falls into one of the observational sectors for the TESS satellite, meaning research on this old little friend a lot more convenient with much of the required data already in hand.
TESS（凌日系外行星勘测卫星），在之前的文章中也有简单介绍过，是NASA于2018年发射的，其主要任务是发现未知的系外行星，这些行星可能会成为下一个地球。在执行任务中，这里它提供了J0500−0930的大部分辐射数据。
TESS, the Transiting Exoplanet Survey Satellite, was launched by NASA in 2018 with its primary mission of finding unknown exoplanets, which could potentially be the next Earths. Along its mission, here it provided much of the radiation data on J0500−0930.
TESS每隔30分钟进行一次光度测量，测量光源发出的光量，并为我们的ELM WD生成光曲线。通过测量光曲线上不同频率的功率，也就是功率谱，澳大利亚的研究小组计算出了这个天体的周期 – 9.46小时。这意味着，这颗白矮星每9.46小时重复一次变化，这通常也是天体的轨道周期。
TESS performs photometry, the measuring of amount of light from the source, in 30 minutes intervals and produced light curves for our ELM WD. So, by measuring the power of the different frequencies on the light curve, also known as the power spectrum, the Australian group were able to calculate a period of 9.46 hours for the object. This means, the WD repeat its variability every 9.46 hours, which is often the time for the orbital period of the object.
图为经过数据总结两个图表：上图是由TESS数据生成的光曲线，每9.46小时会出现一次峰值；下图是J0500−0930 在9.46小时内的平均光值变化。
The plots from TESS' data: Top panel as the power spectrum of the TESS light curve with the peak corresponding to the 9.46 hour period; bottom panel as the light curve phase-folded onto the 9.46 hour period with the orange line as line of best fit.
在获取白矮星在不同时间的光谱后，研究小组还通过多普勒效应计算出了这颗白矮星的速度（有关这方面的细节可以在之前的红移和蓝移文章中找到）。然后通过数据，他们画出了链接不同时间和速度关系图标，也就是就是径向速度曲线。
By taking the spectra of the WD at multiple times, the group also found the velocity of the WD through Doppler Effect (details on this can be found in the previous article of Redshift and Blueshift). They then created a graph of the velocity with various times, creating a radial velocity curve.
用表中结果对功率谱再次计算，他们发现了相同的轨道周期，证实了TESS光曲线的计算结果。
Computing again with these results for the power spectrum, the same orbital period was found, confirming the computed results of TESS’ light curve.
但这还不够，为了更多地了解ELM WD本身，研究小组还需要进一步探索这颗白矮星的其他重要参数和大气成分，比如温度和表面重力。期间，研究小组通过拟合光谱和光谱能量分布两种方法的测量计算来确保了一致性。
In order to understand more of the ELM WD itself, the group also needed to study some other vital parameters of the object and the atmosphere components, such as the temperature and surface gravity. They do this via the fitting of both the spectra (light distribution) and the spectral energy distribution for the WD to check for consistency.
经过实验和计算，小组估算出这颗白矮星在一个双星系统中（两个天体围绕一个中心运行），质量仅有太阳质量的17%，边上是一个微弱的~0.3太阳质量的天体。正如你所看到的，与大多ELM WD相比，我们的邻居特别小，即使他已经在一个很低重量的群体中了。
In doing so, the groups estimated the WD to be only 17% of the Solar Mass with the faint companion of ~0.3 Solar Mass object. As you can see, comparing to the ELM WDs our neighbor is especially small, even among an already low mass group.
图为不同极低质量白矮星的温度和表面引力关系。图中红色点为我们的J0500−0930和他的伴星，蓝色点为GALEX J1717+6757（离我们第二近的 ELM WD）。
The relationship between effective temperature and surface gravity of known ELM WDs. J0500−0930, its companion and GALEX J1717+6757 (the second closest ELM WD to us) are shown by red and blue dots respectively.
消失还是绽放
Fade or Firework
经过了白矮星的双星系统的证实后，下一步便是继续研究这个独特的观测结果。这样的天体系统不仅是一个研究恒星演化的机会，它还可能提供一系列有关宇宙形成的信息。
Now a WD binary system (two objects orbiting a single center) is confirmed, it would only make sense to keep on studying this unique observation. It is beyond an opportunity for just the study of stellar evolution, it may be providing information on the formation of the universe.
研究发现，当两个白矮星相互绕轨道运行时，它们会在时空中产生微小的引力辐射涟漪，慢慢地释放能量，直到耗尽为止。
As two WDs orbit each other, small ripples of gravitational radiation would be caused in space time, which slowly releases the WDs’ energy until there is no more.
为了更好地探测这些辐射，并发现更多类似系统的例子，由欧洲航天局（ESA）领导与 NASA合作了一个名为LISA（激光干涉仪空间天线）的项目，计划在2034年发射。它将是第一个基于太空的引力波观测台，为目前不可见的宇宙天体提供潜在的发现。
To detect these radiations properly and giving more examples of system alike, the ESA (European Space Agency) has led a project with NASA called LISA (Laser Interferometer Space Antenna) planning to launch in 2034. It will be the first space-based gravitational wave observatory, providing potential discoveries on parts of the universe invisible for now.
视频为未来LISA发射后和地球一起的动态模拟。LISA系统将有三个单独的飞船组成，并跟随地球一起围绕太阳旋转。它们离地球距离将超过地球与月亮距离的100多倍。
The simulation of LISA's motion after launched. It is composed of three separate spacecrafts and trailing the Earth's obit of the sun. Their distance to us is more than 100 times the distance between Earth and Moon.
除了发射引力波外，另外科学小组发现1A型（IA）超新星是由白矮星爆炸而产生的。也就是这个恒星生命结束前最后高光时刻的机会。
On the other hand, instead of fading away, it is known that Type one-A (IA) supernova result from WD explosions. The last chance of making some ‘noise’ before the proper end.
这种爆炸的发生有两种可能性，第一种称为单一情况，由一颗白矮星单独爆炸。第二种，也就是更有可能发生的情况，被称为双退化情况，在这种情况下，两个白矮星合并导致了爆炸。
There are two possible ways for this event to occur, the first called single degenerate scenario, which happens with just a single WD. The second, the more likely scenario, is called the double-degenerate scenario, where two WDs merger and caused an explosion.
千万不要低估这些相对较小天体的潜在能量。IA型超新星是所有超新星中最亮的，在其亮度最高的时候，它们能点亮整个星系，并以3%的光速发射物质!
Don’t underestimate how much these comparatively small object can do. Type IA supernovas are the brightest of all supernovas, at the peak of their brightness they are able to outshine an entire galaxy and fire materials at 3% the speed of light!
因此，对我们来说，继续完善我们对这些超新星产生和演变的理解是至关重要的，这样才能增进我们对宇宙的认识。
Therefore, it is vital for us to continue perfecting our understand of how those supernovas actually happen, in order to progress our knowledge of the universe.
视频为白矮星的爆炸过程。这里白矮星通过引力从附近的伴星上获取物质，当质量达到太阳的1.4倍时，由于不再能维持自己的重量而爆炸
Animation of the explosion process of a WD. Here, the its gravity steals material away from a nearby stellar companion. When reaching ~1.4 solar mass, it can no longer sustain its own weight, and blows up
记忆将被永存吗？
Remember or Forgotten Forever?
遗憾的是，由于J0500−0930和他伴星的大小，它们非常微弱的引力辐射低于LISA的探测极限，因此即使在LISA启用后也不可能通过引力波来研究它们。LISA的主要研究对象主要还是那些大人物，比如黑洞们。
Unfortunately, due to the size of J0500−0930 and its companion, their very weak gravitational radiations is below LISA’s detection limit, therefore they cannot be studied via gravitational waves even when LISA is launched. The observatory is designed for larger object, such as black holes.
预测还表明，这两颗ELM WD的合并在未来300多亿年内都不会发生。这个宇宙现在也只有140亿岁，谁知道未来会发生什么呢？即使它们真的合并了，它们的大小可能也会不够大而只产生微不足道的，不至于产生能被探测到的超新星。
Predictions also show that the two ELM WDs’ merge will not happen for more than 30 billion years. With the universe only 14 billion years old, who knows what’s going to happen between then. Even if they do merger then, their size might not cause a large enough explosion to be detectable supernova.
那么，这是否意味着J0500−0930和它的伴星在发现后，就将这样永远地被我们封存在历史的长河中呢?
So, does that mean J0500−0930 and its companion would forever be some objects we’ve detected in history?
不，他们仍然有一个天然的优势亦是它的存在价值。他们离我们很近！为了将来全面解释像LISA这样任务的探测结果和持续IA型超新星进一步研究，事先对类似系统进行详细和精确的测量是至关重要的。
No, they still have one natural advantage over all other. Their close distance to us! For full interpretations of results from missions like LISA and the continual of Type IA supernova study, it is crucial to have detailed and precise measurements on similar systems beforehand.
因此，我们必须珍惜
我们和这位朋友之间‘亲密’的关系，
他为我们提供了一个非常值得学习
和探索的舒适圈!
Therefore, we must cherish
the close relationship we have
with our little friend,
for providing a place
where valuable information can be learnt!
图为一颗艺术绘画的白矮星和他在双星系统里的伴星
An illustration of a white dwarf and its companion star in a binary system
图片和视频来自 NASA, AASNOVA, Astronomy 官网
文中部分英文信息参考来澳大利亚和意大利学者合作， 由 Adela Kawka 带领的团队的 “The closest extremely low-mass white dwarf to the Sun" 论文
其余中英文内容为原创
Pictures and videos from official website of NASA, AASNOVA, Astronomy.
Parts are from “The closest extremely low-mass white dwarf to the Sun" by the scientist team lead by Adela Kawka from Australia and Italy
the rest of the Chinese and English content is original
