Scientists find closest star to the Milky Way’s central black hole

科学家发现银河系中心黑洞附近最近的恒星

Sgr A* is the name we’ve given to the supermassive black hole that sits at the center of the Milky Way. We’ve known about its presence since the 1970s but only managed to image it within the past few years. In the intervening time, most of our understanding of the object was obtained by watching a group of stars that orbit the black hole, helping us get a good estimate of its mass and size. In essence, those stars acted as instruments that let us peer into an environment we couldn’t study any other way. In Wednesday’s issue of Nature, researchers describe a recently discovered star that is on an extremely eccentric orbit that takes it closer to Sgr A* than anything we’ve previously identified. It gets so close that it may help us get our first measurements of the spin of the black hole.

人马座A*（Sgr A*）是我们为位于银河系中心的超大质量黑洞所起的名字。自20世纪70年代以来，我们就已知晓它的存在，但直到最近几年才成功对其进行成像。在此期间，我们对该天体的大部分了解都是通过观测围绕黑洞运行的一群恒星获得的，这帮助我们对其质量和大小进行了准确估算。从本质上讲，这些恒星就像是观测仪器，让我们得以窥探无法通过其他方式研究的环境。在周三出版的《自然》杂志上，研究人员描述了一颗新发现的恒星，它正处于一个极度偏心的轨道上，使其比我们之前发现的任何天体都更接近人马座A*。它离黑洞如此之近，以至于可能帮助我们首次测量出该黑洞的自旋。

Reading the spin

解读自旋

There’s an entire population of stars that orbit relatively close to Sgr A*. We can estimate their mass based on their brightness and spectral features. Using their masses and a reconstruction of their orbits using several years of data, we can figure out just how supermassive Sgr A* is (nearly 10^37 kilograms). Figuring out a black hole’s mass doesn’t seem to tell us much about it. But there isn’t exactly much else to tell. Our understanding of relativity says that, beyond mass, there are only two distinct properties of black holes. One of them is charge, and the other is spin. There’s no way to get information about charge. But spin is a different matter. We know that the materials that form black holes have momentum that has to be conserved by the spin of the resulting body. And our detection of gravitational waves from black hole mergers tells us something about the spin.

有一群恒星在相对靠近人马座A的地方运行。我们可以根据它们的亮度和光谱特征来估算它们的质量。利用这些质量数据以及通过多年数据重建的轨道，我们可以计算出人马座A的超大质量（接近10^37千克）。弄清楚黑洞的质量似乎并不能告诉我们太多关于它的信息，但实际上也没有太多其他信息可供挖掘。根据我们对相对论的理解，除了质量之外，黑洞只有两个独特的属性：一个是电荷，另一个是自旋。我们无法获取有关电荷的信息，但自旋则不同。我们知道，形成黑洞的物质具有动量，这些动量必须通过最终天体的自旋来守恒。此外，我们对黑洞合并产生的引力波的探测也为我们提供了一些关于自旋的信息。

So, we know there’s almost certainly some spin in Sgr A*. And it’s possible to learn something about it. The spin of Sgr A* should influence the precession of the orbit of any stars that pass sufficiently close. Unfortunately, that effect diminishes with the cube of the radius, which means we have to do one of two things: either find a star that goes close enough to the black hole, or do observations for long enough that the effects become apparent. Unfortunately, for all the stars we’re aware of near Sgr A*, “long enough” means thousands of years. So, without a new discovery, we’d either need to up our imaging game in order to get finer-scaled information about the orbits of the stars we know about, or be very, very patient.

因此，我们几乎可以肯定人马座A存在某种自旋，并且有可能对其进行研究。人马座A的自旋应该会影响任何足够靠近它的恒星的轨道进动。遗憾的是，这种效应会随着半径的立方而减弱，这意味着我们必须采取以下两种措施之一：要么找到一颗足够靠近黑洞的恒星，要么进行足够长时间的观测，直到这种效应变得明显。不幸的是，对于我们所知的靠近人马座A*的所有恒星来说，“足够长”意味着数千年。因此，如果没有新的发现，我们要么需要提升成像技术，以获取已知恒星轨道更精细的信息，要么就必须非常有耐心。

An extreme orbit

极端的轨道

The new discovery comes out of a program that uses the GRAVITY instrument on the Very Large Telescope at the European Southern Observatory. GRAVITY combines light from four individual telescopes, giving it the resolution of a single telescope with a 130-meter diameter. Since 2017, a team has been using the instrument to track stars in the immediate vicinity of Sgr A*. While GRAVITY doesn’t natively return image data, the team could convert it to images. In 2023, this allowed them to spot a star moving away from the black hole, which they termed S301. After several months of observations, they were able to estimate an orbit, which they used to predict its position in earlier data, allowing them to confirm they were looking at an actual star.

这一新发现源于一个使用欧洲南方天文台甚大望远镜（VLT）上GRAVITY仪器的项目。GRAVITY将四台独立望远镜的光线结合起来，使其具备了直径130米的单台望远镜的分辨率。自2017年以来，一个团队一直使用该仪器追踪人马座A*附近区域的恒星。虽然GRAVITY本身并不直接返回图像数据，但该团队可以将数据转换为图像。2023年，这使他们发现了一颗正在远离黑洞的恒星，并将其命名为S301。经过几个月的观测，他们估算出了其轨道，并利用该轨道预测了它在早期数据中的位置，从而确认他们观测到的是一颗真实的恒星。

This orbit suggested that, early in 2023, it had its closest approach to the black hole, one that took it closer than any star we had previously identified. The best orbital model suggests that this star takes only 8.7 years to complete an orbit, more than three years less than any other star in the area. The orbit is extremely eccentric, meaning that it is tracing a severely squashed oval. A circle has an eccentricity of zero, while any value over one means that an object isn’t orbiting, but rather taking a one-way trip around a gravitational attractor. S301’s eccentricity is 0.9832, suggesting it is close to escaping the gravitational grasp of Sgr A*. (For context, Pluto’s eccentricity is 0.25.)

该轨道表明，在2023年初，它运行到了距离黑洞最近的位置，比我们之前识别出的任何恒星都要近。最佳轨道模型显示，这颗恒星完成一次轨道运行仅需8.7年，比该区域的其他任何恒星都要快三年多。其轨道偏心率极高，这意味着它运行的轨迹是一个被严重压扁的椭圆。圆的偏心率为零，而任何大于1的值意味着物体不再是绕轨道运行，而是绕着引力源进行“单程旅行”。S301的偏心率为0.9832，表明它已接近逃离人马座A*的引力束缚。（作为参考，冥王星的偏心率为0.25。）

This leads to some pretty outrageous consequences. For example, at its closest approach, the star is moving at about 25,000 kilometers a second, which is over 8 percent of the speed of light. At this closest approach, the star passes 10 times closer to Sgr A* than the previously identified closest star. The researchers estimate that the distance may be as small as 11 Astronomical Units (one AU is the typical distance between the Earth and Sun). That would make the separation between the two just a bit beyond Saturn’s distance from the Sun. That’s close enough that, if S301 were a larger star (it appears to be about 1.5 times larger than the Sun), it would be disrupted by the gravity of the black hole.

这导致了一些相当惊人的后果。例如，在最接近点时，这颗恒星的移动速度约为每秒25,000公里，超过了光速的8%。在这次最接近时，该恒星与人马座A*的距离比之前发现的最接近恒星还要近10倍。研究人员估计，这一距离可能小至11个天文单位（1个天文单位是地球与太阳之间的典型距离）。这意味着两者之间的距离仅略超过土星到太阳的距离。这个距离已经足够近了，如果S301是一颗更大的恒星（它看起来大约是太阳的1.5倍），它就会被黑洞的引力撕裂。

A new tool

一种新工具

The researchers think that S301 got there by being part of a binary system that wandered too close to Sgr A*, with its partner having been ejected from the neighborhood of the black hole. But the more significant aspect of the discovery is what it could tell us in the future. The researchers estimate that we already have instruments that can track S301’s orbit with enough precision to provide an estimate of the spin of Sgr A* with about a decade of data. Further improvements in resolution or even a longer time series would let us start exploring even finer details of the supermassive black hole. These include whether it deviates from a perfect sphere, and whether the black hole has any additional properties beyond spin that influence its behavior. In short, finding S301 may be like developing a higher-precision instrument to study a supermassive black hole.

研究人员认为，S301之所以出现在那里，是因为它曾是一个双星系统的一部分，该系统在游荡时过于靠近人马座A*，而它的伴星则被从黑洞附近弹射了出去。但这一发现更重要的意义在于它未来能告诉我们什么。研究人员估计，我们现有的仪器已经能够以足够的精度追踪S301的轨道，只需大约十年的数据，就能估算出人马座A*的自旋。分辨率的进一步提高，甚至更长的时间序列，将使我们能够开始探索超大质量黑洞更精细的细节。这些细节包括它是否偏离了完美的球体，以及黑洞是否还有除自旋之外的其他影响其行为的属性。简而言之，发现S301就像是开发出了一种更高精度的仪器来研究超大质量黑洞。