黑洞：看见不可见之物！

Black holes 是宇宙中最奇异、最迷人的天体之一。天文学家想研究大量的黑洞，但有一个大问题——黑洞是看不见的！因为它们不发出任何光，所以要在漆黑一片的太空深处找到它们藏身之处，可真是相当困难。幸运的是，我们有几种不同的方法，可以通过观察黑洞如何影响周围环境来间接地“看见”它们。

This animated gif of supercomputer data takes you to the inner zone of the accretion disk of a stellar-mass black hole. Purple and blue-colored swirls spin in toward a central black circle.

这段由超级计算机数据生成的动画，带你进入一个恒星级质量黑洞吸积盘的内区。事件视界（event horizon）是这样一条边界：所有轨迹，包括光的轨迹，都必须向内延伸。NASA’s Goddard Space Flight Center/J. Schnittman, J. Krolik (JHU) and S. Noble (RIT)

飞驰的恒星

如果你曾经花些时间仰望星空，你就会知道我们的宇宙可以多么宁静、祥和。但你知道吗？在我们的银河系（Milky Way）正中心，藏着一个怪物！天文学家注意到，有恒星以每小时约一

A simulation of stars orbiting Sagittarius A*, the black hole at the center of our Milky Way. Several stars and their orbits appear over time, with a counter counting up from 1995 to 2018. As the stars orbit, their orbits are shown as lines, showing that they are clearly orbiting something at the center of our galaxy.

This animation shows orbits of stars near the Milky Way’s central black hole based on images taken between 1995 and 2018. Using these stellar orbits, we can estimate the mass of the supermassive black hole to be 4 million solar masses.

Keck/UCLA Galactic Center Group

Two astrophysicists won half of the Nobel Prize in Physics in 2020 for revealing this dark secret. The black hole is truly monstrous, weighing about four million times as much as our Sun! And it seems our home galaxy is no exception – our Hubble Space Telescope has revealed that the hubs of most galaxies contain supermassive black holes.

Shadowy silhouettes

Technology has advanced enough that we’ve been able to spot one of these supermassive black holes in a nearby galaxy. In 2019, astronomers took the first-ever picture of a black hole in a galaxy called M87, which is about 55 million light-years away. They used an international network of radio telescopes called the Event Horizon Telescope. And in 2022, they used the same network of telescopes to reveal the black hole at the center of the Milky Way.

sing the Event Horizon Telescope, scientists obtained an image of the shadow of the black hole at the center of galaxy M87. This donut-shaped image glows in orange against a black background. The bottom portion of the “dount” is brighter, appearing almost white in places. The image shows emission from hot gas swirling around the black hole under the influence of strong gravity near its event horizon.

Using the Event Horizon Telescope, scientists obtained an image of the black hole at the center of galaxy M87 outlined by hot gas swirling around it under the influence of strong gravity near its event horizon.

Event Horizon Telescope collaboration et al.

In the image, we can see some light from hot gas surrounding a dark shape. While we still can’t see the black hole itself, we can see the “shadow” it casts on the bright backdrop.

Shattered stars

黑洞也有较小的种类。当一颗大质量恒星耗尽了用来发光的燃料时，它就会向内坍缩。这些轻量级或“恒星级质量”黑洞的质量是太阳的几倍到几百倍。它们散布在星系中我们发现恒星的那些地方，因为它们就是这样开始生命的。其中一些最初有一颗伴星，到目前为止，这是我们找到它们的最佳线索。

A large pancake-shaped disk of material swirls in toward a black hole at the center of the disk. An orange star enters the image on the right and behind the disk, but a stream of gas connects to the disk in a long, thin arc. A blue flame-shaped region rises above the disk, from near the black hole. A similar region is just visible protruding below the disk as well.

在这幅 MAXI J1820+070 的示意图中，一个黑洞从邻近恒星拉走物质，并将其吸入吸积盘。盘上方是一个称为日冕的区域。Aurore Simonnet 和 NASA 戈达德太空飞行中心

一些黑洞会窃取伴星的物质。当物质落到黑洞上时，它会变得超热并在 X-rays 中发光。天文学家发现的第一个被确认的黑洞，称为天鹅座 X-1，就是这样被发现的。

Watch how monster black hole rip apart a star in this animation.
NASA’s Goddard Space Flight Center

如果一颗恒星离超大质量黑洞太近，效果会更加戏剧性！超大质量黑洞不会像较小黑洞那样只是从恒星吸走物质，而是会把恒星完全撕碎成一股气流。这被称为 tidal disruption event。

掀起波澜

但如果两颗伴星都变成黑洞呢？它们最终可能会相互碰撞，形成一个更大的黑洞，并在时空——宇宙的结构——中激起涟漪！

In this simulation, a pansy-shaped structure emits vertical and horizontal purple ripples. Yellow structures, making up the pansy’s center, surround each black hole and illustrate the strong curvature of space-time. The purple of the petals and the purple waves emanating outward represent gravitational waves.

这幅可视化图显示了两个质量几乎相等的黑洞在相互盘旋并合并时发出的引力波。NASA/Bernard J. Kelly（戈达德和马里兰大学巴尔的摩县分校）、Chris Henze（艾姆斯）和 Tim Sandstrom（CSC Government Solutions LLC）

这些涟漪被称为 gravitational waves，以光速在太空中传播。到达我们这里的波极其微弱，因为时空非常坚硬。

三位科学家获得了 2017 Nobel Prize in Physics，因为他们使用 LIGO 观测到了由恒星级质量黑洞碰撞发出的引力波。尽管引力波很难探测，但它们提供了一种无需看到任何光就能找到黑洞的方法。

我们正与 European Space Agency 合作开展一项名为 LISA 的任务，LISA 代表激光干涉仪空间天线。当它在 2030 年代发射时，它将探测来自合并超大质量黑洞的引力波——这很可能是星系碰撞的信号！

Simulation of two colliding galaxies. Two galaxies, seen edge on, are in the last stages of colliding. The two, coin-shaped galaxies have bright centers that approach each other, and then quickly orbit each other a few times as they get closer and closer until the merge together. Streams of gas, dust and stars stream behind the two galaxies, ultimately tangling together into a single giant ellipse around the merged galactic centers.

在这幅模拟被称为“老鼠”的星系形成过程的图中，两个星系碰撞并合并。Josh Barnes（夏威夷大学）和 John Hib

Rogue black holes

So we have a few ways to find black holes by seeing stuff that’s close to them. But astronomers think there could be 100 million black holes roaming the galaxy solo. Fortunately, our Nancy Grace Roman Space Telescope will provide a way to “see” these isolated black holes, too.

Animation showing the concept of gravitational microlensing. On the left is a camera, on the right the apparent position of a background star it is imaging. Between the camera and the star, a black hole passes through. The light from the background star bends around the black hole, making the star appear as though it is in a different position. The process also acts as a virtual magnifying glass, amplifying the brightness of the distant background star.

When one star in the sky appears to pass nearly in front of another, the light rays of the background source star become bent due to the warped space-time around the foreground star. This star is a virtual magnifying glass, amplifying the brightness of the background source star.

NASA’s Goddard Space Flight Center/CI Lab

Roman will find solitary black holes when they pass in front of more distant stars from our vantage point. The black hole’s gravity will warp the starlight in ways that reveal its presence. In some cases, we can figure out a black hole’s mass and distance this way, and even estimate how fast it’s moving through the galaxy.