当死星相撞！

引力一直在掀起波澜——真的是这样。2017 年 10 月，诺贝尔物理学奖被授予两年前首次直接探测到引力波（gravitational waves）的成果。同样在那个月，astronomers announced a huge advance in the field of gravitational waves：科学家们第一次观测到来自同一来源的光和引力波。我们来看看发生了什么。

Two glowing stars with red and black surfaces sit in the middle of a starfield.

在这幅示意图中，两颗中子星（neutron star）即将相撞。NASA 戈达德太空飞行中心

在一个星系（名为 NGC 4993）中，有一对相互绕转的中子星。Neutron stars是大质量恒星（质量超过我们太阳 8 倍的恒星）很久以前作为超新星（supernova）爆炸后留下的压碎残核。在这个星系中，以及在我们能看到的 all 星系中，有许多这样的双星对，但这一对特殊的中子星即将发生一件特别的事。

Two blue spheres circle each other on a grid representing space-time. As the spheres orbit, ripples propagate outward along the grid, representing gravitational waves.

引力波传播的动画。R. Hurt/加州理工学院/JPL

每当这些中子星绕转时，它们

Two bright spheres orbit each other, and pale arcs of blue, representing gravitational waves, ripple away from the spheres. The spheres get closer with each orbit, and as they do they distort, turning into teardrop shapes, with the points pointing toward the center. Then they touch and finally merge in a bright, white explosion.

Doomed neutron stars whirl toward their demise in this illustration. Gravitational waves (pale arcs) bleed away orbital energy, causing the stars to move closer together and merge.

NASA’s Goddard Space Flight Center/Conceptual Image Lab

The teeny energy loss caused the two neutron stars to get a teeny bit closer to each other and orbit a teeny bit faster. After hundreds of millions of years, all those teeny bits added up, and the neutron stars were very close. So close that … BOOM! … they collided. And we witnessed it on Earth on August 17, 2017.

At the center of this illustration is a bright region of light that looks like two balls that haven’t quite merged into one. Two rays of white and orange light emanate from that central collision, one up and to the right, the other down and to the left, though you can’t see the one to the left quite as well because there is also a disk of swirling material blocking the view. There is also a faint grid across the entire image, representing space-time. Ripples in the grid can be seen at the edges of the image, showing gravitational waves that had been emitted by the merger.

Illustration of two merging neutron stars. The rippling space-time grid represents gravitational waves that travel out from the collision. Narrow beams show the burst of gamma rays that are shot out just seconds after the gravitational waves. The swirling clouds of material are ejected from the merging stars.

National Science Foundation/LIGO/A. Simonnet (Sonoma State Univ.)

A couple of very cool things happened in that collision, and we expect they happen in all such neutron-star collisions. Just before the neutron stars collided, the gravitational waves were strong enough and at just the right frequency that the National Science Foundation’s Laser Interferometer Gravitational-Wave Observatory (LIGO) and European Gravitational Observatory’s Virgo could detect them. Just after the collision, those waves quickly faded out because there are no longer two things orbiting around each other!

LIGO and Virgo are ground-based detectors waiting for gravitational waves to pass through their facilities on Earth. When it is active, it can detect them from almost anywhere in space.

This animation of a gamma-ray burst shows two jets of material that look like two orange cones connected by their points and facing in opposite directions, one opening up and to the right of the center, the other opening down and to the left. The ends of the cones have bright magenta light, which represent an expanding shock wave. At the center is a wrapped-candy-shaped blue structure lined up with the jets which represents the kilonova, the neutron-rich debris of the explosion.

This illustration shows a snapshot of a gamma-ray burst caused by the merger of two neutron stars. Powerful jets (orange) emerge and plow into their surroundings, causing shock waves (pink). Just emerging at the center is the kilonova, the neutron-rich debris of the explosion (blue) powered by the decay of newly forged radioactive elements.

NASA’s Goddard Space Flight Center/Conceptual Image Lab

另一件发生的事就是我们所说的伽马射线暴。当它们靠得非常近时，中子星会碎裂开来，产生一场壮观但短暂的爆炸。在几秒钟内，我们的Fermi satellite观测到了那次爆炸发出的伽马射线。费米伽马射线暴监测器是我们观测天空的眼睛之一，专门寻找科学家们希望一发生就能捕捉到的这类伽马射线暴。

而那些伽马射线在引力波信号之后仅仅1.7秒就到达了。这次事件所在的星系距离我们1.3亿光年，所以光和引力波在被我探测到之前已经旅行了1.3亿年。

This animation GIF has text “Swift Ultraviolet light,” and shows the UV light Swift detected on August 18 and 29, fading between the two. The image shows the sky, black with several small circles of light. On August 18, the central source appears as a small yellow blob with a second white ball just to the side of it. On August 29, the white ball has disappeared, leaving just the larger yellow source.

NASA的尼尔·格雷尔斯雨燕天文台于2017年8月18日拍摄了星系NGC 4993（方框内）中中子星并合产生的千新星，此时距离探测到引力波和伽马射线暴大约过去了15个小时。插图：该星系的放大视图。NASA/Swift

在最初的伽马射线暴之后，爆炸产生的碎片继续发光，并随着向外膨胀而逐渐变暗。我们的Swift、Hubble、Chandra和Spitzer望远镜，以及多台地面天文台，都准备好用紫外线、光学、X射线和红外线来观测这次爆炸的余辉。这种卫星之间的协同观测，是我们几十年来一直与国际伙伴合作进行的事情，这样我们就能尽可能快地、尽可能多地用不同波长捕捉到这样的事件。

This animated GIF shows the region of the sky where the gravitational waves and gamma-ray burst were detected as seen by Hubble in visible light and Chandra in X-ray light, fading between the two. In visible light, there is a bright oval-shaped galaxy that takes up most of the image with a bright, white center region that fades into gray clouds around it. The site of the gamma-ray burst is outlined in a box, and shows a dim source in visible light about half way between the center of the galaxy and its edge. In X-ray light, the galaxy’s center and a couple of other sources appear as dots encircled in blue. The galaxy itself does not show up in X-rays. The site of the gamma-ray burst is a bright blue source.

与GW170817相关的千新星（方框内）由NASA的哈勃太空望远镜和钱德拉X射线天文台观测到。哈勃探测到了来自炽热膨胀碎片的光学和红外光。九天后，钱德拉探测到了喷流在扩散进入我们视线方向后朝地球方向发出的X射线余辉。NASA/CXC/E. Troja

天文学家一直认为中子星并合是某一类伽马射线暴的成因——即短伽马射线暴，就像他们在8月17日观测到的那次一样。直到我们能够把卫星数据与LIGO/Virgo的信息结合起来，我们才能直接证实这一点。

When this animation opens, there are concentric rings of pale blue the expand away and off the screen. At the center is a bright ball of light with two narrow cones of orange, fiery-looking material extend in opposing directions, tilted just to the right. During the first few seconds, there are magenta flashes of light that seem to be pushed along with the ends of the orange cones. The central ball expands into a puffy, electric blue cloud. The sequence represents the events that happened after two neutron stars merged, exploding in a gamma-ray burst.

这段动画呈现了2017年8月17日探测到的中子星并合事件GW170817之后九天里观测到的各种现象。它们包括引力波（浅色弧线）、产生伽马射线的接近

That event began a new chapter in astronomy. For centuries, light was the only way we could learn about our universe. Now, we’ve opened up a whole new window into the study of neutron stars and black holes. This means we can see things we could not detect before.

This animated GIF shows a fun animation of what happened on Aug. 17, 2017. The scene shows Earth on the left side with a cartoon depiction of the Fermi satellite near the center. The image appears to ripple, starting from a dot on the upper right of the image. A speech bubble raises from two site on Earth that says, “Did you hear that?” Then, 1.7 seconds after the ripple, a magenta “blast” of light appears where the ripple originated, and the Fermi telescope has a speech bubble that says, “I sure saw it!”

On Aug. 17, gravitational waves from merging neutron stars reached Earth. Just 1.7 seconds after that, NASA’s Fermi saw a gamma-ray burst from the same event. Now that astronomers can combined what we can “see” (light) and what we can “hear” (gravitational waves) from the same event, our ability to understand these extreme cosmic phenomena is greatly enhanced.

NASA’s Goddard Space Flight Center

The first LIGO detection was of a pair of merging black holes. Mergers like that may be happening as often as once a month across the universe, but they do not produce much light because there’s little to nothing left around the black hole to emit light. In that case, gravitational waves were the only way to detect the merger.

不过，中子星并合有大量物质可以发光。通过将不同种类的光与引力波结合起来，我们正在了解物质在极端环境下的行为。我们正在进一步了解引力波信息如何与我们已经从光中获得的知识相吻合——在此过程中，我们正在解开一些长期存在的谜团！