The Galaxy’s Fastest Star Could Reveal the Secrets of a Supermassive Black Hole
The Galaxy’s Fastest Star Could Reveal the Secrets of a Supermassive Black Hole
银河系最快恒星或将揭示超大质量黑洞的秘密
A team of astronomers has just detected the fastest star in the galaxy. Its anomalous speed has a more unsettling explanation: It orbits Sagittarius A*, the supermassive black hole at the center of the Milky Way, the mass of which is equivalent to four million times that of the Sun. 天文学家团队刚刚探测到了银河系中速度最快的恒星。其异常的速度背后有着一个令人不安的解释：它正绕着银河系中心的超大质量黑洞——人马座 A* (Sagittarius A*) 运行，该黑洞的质量相当于四百万个太阳。
S301, as astronomers have named the star, reaches 25,000 kilometers per second (km/s) at its maximum speed and takes 8.7 years to complete one orbit around Sagittarius A*. (By comparison, the Sun orbits the center of the galaxy at about 230 km/s.) At its peak speed, S301 moves more than 100 times faster than our star, and reaches 8 percent of the speed of light. 这颗被天文学家命名为 S301 的恒星，其最高速度可达每秒 25,000 公里，绕人马座 A* 运行一周仅需 8.7 年。（相比之下，太阳绕银河系中心运行的速度约为每秒 230 公里。）在峰值速度下，S301 的移动速度比太阳快 100 多倍，达到了光速的 8%。
The key to S301’s speed lies in how close it passes to the supermassive black hole. According to the European Southern Observatory (ESO), at its closest approach, the distance between the two is comparable to that between Saturn and the Sun. S301 速度的关键在于它与超大质量黑洞的近距离接触。据欧洲南方天文台 (ESO) 称，在最接近时，两者之间的距离与土星到太阳的距离相当。
The enormous gravitational pull of Sagittarius A* accelerates the star as it approaches the black hole. S301, however, does not fall directly into it, but instead follows an extremely elongated orbit around it. 人马座 A* 巨大的引力在恒星靠近时对其进行了加速。然而，S301 并没有直接坠入黑洞，而是沿着一条极其细长的轨道绕其运行。
The shape of its orbit also explains why its speed varies so much. At its closest approach, it moves faster, while when it is farther away, it moves more slowly. Something similar happens in the solar system with some comets, such as Halley’s Comet. 轨道的形状也解释了为什么它的速度变化如此之大。在最接近时，它移动得更快；而在远离时，它则移动得更慢。太阳系中的一些彗星（如哈雷彗星）也存在类似的情况。
S301 is the known star with the highest orbital velocity in the Milky Way, and also the one that comes closest to Sagittarius A*. It likely did not form where we see it today, and may have lost a companion along the way. S301 是目前已知银河系中轨道速度最快的恒星，也是最接近人马座 A* 的恒星。它很可能并非形成于我们今天所见的位置，并且在演化过程中可能失去了一颗伴星。
“S301’s orbital properties, and the fact that stars cannot form so close to a massive black hole, indicate that the star was likely part of a binary pair that was torn apart by the tidal forces of Sagittarius A*. In the process, S301 became trapped by the black hole’s gravity while its companion star was kicked out with high velocity, most likely enough to leave the galaxy altogether,” explains the ESO. 欧洲南方天文台解释道：“S301 的轨道特性，以及恒星无法在如此靠近大质量黑洞的地方形成这一事实，表明该恒星很可能曾是一个双星系统的一部分，后被人马座 A* 的潮汐力撕裂。在此过程中，S301 被黑洞的引力捕获，而它的伴星则以极高的速度被甩出，极有可能已经彻底离开了银河系。”
Astronomers believe that within 10 years they will be able to determine the rotation of Sagittarius A*. Mass and rotation, or spin, are two of the fundamental properties that scientists use to describe a black hole astrophysically. So far, there are signs that Sagittarius A* is spinning, but it is not yet possible to determine precisely how much or in which direction it is spinning. 天文学家认为，在未来 10 年内，他们将能够确定人马座 A* 的自转情况。质量和自转（或称旋转）是科学家在天体物理学中描述黑洞的两个基本属性。目前已有迹象表明人马座 A* 正在旋转，但尚无法精确测定其旋转的程度或方向。
According to the theory of general relativity, a rotating black hole drags the spacetime around it along with it. That effect should slightly alter the orbit of S301. If astronomers are able to measure these small deviations over the next few years, they will be able to determine the spin of Sagittarius A*. 根据广义相对论，旋转的黑洞会拖拽周围的时空。这种效应应该会轻微改变 S301 的轨道。如果天文学家能够在未来几年内测量出这些微小的偏差，他们就能够确定人马座 A* 的自转。
A black hole’s spin would warp the fabric of spacetime. Tracking S301 could provide more insight into this phenomenon. 黑洞的自转会扭曲时空结构。追踪 S301 或能为这一现象提供更多见解。
Astronomers first detected S301 in 2023 using GRAVITY, an instrument on ESO’s VLTI in Chile. Since then, they have tracked its movement and, after identifying it, were able to trace it backward through observations dating back to 2017. By combining these data, they reconstructed its orbit with greater precision and determined that the star passed its closest approach to Sagittarius A* in early 2023. It will approach the black hole again in 2031. 天文学家于 2023 年首次利用位于智利的欧洲南方天文台甚大望远镜干涉仪 (VLTI) 上的 GRAVITY 仪器探测到了 S301。此后，他们一直追踪其运动轨迹，并在确认其身份后，通过追溯 2017 年以来的观测数据，以更高的精度重建了它的轨道，并确定该恒星在 2023 年初最接近人马座 A*。它将于 2031 年再次靠近该黑洞。
S301 is not the only known star orbiting close to Sagittarius A*. For decades, stars such as S2 have allowed astronomers to determine the black hole’s mass and test predictions of general relativity. The difference is that S301 passes so close to Sagittarius A* that it could allow them to directly measure its spin over the next decade—something that would require several more decades of observations with other stars. S301 并不是唯一一颗已知绕人马座 A* 运行的恒星。几十年来，像 S2 这样的恒星使天文学家能够确定黑洞的质量并验证广义相对论的预测。不同之处在于，S301 经过人马座 A* 的距离非常近，这可能使科学家在未来十年内直接测量出其自转——而使用其他恒星则需要再观测几十年才能做到。