Space Stories - The Fastest Star in the Milky Way

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0:20Welcome to the 365 Days of Astronomy. I'm Aviva, your host. Today we're going to talk about the fastest star in the Milky Way. There is a star racing through the center of our galaxy at 25,000 km per second. That is more than 8% of the speed of light. Let me introduce you to S301, the fastest known star in the Milky Way. But its extraordinary speed is not actually the most interesting thing about it. S301 orbits Sagittarius A star, the 4 million solar mass supermassive black hole at the center of the Milky Way.

0:58Astronomers discovered S301 with Isovery Large Telescope Interferometer, or VLTI, in Chile, using gravity instrument now being upgraded to Gravity Plus. And finding this star was not easy. In our sky, S301 appears about 2 billion times fainter than Betelgeist, the bright orange star in Orion constellation. The VLTI makes observations like this possible by combining the light from four 8-meter telescopes. Together, they work as an interferometer, providing extraordinarily sharp views of the crowded center of our galaxy.

1:34S301 was first recognized in observations from 2023. Once astronomers knew its orbit, they went back through older observations and found traces of the star all the way back to 2017. And what they found was remarkable. S301 takes only 8.7 years to orbit Sagittarius A star. Its orbit is extremely elongated. At its closest approach, the star comes within only about 12 astronomical units, or 12 times the average distance between Earth and the Sun.

2:14That is roughly the scale of Saturn's neighborhood in our solar system. Except, of course, there is no sun at the center of this system. There is a black hole with a mass of more than 4 million solar mass. And when S301 dives toward the black hole, its speed climbs to around 25,000 kilometers per second. If we could somehow travel at that speed, the trip from Earth to the Moon would take only about 15 seconds. So, what exactly is S301 star?

2:47It belongs to a group astronomers call the S star. Now you might wonder, doesn't our Sun also orbit the center of the Milky Way? Yes, it does. But the Sun is about 27,000 light years from the galactic center, and it takes roughly 230 million years to complete one orbit. Its motion is governed by the combined gravity of the entire Milky Way. The S stars are different. They live in the immediate neighborhood of Sagittarius A star, where the black hole dominates their motions.

3:21Instead of hundreds of millions of years, their orbit takes only years or decades. For decades, astronomers have followed these stars as natural test particles for studying gravity in one of the most extreme environments we can observe. The most famous of them is S2, which completes an orbit every 16 years. Observations of S2 have already allowed astronomers to detect gravitational redshift and Schwarzschild precisions, both predicted by Einstein's general theory of relativity.

3:58Instead of tracing the same perfect ellipse again and again, S2 orbits shift by about 0.2 degrees per revolution, because space-time itself is curved around Sagittarius A star. S301 experiences the same effect, but more strongly. It orbits shift by about 1.9 degrees every revolution, and that is because S301 ventures much deeper into the black hole's gravitational field.

4:29But there is another mystery. How did a star get there in the first place? This is a really extreme and dangerous environment. An S301 is not particularly massive. It is probably an early F-type main sequence star with less than about 1.5 times solar mass. And forming a star this close to supermassive black hole is extremely difficult. Stars normally form when clouds of cold gas and dust collapse under their own gravity.

5:02Near Sagittarius A star, however, the enormous tidal force from the black hole can pull those clouds apart before they have chance to collapse. So, S301 probably formed somewhere else. Its extremely elongated orbit gives us a clue to what may have happened. S301 may once have been part of a closed binary system, two stars orbiting each other. Then, the pair wandered too close to Sagittarius A star.

5:34The black hole's tidal gravity tore the binary apart. One star thrown away at enormous speed, possibly fast enough to escape the Milky Way altogether. The other star was captured. And that captured star may be S301. But being trapped by a black hole has turned S301 into something astronomers desperately wanted. A probe of the black hole's spin. According to general relativity, a rotating black hole does more than curve space-time.

6:06It also drags space-time around with it. This is called frame-dragging or the lens-stirring effect. One way to picture it is to imagine stirring a spoon through a thick liquid. As the spoon rotates, some of the liquid around it is dragged along. Space-time is not a liquid. Of course, we know that. But the analogy gives us an idea of what a rotating black hole does to its surroundings.

6:36For S301, the effect of the black hole's spin on its orbit is tiny, potentially around a tenth of a degree per orbit, depending on how fast Sagittarius A star spins and how its spin is oriented. But S301 comes so close to the black hole that this tiny effect may finally be measurable. And that is the real significance of this discovery. S301 is currently the first practical stellar probe that could allow astronomers to directly measure the spin of Sagittarius A star through stellar dynamics.

7:13The astronomers hope that follow-up observations over roughly the next decade could reveal that signal. Gravitif Plus will keep tracking S301. And the future observations with instruments such as Mikado on ESO Extremely Large Telescope will provide even more precise measurement. One date will be particularly important, 2031, when S301 makes its next close approach to Sagittarius A star.

7:44So yes, S301 is the fastest known star in the Milky Way. But its speed may not be what makes it famous. A star that may once have been torn away from its companion and captured by the black hole at the center of our galaxy, could now become the tool that tells us how that black hole itself is spinning. And that is S301, a small, faint star living in one of the most extreme and dangerous places in the Milky Way. Thank you for listening. This is 365 Days of Astronomy.

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8:22The 365 Days of Astronomy podcast is produced by the Planetary Science Institute. Audio post-production is by me, Richard Drumm. Project management is by Aviva Yamani. And hosting is donated by LibSyn.com. This content is released under a Creative Commons attribution, non-commercial 4.0 international license.

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9:33With every new discovery from ground-based and space-based observatories and each milestone in space exploration, we come closer to understanding the cosmos and our place within it. Until next time, let the stars guide your curiosity. Curiosity Curiosity You