Fastest Star Ever Observed Orbiting Milky Way's Central Black Hole

Astronomers have discovered S301, the closest and fastest star ever observed orbiting Sagittarius A*, the supermassive black hole at the center of the Milky Way. This discovery provides a critical new tool for testing the limits of Albert Einstein's general theory of relativity.

Using the European Southern Observatory's Very Large Telescope Interferometer (VLTI), researchers identified the star traveling at approximately 25,000 kilometers (15,500 miles) per second. This velocity represents roughly 8% of the speed of light, making S301 the fastest known star in the galaxy. According to Felix Mang, a PhD student at the Max Planck Institute for Extraterrestrial Physics (MPE), the star's tight orbit is unprecedented in its proximity to the galactic center.

The Challenge of Black Hole Spin

Sagittarius A* possesses a mass roughly 4 million times that of the Sun, but measuring its spin—a phenomenon known as the Lense-Thirring effect or frame-dragging—has historically been a slow process. Because the effects of a black hole's spin diminish rapidly as distance increases, astronomers typically have to track multiple stars over several decades to gather sufficient data.

S301's extreme proximity to the event horizon changes this timeline. By observing a star that ventures so deep into the black hole's gravitational well, scientists can observe the warping of spacetime more acutely than with previous stellar targets.

Testing General Relativity

The discovery is expected to significantly accelerate the timeline for measuring the spin of Sagittarius A*, potentially reducing the required observation period from decades to just ten years. This measurement serves as a high-stakes verification of general relativity, which predicts how massive spinning objects drag the fabric of space around them.

"For the first time, we would actually be able to measure very directly the spin of a massive black hole, which would be a key test of Einstein’s theory," said Dr. Stefan Gillessen, a senior scientist at MPE.

Future Implications

Beyond the immediate test of relativity, understanding the spin of the Milky Way's central black hole offers deeper insights into the evolution of the galaxy itself. The spin provides clues about how the black hole grew over billions of years and how it interacted with surrounding matter.

Researchers will now focus on long-term monitoring of S301's eccentric path. While the star's speed and proximity are confirmed, the precise measurements of its spin-induced orbital precession will remain the primary objective for the team at MPE and the VLTI in the coming years.