A star so close to the Milky Way’s black hole it feels its spin
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Overview
S301, a newly identified star orbiting the Milky Way’s 4.3-million-solar-mass central black hole, reaches 25,000 km/s and passes close enough for its orbit to reveal frame dragging. The Gravity Collaboration tracks S301 with the Very Large Telescope Interferometer, but measuring the black hole’s spin directly will require observing more of its 8.7-year orbit, likely over another 10 to 15 years.
Key takeaways
- S301 is the closest identified star to the Milky Way’s central black hole and orbits it every 8.7 years, reaching roughly 25,000 km/s at closest approach.
- The black hole’s spin can reveal how it grew: consistently directed inflows of material can spin it up, while inflows from varying directions can counteract one another.
- X-ray measurements infer spin from how close hot accretion-disk material can orbit and how strongly its iron emission is smeared, but the method relies on complex models.
- Frame dragging offers a more direct route to measuring spin because a rotating black hole twists spacetime and causes nearby stellar orbits to precess.
- The Gravity Collaboration’s Very Large Telescope Interferometer combines four 8-meter telescopes over baselines up to about 130 meters to resolve faint stars in the crowded Galactic Center.
- A direct spin measurement from S301 requires tracking two complete orbits; the Extremely Large Telescope may help identify and follow additional faint stars near the black hole.
Chapters
- A black hole’s mass and spin are among the few properties that preserve clues about how it accumulated material.
- Material arriving from many directions can leave a black hole spinning less rapidly, while material funneled in a consistent direction can spin it up.
- Measuring spin could test predictions about black holes in unmerged spiral galaxies, where gas may flow toward the center along spiral arms.
- The innermost stable circular orbit moves closer to a faster-spinning black hole, allowing spin estimates from X-ray emissions such as Doppler-smeared iron lines.
- X-ray spin estimates are indirect and depend on complex accretion-disk models; collecting enough data can take weeks for each black hole.
- Lense-Thirring frame dragging predicts that a spinning object twists nearby spacetime, causing orbital paths to precess; Earth-orbiting satellites have demonstrated the effect.
- Measuring a black hole’s spin through frame dragging requires resolving and tracking an individual object on a close orbit.
- The Gravity Collaboration uses the Very Large Telescope Interferometer in Chile, combining four 8-meter telescopes across baselines up to about 130 meters to resolve stars near the Galactic Center.
- First spotted in 2023, S301 is about 2 billion times fainter than Betelgeuse; its estimated 8.7-year orbit brings it within 1.78 billion kilometers of the black hole.
- S301 reaches about 25,000 km/s, or 8% of the speed of light, close enough for frame dragging to measurably affect its orbit.
- Only about half an orbit has been observed; measuring spin requires two full orbits, so the team expects another 10 to 15 years of observations, with the 39-meter Extremely Large Telescope potentially helping.
Transcript excerpt
0:00 We thought this was the closest star to the center of our galaxy, the Milky Way. Turns out we were wrong. It's actually this one, S301. [music] 2 billion times fainter than the brightest stars in the night sky. It comes closer to the black [music] hole at the center of the Milky Way than Uranus does to the [music] Sun, traveling at over 25,000 km per second. That's 8% [music] of the speed of light. But, it's not getting torn apart and it's not getting sucked in by the black [music] hole. It's on a stable orbit and that is really, really good news for us astrophysicists. [music] Because with this star, we
0:46 should be able to directly measure how fast the Milky Way's black [music] hole is spinning for the first time. Because spin, mass, and charge, those are the only three properties that any [music] black hole has. And we've only ever directly measured one of them for the black hole at the center of the Milky Way. But, all of that is about to change thanks [music] to this new discovery by the Gravity Collaboration. But, why do we even care about black hole [music] spin? Well, that's something we're going to dive into in this video along with how we measure black hole spin, including directly through the >> [music] >> and how this newly discovered star holds the key to doing just that. So, let's dive into why we care about black hole
1:34 spin. A black hole is an object so dense that nothing, not even light, can escape it. And the event horizon marks the sphere within which everything is trapped, whether that's matter, light, or information of all kinds. That means that as a black hole grows, drawing in more and more material, all the
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Related notes
Summary, takeaways, and chapters were generated by AI from the video's transcript and may contain errors. The video belongs to its creator, Dr. Becky.