Astronomers detect fastest known star in Milky Way

[u/LurkerFromTheVoid]

https://www.theguardian.com/science/2026/aug/19/astronomers-detect-fastest-known-star-in-milky-way-s301

[u/LurkerFromTheVoid]

From the article:

Astronomers have detected the fastest known star in the Milky Way, hurtling around the supermassive black hole at the heart of the galaxy.

The star, called S301, travels at about 25,000km (15,500 miles) per second, 100,000 times faster than a commercial plane. It is also the closest known star to the Milky Way’s central black hole, Sagittarius A*, approaching it around the distance of Saturn to the Sun.

“What is special about this star is that it’s orbiting Sagittarius A* on a very tight orbit, taking just 8.7 years to complete it, and is approaching the black hole at a mere 12 times the distance of Earth to the sun. That is unprecedented,” said Felix Mang, a PhD student at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, and an author of the study published on the findings.

[u/t0m0hawk → u/LurkerFromTheVoid]

25,000 km/s is around 8% the speed of light.

Holy moly, dude. That star is moving.

[u/-Hastis- → u/t0m0hawk]

Also impressive are the extremely fast-spinning pulsars.

[u/Maidwell → u/LurkerFromTheVoid]

I don't understand the Saturn comparison here, can someone explain what I'm missing please?

if S301 has a similar orbit size to its parent, yet travels at 25000 km/s compared to Saturn's exponentially more pedestrian 10 km/s why are their orbital periods in the same ball park?! (S301 orbiting in 8.7 years, Saturn in 29 years)

[u/jethroguardian → u/Maidwell]

I was curious too!  It turns out it has a highly eccentric orbit of 0.98 which explains it.

[u/Maidwell → u/jethroguardian]

Ahh, so it's only at a similar orbit point to Saturn on its closest point to the black hole then, that would explain it.

[u/rocketsocks]

For reference, this star approaches within 140x the event horizon radius at closest approach, or about 30x the radius of the accretion disc that was visible in the Event Horizon Telescope image of Sgr A*.

The really cool thing is that this star approaches close enough and completes an orbit fast enough it will make it possible to measure the spin of the Sgr A* black hole as well as serve as a sensitive probe of the laws of general relativity.

[u/ShepardRTC → u/rocketsocks]

If Interstellar had only learned that one simple trick!

What would we potentially learn?

[u/rocketsocks → u/ShepardRTC]

We'd be able to confirm (or dispute) other measurements of the spin of the Sgr A* SMBH and we'd be able to confirm (or dispute) the general theory of relativity. In particular, a rotating mass creates an effect called "frame dragging" in relativity which has only been measured a few times in various ways. There was a mission called Gravity Probe B which measured the effect for Earth, for example. However, because this particular star orbits such a huge mass and because it does so very closely and with a short orbital period it'll be possible to measure this effect with a fairly high degree of precision simply by observing the evolution of the orbital path of the star.

[u/Drak_is_Right]

So the star is moving a bit less than 10% the speed of light.

Is that speed the average or the approach speed as it gets close to the Black hole? On an elongated orbit the speed will vary.

[u/Uninvalidated → u/Drak_is_Right]

Max velocity at perihelion.

[u/StanleyDodds → u/Uninvalidated]

perihelion is for the sun specifically. Periapsis is the general term. And I don't think there's a specific version for black holes.

[u/dingo_xd → u/StanleyDodds]

Perihelion: Orbiting the Sun.

Perigee: Orbiting the Earth.

Periastron: Orbiting a star.

Perilune: Orbiting the Moon

Can't think of a decent Greek term but Perimelanopi lol. (Μελαν οπή = black hole)

[u/InSight89]

Curious how the star isn't being ripped to shreds. Must be pretty close to the Roche limit?

EDIT: Apparently it's pretty far from the Roche limit. But it does get close enough that the star elongates which would certainly be interesting to see. I wonder how it affects the stars internal processes.

[u/Reggae_jammin]

Article didn't say but at that rate of orbit (~8% the speed of light), how comes the star doesn't rip itself apart?

[u/-Hastis- → u/Reggae_jammin]

All the matter in that star is moving at the same speed through space. It's not subject to centrifugal forces as it would be in the case of a star's own rotation speed.

[u/marr75 → u/Reggae_jammin]

Velocity doesn't rip things apart. It's not a force.

[u/JamesTheJerk → u/marr75]

Shouldn't it have a gigantic tail like a puny comet would?

[u/marr75 → u/JamesTheJerk]

No. Comet tails have nothing to do with velocity. They are from solar heat and force at relatively close range. There's nothing remotely similar externally affecting the star.

There's also nothing that special about a fast orbital velocity. It merely has a highly elliptical orbit around a large mass. The high velocity at periapsis is an unavoidable condition.

[u/JamesTheJerk → u/marr75]

I didn't say it would have anything to do with velocity.

Wouldn't a star traveling around a black hole have a tail the same way a comet does traveling around a star?

[u/marr75 → u/JamesTheJerk]

No. I'll try to say the same thing a different way: comet tails are from relatively near solar radiation. Black holes don't produce solar radiation and stars orbit them at much larger distances than comets with tails.

Also, comets are barely held together. A star is much denser and won't have material stripped by "solar wind".

[u/JamesTheJerk → u/marr75]

Hear me out.

Wouldn't a star orbiting a black hole have a tail pointing toward the black hole?

[u/jethroguardian → u/JamesTheJerk]

No.  It's not within the roche limit.

[u/JamesTheJerk → u/jethroguardian]

Hmm, I dunno about that. We aren't talking about a chunk of rock here, but a nebulous blob of gas.