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Fastest star ever seen is moving at 8% the speed of light:
In the center of our galaxy, hundreds of stars closely orbit a supermassive black hole. Most of these stars have large enough orbits that their motion is described by Newtonian gravity and Kepler's laws of motion. But a few orbit so closely that their orbits can only be accurately described by Einstein's theory of general relativity. The star with the smallest orbit is known as S62. Its closest approach to the black hole has it moving more than 8% of light speed.
[...] For years, S2 was thought to be the closest star to SgrA*, but then S62 was discovered. As a team recently discovered, it's a star about twice as massive as the sun that orbits the black hole every 10 years. By their calculations, at the closest approach, its speed approaches 8% of the speed of light. That's so fast that time dilation comes into play. An hour at S62 would last about 100 Earth minutes.
Because of its proximity to SgrA*, S62 doesn't follow a Keplerian orbit. Rather than being a simple ellipse, it follows a spirograph motion by which its orbit precesses about 10 degrees with each cycle. This kind of relativistic precession was first observed with the orbit of Mercury, but only as a small effect.
Journal Reference:
Florian Peißker, Andreas Eckart, and Marzieh Parsa. S62 on a 9.9 yr Orbit around SgrA* - IOPscience, The Astrophysical Journal (2020) (DOI: 10.3847/1538-4357/ab5afd)
(Score: 3, Interesting) by Common Joe on Monday August 17 2020, @09:44AM (18 children)
I'm not a physicist nor an astronomer, but the first question that pops into my head is "8% of the speed of light relative to what?" At that kind of speed, there's should be noticeable time dilation, shouldn't there? It seems there should be a better way to describe this.
Perhaps someone more knowledgeable than me can comment on this part of the article.
(Score: 3, Informative) by FatPhil on Monday August 17 2020, @10:08AM (14 children)
Or the CMB. That would make sense too.
But who cares - do you know the difference between the two?
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(Score: 2) by inertnet on Monday August 17 2020, @11:49AM (2 children)
The galactic center would be logical to me too. But they give the time dilation relative to Earth, which also makes sense.
But the 100 minutes for an Earth hour doesn't seem right to me. Time dilation at 8% of light speed would be almost one third percent. Speed would have to be 70% of light speed for one hour to last 100 minutes.
(Score: 3, Informative) by inertnet on Monday August 17 2020, @12:15PM
I realized that gravity must also contribute heavily to the time dilation. So those 100 minutes could be right after all.
(Score: 2) by tizan on Monday August 17 2020, @03:05PM
Relative to the black hole SGR A* is what matters in the end. Basically you want to find the velocity and gravity w.r.t the black hole and see when the star will fall in or escape.
But as other people are saying...the values will not change much w.r.t earth as the velocity of the sun or earth w.r.t SGR A* is small w.r.t to 8% speed of light ...so value of 8% most probably is correct whether the reference is galactic or earth.
For example the sun motion w.r.t CMB is of the order of 600 km/s ...
It does not matter for the magnitude of 8% of speed of light which is roughly which will be 30000 km/s (10% speed of light)
(Score: 0) by Anonymous Coward on Monday August 17 2020, @12:31PM
Compared to something moving at 8% the speed of light, those references are all the same.
(Score: 2) by PiMuNu on Monday August 17 2020, @01:23PM (9 children)
> Or the CMB
The CMB is travelling at the speed of light relative to everything - it is infrared light.
(Score: 1, Informative) by Anonymous Coward on Monday August 17 2020, @02:19PM
True, but it still has a reference frame, defined by the matter that emitted the radiation once the universe became transparent.
https://physics.stackexchange.com/questions/25928/is-the-cmb-rest-frame-special-where-does-it-come-from [stackexchange.com]
(Score: 5, Insightful) by maxwell demon on Monday August 17 2020, @03:41PM (4 children)
First, the CMB is not infrared light, it's microwaves (that's why it is called the cosmic microwave background).
Second, while the individual photons of course move with the speed of light, the same is not true for the CMB as a whole. This is because not all photons fly in the same direction.
One way to understand the rest frame of the CMB is to imagine some object in space that is opaque in the relevant frequencies, and that is not subject to any force except for the interaction with the CMB. If that body is in the rest frame of the CMB, the radiation pressure from all sides will be the same, and therefore the body will not feel a net force. Otherwise the radiation pressure will be larger in the direction it is moving relative to the CMB frame, and therefore it will experience a (very small) force in the opposite direction. You could consider it to be the friction of the CMB.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by PiMuNu on Monday August 17 2020, @03:58PM (1 child)
> the CMB is not infrared light, it's microwaves
Semantics!
> One way to understand the rest frame of the CMB...
Very interesting! I should have thought of that. Presumably it is also that frame of reference where the CMB is to first order (doing a multipole expansion or something) uniform frequency in all directions.
(Score: 3, Informative) by maxwell demon on Monday August 17 2020, @04:54PM
Yes. Indeed that is how you would actually measure the speed relative to the CMB. But I think the friction view is more intuitive, as it relates to something about “at rest relative to” from our everyday experience: We experience friction with another object only if we are not at rest relative to it.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 0) by Anonymous Coward on Monday August 17 2020, @04:43PM (1 child)
So the rest frame is the velocity at which the CMB frequency is the same in all directions. Nice. What's our current V wrt that?
(Score: 3, Informative) by maxwell demon on Monday August 17 2020, @05:06PM
According to this article, [scientificamerican.com] our speed relative to the CMB is 390 km/s. While it isn't written there, I guess it's an average, as the orbital speed of the earth is around 30 km/s, which is a significant fraction of the above speed, and according to the article linked above, the direction of movement relative to CMB is towards Leo, which is in the ecliptic (so at some time in the year, the orbital speed will be added to the sun's speed, and half a year later it will be subtracted from it).
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by FatPhil on Tuesday August 18 2020, @12:15PM (2 children)
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(Score: 2) by PiMuNu on Tuesday August 18 2020, @01:02PM (1 child)
Thanks for that. I just hadn't ever appreciated that there is a universal rest frame. Despite studying cosmology at undergraduate. Fallacy of being a plonker :)
(Score: 2) by FatPhil on Tuesday August 18 2020, @06:58PM
Yes, I have been reading Sabine Hossenfelder's blog all day, how did you guess?
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(Score: 2) by DannyB on Monday August 17 2020, @05:07PM (2 children)
Would it be relative to the observer making the measurement?
Some other observer might measure a different speed for that same star.
Is there a chemotherapy treatment for excessively low blood alcohol level?
(Score: 2) by Common Joe on Tuesday August 18 2020, @09:04AM (1 child)
That's my point. It's not explicit, but I assume the observer would be Earth, so it's moving 8% the speed of light relative to Earth.
That's assuming we have a top-down view of the star. If the star wobbles back and forth from and to us, then that would change the speed at which we see it at (faster when coming towards us and no speed when going moving sideways), so 8% doesn't seem accurate anymore.
This is the reason I asked my question. The whole "8%" measurement seems not precise to me.
(Score: 2) by DannyB on Tuesday August 18 2020, @01:43PM
Even if the measurement of 8% were off by about +/- 12 % (eg, +/- 1, making it either 7% or 9% the spoon of light), that still says that it is an incredibly fast motion of a star, and is noteworthy.
Is there a chemotherapy treatment for excessively low blood alcohol level?
(Score: 3, Interesting) by FatPhil on Monday August 17 2020, @10:06AM
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(Score: 0) by Anonymous Coward on Monday August 17 2020, @01:22PM (4 children)
If your space ship hyperspace jumped to this point of closest approach, what escape velocity do you need to escape the blackhole?
(Score: 0) by Anonymous Coward on Monday August 17 2020, @03:08PM
And when can we start mining black holes so that tech billionaires can keep little trophies made out of black hole pixie dust on their mantelpieces?
(Score: 0) by Anonymous Coward on Monday August 17 2020, @03:11PM
Only a little faster than the star is moving. I used a quick online calculator given the star's periapsis (closest approach) distance from Wikipedia, and the mass of the black hole and it came up with 21,000 km/sec, which is about 7% of the speed of light. Of course, the online calculator is just a simple Newtonian calculation and doesn't take into account all the factors going on with a supermassive black hole. But in general, speed at the periapsis of a highly elliptical orbit, which this one is, is only a little slower than the escape velocity.
At periapsis, the star is only about 2.4 billion km away from the black hole - that's closer than Uranus is to the Sun.
The linked article gives the speed of the star at about 8% of light speed - but both Wikipedia and the source paper list it at 10%. It's fast.
(Score: 3, Insightful) by https on Monday August 17 2020, @05:00PM (1 child)
A bit more than 0.08c, it looks like. Else that star wouldn't be in orbit.
Offended and laughing about it.
(Score: 2) by FatPhil on Wednesday August 19 2020, @05:45AM
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(Score: -1, Spam) by Anonymous Coward on Tuesday August 18 2020, @05:47AM
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OR
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APK
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