122 points by belter 10 days ago | 8 comments
SiempreViernes 9 days ago
Journal article for those that are a bit wary of MIT press releases by now: https://academic.oup.com/mnras/article/530/4/4712/7667655
ck2 10 days ago
So a vaguely related question for an astronomy thread about our galaxy since smart people lurk here:

If the center of most galaxies is a super-massive black hole, including the Milky Way, and most of those SMBH have relativistic jets with lobes throwing out particles near light speed

1. Have we detected such lobes in the milky way? why not?

2. If those particles are going near the speed of light yet have no reason to slow down unless captured, unlikely outside of their original galaxy, they are still going for billions of years? (wow if so!)

3. If some of those jets from other galaxies are pointed at earth and contain physical particles with mass near the speed of light, why don't they do measurable damage?

reference: https://www.nustar.caltech.edu/page/relativistic_jets

Conasg 10 days ago
Those lobes have been detected in our galaxy, yes. There’s a page from 2012 by NASA talking about it: https://svs.gsfc.nasa.gov/10918

As I understand it, recent research suggests the last time our SMBH consumed enough matter to erupt was millions of years ago, so the lobes have cooled down and are difficult to detect.

SiempreViernes 9 days ago
No, the Fermi lobes are not what is meant when talking about jetted AGNs, though they are plausibly the remnants of past episodes where Sgr A* did have jets.
hannasanarion 10 days ago
> and most of those SMBH have relativistic jets with lobes throwing out particles near light speed

This is not correct. Most SMBH do not have relativistic jets. The jets only form when the black hole is actively consuming a large quantity of matter.

The Milky Way's SMBH Saggitarius A* is not actively eating anything, so it is not producing a jet.

colechristensen 10 days ago
>3. If some of those jets from other galaxies are pointed at earth and contain physical particles with mass near the speed of light, why don't they do measurable damage?

When they hit Earth (from these kinds of jets and other sources) they're cosmic rays. But it isn't a whole beam of them, it's individual particles way up near light speed. We can detect them, they can flip bits in computer memory, but they don't do a lot of damage because even at their speeds, a single proton, electron, or two or more protons as a bare nucleus still doesn't have a particularly large amount of energy on a human scale.


notaustinpowers 10 days ago
> 1. Have we detected such lobes in the milky way? why not?

Sag A* (our black hole in the center of the Milky Way) isn't considered "active" right now. We don't notice it gobbling up stars and gasses, which would be necessary for the jets to be possible. I remember back in 2020 or 2021 there was an article that we're noticing a jet from Sag A*, which we're still trying to understand why because we don't expect Sag A* to be active. It's also super difficult to monitor Sag A* since there is so much dense dust, gas, etc in the way between us and the SMBH.

> 2. If those particles are going near the speed of light yet have no reason to slow down unless captured, unlikely outside of their original galaxy, they are still going for billions of years?

Generally speaking, yeah, they are! If we're looking at photons though, they will eventually get red-shifted so much that they'll become infrared (invisible to us), until their energy is so low that it'll be near impossible to see without telescopes more powerful than anything we have right now.

> 3. If some of those jets from other galaxies are pointed at Earth and contain physical particles with mass near the speed of light, why don't they do measurable damage?

Space is a vacuum, but there are still things that can slow these particles down (loss of energy like photons, gravity wells from other massive objects, running into a spec of space dust, etc. Also, space is very empty, and statistically, it's incredibly improbable that one of these jets could be aimed directly at us, while also being close enough to us, to cause damage. We do notice them though! They're powerful enough to get picked up by scientific instruments, but are not concentrated enough or powerful enough to cause damage to us or Earth.

dylan604 10 days ago
> Generally speaking, yeah, they are! If we're looking at photons though, they will eventually get red-shifted so much that they'll become infrared (invisible to us), until their energy is so low that it'll be near impossible to see without telescopes more powerful than anything we have right now.

Isn't the precisely what JWST is built for?

notaustinpowers 10 days ago
Even JWST has it's limits. There are some very, very, very, very old galaxies that are so red-shifted, JWST is only able to see them thanks to gravitational lensing amplifying the energy of the light. https://www.space.com/james-webb-space-telescope-distant-gal...
PaulHoule 10 days ago
When it comes to the “oldest stars” there is reason to believe that very early there were very big Population III stars that formed very quickly and burned out fast leaving nothing but black holes and there is hope JWST will see some.

In general there are multiple recent observations that things seemed to happen much more quickly in the early universe than we expected so maybe what we think was the first 1 billion years was really the first 10 billion years or there is another big secret to be discovered in cosmology.

AnimalMuppet 10 days ago
I'm not an astronomer, and may or may not be smart.

1. I don't know; Google probably does know.

2. Those jets aren't in a complete vacuum. They're running into galactic gas, of which, on a galactic scale, there is quite a bit.

3. Several reasons. One, they aren't a perfect "beam". They spread out. If you're a few billion years away, they spread out quite a bit in that distance. Then, to get to us, they go through their galaxy's gas, intergalactic space (not totally empty), our galaxy's gas, and finally our atmosphere. Each of those reduces the amount of radiation. Oh, yeah, our magnetosphere deflects charged particles, too.

jwells89 10 days ago
For #3, I think I remember reading that the Sun’s heliosphere (which contains the entirety of the familiar Solar System) also plays a role in cutting down what gets to Earth, but I may be misremembering.
readams 9 days ago
Some very recent evidence suggests that such jets may exist for Sagittarius A*: https://www.livescience.com/space/black-holes/milky-ways-mon...
Dylan16807 10 days ago
They do do measurable damage!

The amount we measure is extremely small because of how wide the beams are by the time they reach earth.

SiempreViernes 9 days ago
Hey! Making extragalactic gamma-ray astronomy possible is not damage!
TheBlight 10 days ago
FWIW, it's not clear that SgA* is actually a canonical black hole (nor that even such a thing truly exists in nature.)

The EHT image is taken as confirmation but the accuracy of that technique has been called into question: https://arxiv.org/abs/2205.04623

Edit: In response to the downvotes, here are 2 very good sources who at least argue against the existence of singularities and their event horizons.

1: https://arxiv.org/pdf/2312.00841

2: https://uncnewsarchive.unc.edu/2014/09/23/carolinas-laura-me...

I don't know exactly when science discussion turned into rigid dogma enforcement but we are certainly in that era presently.

cthalupa 10 days ago
Kerr's paper is quite specifically about not believing in singularities, not about not believing in black holes. It's hardly a controversial opinion in the science community to believe that singularities in black holes are an artifact of our incomplete mathematical representation of how gravity works. That is not the same as suggesting that black holes or event horizons don't exist. Kerr's solution to the field equations involves two event horizons to begin with, and his argument in the new paper is based on Kerr black holes and explicitly talks about event horizons in multiple places.

I'm very confused as to why you believe that paper provides significant argument as to why calling SgA* a black hole would be jumping the gun.

TheBlight 9 days ago
dekhn 10 days ago
Neither of those are "very good sources". Further, they are basically mathematical modelling papers. We have a lot of experimental evidence about the nature of black holes. If you argument is just "black holes might not be a true singularity", well, nobody is strongly disagreeing with that, we just don't have evidence or good support for alternative models. People aren't being dogmatic, they just don't have any better models that explain the observations.
TheBlight 10 days ago
Laura Mersini-Houghton and Roy Kerr seem like very good sources to me. Are you familiar with their work? It seems not.

A "black hole" implies a singularity behind an event horizon not even light can escape from. There isn't any proof that such a thing exists in nature. You're correct in saying that we see the indirect gravitational effects of something that doesn't fit any model our imaginations have conjured up to date except for "black hole." That doesn't mean it's clear that black holes are a real thing.

wizzwizz4 10 days ago
> You're correct in saying […] a real thing.

This is, indeed, how science works. "All models are wrong", as they say. I'm not sure what you're trying to push back against.

TheBlight 10 days ago
That would be perfectly lovely for me if that was actually the attitude of most people weighing in on these discussions. But in practice, when it's insinuated that a more beloved sci-fi model may perhaps be incorrect/incoherent it's met with people being upset that it's being pointed out.
dekhn 9 days ago
I would not be upset if black holes are not singularities although I would be surprised if they didn't have event horizons.

IIUC you're basically talking about this: https://en.wikipedia.org/wiki/Nonsingular_black_hole_models which don't seem to be fully mainstream, nor supported by any observed evidence.

TheBlight 9 days ago
I'm not advocating for any given alternative model really. I primarily want to call out that we're not really sure what we mean when we say "black hole" and there is good reason to be at least a little skeptical-- especially of models invoking a singularity. The vast majority of people take it as a given that black holes as depicted by popular sci-fi films/TV exist in nature and we're far from being sure of that.

Somewhat related and maybe interesting to you: https://www.llnl.gov/article/40576/black-hole-loses-its-appe...

dekhn 8 days ago
I read the linked news article and the related paper and it just says that they're struggling to figure out what one of the light sources that's orbiting the hole is made of. The article doesn't really say much that isn't mainstream understanding of black holes.
TheBlight 8 days ago
You sort of have to read between the lines with popular science reporting. Prior to the event, the gas cloud was spotted and it was big news in the media as this would effectively be an empirical test of black hole theory and we'd get to see it in action. But the event came and went and nothing remarkable happened. You had to look it up to find out what ended up happening because it wasn't widely reported. We're still trying to come up with answers for why the experiment didn't turn out as expected.

So we either don't understand the gas cloud or our SgA* black hole model isn't correct (or some combination of both.) In either case, we seemed quite sure prior to the event. I think anomalies like this are where the really interesting information lies and we should be more humble wrt the veracity of our current models.

cthalupa 5 days ago
You're making a lot of arguments and linking a lot of things that are only loosely related and definitely are not actually supporting evidence for your argument.This article (and event in general) also have very little to say about whether or not black holes exist.

The galactic center is one of the noisiest portions of the galaxy and most difficult for us to look at. (Relatively) high densities of gas further obscure things within them. It's a pretty huge leap to go from the fairly obvious answer - we weren't quite right about everything inside of G2 - to "Black holes as we think of them don't exist."

And, the most recent evidence we have is... that we got the composition of G2 wrong. https://arxiv.org/pdf/2112.04543

10 days ago
mr_toad 9 days ago
GR implies a singularity, but there are other possible forms of black holes e.g. Hayward black holes. They would require a new theory of gravity.
sergent_moon 10 days ago
I think some of the apparently dogmatic attitude is from exhaustion. Usually (I'm not implying you) someone calling commonly accepted science into question are just waiting for a moment to drop something about Jesus, or Chemtrails or some other nonsense.
dotnet00 9 days ago
Or even more commonly with astronomy, are misinterpreting the material to prop up their pop science influenced preconceived notions.

I think everyone in the hard sciences has had some experience of having to deal with someone with no technical insight, arguing a point that has already been discussed in far more detail by experts and claiming that scientists are just being dogmatic because they don't care to repeat the same points over and over again.

ErigmolCt 10 days ago
Wow... discovering some of the oldest stars in our galactic neighborhood is a remarkable thing
marcosdumay 10 days ago
Shouldn't they be distributed everywhere?

Finding some close to us is just expected.

Sharlin 10 days ago
Indeed. And there should be plenty of them, given that a large fraction of all stars (evidently including these three) are K-type orange dwarfs slightly smaller and cooler than the sun. They last tens of billions of years on the main sequence, burning their hydrogen at a leisurely pace. A large majority of stars born back then should still be alive and kicking long after the sun is gone.
xenadu02 9 days ago
They are distributed everywhere. These three are among the oldest and presumably of similar age to other "oldest" stars in other galaxies.

I take it to mean many galaxies have surviving stars from the population of stars expected to still exist. Honestly it seems obvious. Any given galaxy will either be as old as those stars, formed from the merger of other galaxies some of which are old, or will have stolen some stars from an old galaxy during a flyby.

This story boils down to "at large scale stars are roughly evenly distributed".

caf 9 days ago
That plays it down far too much, because what's actually interesting here is that we have been able to identify three specific nearby examples.
SiempreViernes 9 days ago
Sure, it's not surprising that some exist in our galaxy, but actually finding some is still remarkable!
mr_toad 9 days ago
One of the oldest is only 200 LY away.


actionfromafar 10 days ago
Aren't some galaxies older than other.
bufferoverflow 9 days ago
Considering there are 200 to 1000 billion galaxies (based on what we can see), the odds of that is not just astronomically low, it's basically impossible.

Which tells you that the method for determining the age of stars is wrong.

Aloisius 9 days ago
Why exactly? It's three of the oldest stars, not the three oldest stars.

There could be a quadrillion stars of a similar age and the statement could still be true.

bufferoverflow 9 days ago
Ok, then the wording is horrible. As you can consider all the stars except the youngest one as "the oldest stars".
jbstack 9 days ago
I think the wording is fine. While technically you are correct, most people will not interpret "three of the oldest" to mean that they could be 2 seconds old as long as there is another star which is 1 second old.
thegrim33 9 days ago
It's a common journalist tactic that allows for painting a misleading picture. E.g., I read a story earlier today about "State X among the worst states for Y". The state could be #2 out of 50, and they'd be "among the worst 49". The state could be #49 out of 50, and they'd be "among the worst 2".

Same thing here. Three of the oldest stars? It means literally nothing but it paints a picture.

jeremyjh 9 days ago
“The oldest stars in the universe” are a cohort, not a ranking, and these three are members of it.
andrewstuart 9 days ago
The big bang happened right here, not in some distant galaxy.

Thus if there are old stars then they'd be ..... right here.

mr_toad 9 days ago
There are estimates that half the matter, including the older stars in our Galaxy came from other galaxies that were absorbed long ago (10B+ years) by the Milky Way. The younger stars have been formed here in the disk.

So on average a large portion of the oldest stars tend to have been from elsewhere, whereas the newer stars were born here.

SiempreViernes 9 days ago
They have just dropped the qualifier "known" from the title, as in it should be read "Three of the oldest known stars found in our galaxy".

We obviously don't know the age of each and every star in the entire universe, calm down dude.

munk-a 9 days ago
I feel like it's quite reasonable to criticize them for that though. "Known" isn't something we can automatically assume - there could be theoretical boundaries on star life and these objects were detected as being extremely close to that boundary.
verzali 9 days ago
There are limits on how old stars can be. They obviously can't be older than the universe itself. It also took time for star formation to be possible. The paper itself argues these are among the oldest stars that can exist in the galaxy, although it doesn't say they are actually the "oldest", just members of the group.
dotnet00 9 days ago
Known kind of by definition implies that it's based on our current knowledge.
munk-a 9 days ago
I'd clarify - there's a distinction between "oldest observed" and "oldest we think are possible" and the known in this context (for me at least) leans strongly into the "oldest observed" camp for me - though, as with all things english, technically it could mean anything.
alexander2002 9 days ago
(Disclaimer: I am not an expert) In my humble opinion, because the black hole calculations and other important proven scientific equations must have taken into account the age of stars, the age of a star could be imprecise but not outright wrong.
rthnbgrredf 10 days ago
> Interestingly they’re all quite fast — hundreds of kilometers per second

I bet that the milky way could only capture these ultra fast moving stars because of dark matter.

klyrs 10 days ago
This echoes a thought I had recently -- stars traveling at relativistic speeds should look deceptively young due to time dilation. But while this is certainly speedy, mere hundreds of km/s isn't enough to significantly prolong the observed lifetime of a star.
chasil 10 days ago
The stars that are going beyond our cosmological horizon are effectively traveling away from us faster than their light towards us.


"Or, more precisely, there are events that are spatially separated for a certain frame of reference happening simultaneously with the event occurring right now for which no signal will ever reach us, even though we can observe events that occurred at the same location in space that happened in the distant past.

"While we will continue to receive signals from this location in space, even if we wait an infinite amount of time, a signal that left from that location today will never reach us."

pdonis 10 days ago
> stars traveling at relativistic speeds

So far, to my knowledge, we have not observed any. As you note, hundreds of km/s is way too small to have any appreciable effect.

ChrisGranger 10 days ago
"The fastest star ever seen is moving at 8% the speed of light"


AnimalMuppet 10 days ago
0.08c translates into a gamma of 1.0032. That's not enough for relativity to be doing much...
bloopernova 10 days ago
How much relativity effects occur at 0.2c? A sci-fi book series I'm reading has warships unable to hit other ships if their relative velocity is above 0.2c. I was wondering if that was realistic.
pdonis 9 days ago
The gamma factor at 0.2c is only 1.02, which isn't much of an effect. The main difficulty with aiming at something moving that fast wouldn't be time dilation, it would be light travel time delay.

For example, suppose the other ship is one light-second away from yours and is moving at 0.2c. You're seeing the other ship where it was one second ago, and in that time it has moved 1/5 of a light second. Plus, supposing you're firing a laser at it, the laser will take another second to get to the target, so you have to aim 2/5 of a light second away from where you're seeing the other ship. And during that time the other ship could change direction and spoil your aiming calculation. The sci-fi book series probably is making some assumptions about how fast the ships can change direction, how fast and accurately they can aim, and typical distances between ships during combat to come up with the 0.2c number.

klyrs 10 days ago
I found this to help me not sound completely ignorant in this thread


luyu_wu 10 days ago
As the lorentz factor calculated above, not much! Time would only be dilated by that much, and length by the the same amount. It'd definetly be hard to hit something at that speed, but not due to relativity!
malfist 10 days ago
> should look deceptively young due to time dilation

That's not how we date stars. We typically date the star by it's metallic content. More non-hydrogen elements in it's spectrograph, then we know it's an older generation of stars.

Starlevel004 10 days ago
> More non-hydrogen elements in it's spectrograph, then we know it's an older generation of stars.

It's actually the exact opposite, but yes.

malfist 9 days ago
Yes thank you, got it backwards
pfdietz 10 days ago
There's some weird caveats, like so-called "Lead Stars". These are aged stars with very large amounts of lead relative to other heavy elements. This happens because lead is the end nuclear of the so-called s-process, where neutrons are captured at a slow rate in large stars whose internal processes produce some neutrons by (alpha,n) reactions.

In lead stars, there are so few seed nuclei (those metals) that each of the seed nuclei that are there can capture many more neutrons.

jddj 10 days ago
Does it matter in this context whether it's metals or tree rings or wrinkles around the eyes?

It's all time, right?

klyrs 10 days ago
> It's all time, right?

Sure, but we can measure time dilation between the peak of a mountain and the base of a mountain, due to the differing velocities. Time is relative!

AnimalMuppet 10 days ago
But they're in the halo. Can they actually be gravitationally bound at that distance at those velocities? Is even the (computed by other means) amount of dark matter enough for them to be captured?
Keysh 9 days ago
If they weren’t gravitationally bound, they would have left the Milky Way billions of years ago.

Also, “in the halo” doesn’t necessarily mean “far from the center of the galaxy”; it means “not orbiting in the disk”.[1] Some of these stars are closer to the galactic center than the Sun.

[1] And further away from the center of the galaxy than the bulge — but the bulge is much smaller than the orbit of the Sun.

jajko 10 days ago
Sun is moving cca 230 km/s around Sagittarius A*, so something even further from the center would have even higher speed if rotating at same speed (don't know if that's the trend in our galaxy, very much a tourist in astronomy)... doesn't sound that unusual unless its those 999km/s corner cases
deciduously 10 days ago
Except these stars are rotating the other direction
ashes-of-sol 10 days ago
Orbital velocity increases as you get closer to the middle, not the other way around.

An example closer to home, our orbital velocity around the Sun is 29.8km/s. Mercury is 47.9km/s (on average, it actually varies throughout its orbit). Neptune is 5.4km/s.

shrimp_emoji 10 days ago
This doesn't apply to stars in the Milky Way. Unlike planets around a star, stars in the Milky Way don't follow Keplerian physics in their orbit around the galactic center.



The rotational/orbital speeds of galaxies/stars do not follow the rules found in other orbital systems such as stars/planets and planets/moons that have most of their mass at the centre. Stars revolve around their galaxy's centre at equal or increasing speed over a large range of distances. In contrast, the orbital velocities of planets in planetary systems and moons orbiting planets decline with distance according to Kepler’s third law.


9 days ago
ChrisMarshallNY 10 days ago
> are still intact today

I would suggest, instead, that we can still see their light.

They may have popped their clogs, long ago, and we would not have known, as we're seeing their old videotapes.

There's something called the "Cosmic Event Horizon," or somesuch. It's the distance from us, that we'll never be able to see, because it is more than 13.8 billion light-years away, and we'll never see anything beyond.

Every time I think about the distances and scales of the universe, I get a headache.

[EDITED TO ADD] I wasn't talking about the nearby stars, and neither were they. That quote was talking about distant, red-shifted galaxies.

Starlevel004 10 days ago
> They may have popped their clogs, long ago,

Nearly every single star we can see in the entire Laniakea supercluster is still shining today.

The universe is big, but stars live for a really long time.

cvoss 10 days ago
> still shining today

It's an interesting quirk of these discussions of events at relativistic scales that it's very hard to precisely speak about what we mean whenever we reference time.

For all of us "here", who are within non-relativistic distance of each other, "today" is a meaningful point in time. But what does our "today" mean for that far-away star? I think you are trying to articulate that, if the star is X light years away from us, after X years from "today" we will still be receiving light that has traveled from the star to "here". But you might instead mean that if a traveller were to depart from "here" "today" at near relativistic speed, when he arrives at the star he will find it still shining "there" at "that time".

But notice those are definitely not the same data point about the star. The first data point will arrive here in X years to show us the star was still shining X years previously. But the traveler will collect the second data point (almost immediately for himself, by the way) and may find the star dead. This can happen if he and the star's last light cross paths in flight.

dhosek 10 days ago
My favorite thing along these lines is a question from my undergrad special relativity textbook:

A pole vaulter carrying a 40m pole is running at a speed such that to an observer, he appears contracted by ½. He runs through a barn of length 20m and the doors at each end of the barn are closed simultaneously.

But to the pole vaulter, the barn appears contracted by ½ and thus appears to be 10m long to him. What does he see when the doors are closed?

dhosek 10 days ago
Of course, just by virtue of context in this discussion, the answer is kind of given away.
Starlevel004 10 days ago
> It's an interesting quirk of these discussions of events at relativistic scales that it's very hard to precisely speak about what we mean whenever we reference time.

No it isn't. This is a stupid psued talking point.

ChrisMarshallNY 10 days ago
Depends on what type of star.

The only stars that have never been observed to die, are red dwarfs.

I think blue giants are the shortest-lived ones.

Ours is in the middle. I think they give the Sun about four billion more years.

BTW: That was a rhetorical statement. The issue is, we don't actually know what's going on, today.

colechristensen 10 days ago
If they're circling the Milky Way, they are close, in a cosmic sense.

The milky way is only 100,000 light years across.

Stars of a certain small size will also continue shining for a trillion years.

ChrisMarshallNY 10 days ago
Yeah, it's interesting that folks seemed to take offense at what I said. It's really pretty much exactly what you'd hear, from any astronomer (which I'm not, but one of my favorite shows is How the Universe Works).

Not all stars are created equal. Blue giants may only live a few billion years, while red dwarfs will last practically forever.

colechristensen 10 days ago
These are 13 billion year old stars that are only 30,000 light years away, that’s like one part in 40,000 of how long they’ve lived. They are most certainly still there shining. Is just incorrect to be pedantic about there light being there but maybe not them.
ChrisMarshallNY 10 days ago
That was a rhetorical statement. It's a "Here, there be dragonnes" type of thing. We say stuff like that, hereabouts, all the time. My statement was incredibly mild, compared to some of the crazy stuff people say, here.
colechristensen 10 days ago
Shrug, unnecessary pedantry is indeed a theme around here, I try to resist the urge but am guilty of it myself from time to time.
ChrisMarshallNY 10 days ago
I suspect that a number of folks didn’t realize where the quote came from.

They were talking about distant galaxies, not the relatively nearby stars.

Keysh 9 days ago
Blue giants live only a few million years to a few tens of millions of years.
labrador 10 days ago
They should name them the Father, the Son and the Holy Ghost