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?
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.
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.
>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.
> 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.
> 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.
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...
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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."
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.
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.
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.
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".
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.
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.
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.
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.
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.
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.
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.
(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.
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.
"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."
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.
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.
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!
> 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.
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.
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?
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.
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
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.
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.
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.
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.
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?
> 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.
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.
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.
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.
