# Would a star's spectroscopy be stable enough on approach to use it as a navigational reference from a great distance?

NOTE: The offered "duplicate" question IS NOT WHAT I AM ASKING. Alternative methods of navigation ARE NOT ACCEPTABLE as answers. I am specifically asking whether or not a ship traveling in the method described could re-identify the destination star on approach because the perceived light of the star with each jump is "aging" as the ship approaches the "current age" of the star. Don't let @SlowlySwift's obsession with navigation fool you (he deleted his answer) and don't let the reason why I'm asking the question distract you.

In the future, humanity develops what we'll call "hyperspace technology." Basically, a starship can "jump," whether it be via wormhole or any other fanciful idea, from one location to the next a considerable distance away in a very short period of time from Earth's frame of reference. Please accept this as a given. The technology and the rules of that technology are not relevant to the question.

A ship decides to travel from Earth to a world eight-billion light years away. From Earth at the time the ship is launched, we're seeing light that's been in transit for eight-billion years.

The ship must make 100 jumps to get from Earth to that distant world (an arbitrary number for the purpose of the question). No navigational technology is perfect because no knowledge of everything between here and there can be perfect. Therefore, the ship arrives at the termination of each "jump" and must re-determine its location in space before plotting and executing the next jump.

If you're thinking this sounds a lot like space travel in Asimov's "Foundation" series... I've been re-reading it recently and it's definitely an influence on the question.

With each jump of approximately 80 million light years, the light from the star the ship is approaching gets "younger," meaning that it's traveled less distance, and it represents a star that's getting perceptually "older," meaning it's true age from day #1 of the universe vs. the very young age we see through a telescope from Earth.

Question: Since the age of the star in question is getting older as the ship gets nearer jump-by-jump, is the spectral description of the star stable enough or predictable enough to remain entirely identifiable and therefore useful for navigation? In short, can I find it and point my ship at it after each jump?

My goal is to choose how "local" a star chart must be based on today's understanding of solar physics. In short, can someone travel the entire distance without the need of a star chart other than the data acquired on Earth? Or must they pick up the local star chart at, worst case, each jump to have their destination re-identified?

I recognize that a second problem is that over that distance, the star is "moving," meaning where is is "today" when the ship launches from Earth is very different from where it is "then" when the ship arrives, eight billion years into the star's future as perceived by its light from Earth. I'm ignoring that right now because (in my world, if not in reality) the motion of stars is more predictable than the motion of a space ship. I recognize that if the answer is along the lines of, "finding that star at each jump would be whomping difficult," that it would means very-long-distance travel without the aid of localized/regionalized star charts from earlier (and much slower) exploration would be next to impossible as all of the stars's apparent ages and locations are changing substantially over that distance.

I also recognize that at that distance it's plausible that the star has gone nova or some such fairly inconvenient thing, which the travelers will discover as they jump. Or would they? As a bonus question, but one no one is obligated to answer, can the original star be detected in the nova remains for the purpose of navigation? We'll assume the travelers still want to get there. Hopefully they'll be in a hyperspatial jump when passing the advancing cloud of destruction.

• This is a really interesting idea; +1. Are you okay with hand-waving their ability to detect the star at all? At distances of billions of light-years, we can only detect stars that are gravitationally lensed (and luminous, which means high-mass and short-lived). Gravitational lensing required a lucky alignment; if you keep shifting your position, though, the light will no longer be focused near your location, and it won't be possible to detect the star. Sep 2, 2023 at 18:23
• The star will move appreciably after each 80 Mly jump. Consider how the Sun's orbit around the galaxy is some 230 My. It's my understanding they're not very nice orbits, either, but highly perturbed. The question really ought to be "how accurately can we isolate a single star in a galaxy with time jumps?" to cover both bases time (spectroscopy) and space (position).
– BMF
Sep 2, 2023 at 19:39
• Another question is why? Is there some reason the first jumps need to be more than just in the right general direction? You'll have a hundred opportunities to refine the navigation as you approach your target. As you're setting up your last jump, there'll also be a vast range of potential targets that didn't even form early enough to be visible from the start, which you could never visit by the described approach. Sep 2, 2023 at 23:31
• I believe you can get answers more closely to your wishes by stripping away the FTL context and asking on astronomy.SE about whether star spectrometry is stable. You're giving an immense amount of context and then telling us to ignore 90% of it and just answer a purely scientific star property question, whose answer frankly has nothing to do with your world. Sep 9, 2023 at 8:56
• Does this answer your question? How can I know where to point my spaceship? Sep 17, 2023 at 20:14

Let's start with what we know about stellar evolution. Stars coalesce from a mass of gas, mostly Hydrogen, with some Helium and other trace elements, and begin to fuse Hydrogen to Helium. As they age, the Hydrogen runs out, and they begin to fuse their Helium to heavier elements, until they get to Iron. Depending upon their mass, they then either become a dwarf star, or a supernova that leaves behind a neutron star or a black hole.

The time that it takes a star to do all of this depends largely upon its mass; the more massive a star, the faster it fuses its fuel, and the shorter its life. Stars with classes O, B A and some F - blue to white stars - have been determined to have lifespans under 8 billion years.

Thus, if one of these stars was selected as the target for our ship, it would most likely no longer exist by the time the ship arrived. Whatever was left would be so different that there would be no practical way of determining that it had once been the star the ship was aiming for.

Alternatively, if the navigator on the ship was aware of this, and aimed for a high-F or any G, K or M star, whose lifespans are expected to exceed 8 billion years, and in the case of M-class stars may be trillions of years, then matters are a little different.

To answer the question if one of these stars is selected, we must understand stellar spectroscopy. Put in simple terms, the elements present in the atmosphere of a star emit and absorb radiation in such a way that its spectrum can be used to determine its chemical composition. We can say that a star contains whatever percentages of H, He and other elements.

However, we know that over the lifetime of a star, fusing of elements from Hydrogen to Iron results in lighter elements being consumed and heavier elements being created. This will result in the spectrum of the star changing over time.

Now, humans have been studying the spectra of stars since the 1800s... for maybe 200 years. That is a miniscule drop in the bucket of even a short-lived star's lifespan. We can speculate but cannot know how the spectrum of a star will change over its lifespan, because we have not been studying stars long enough to observe any significant changes. M-class stars, being very long lived may not change significantly, but hotter stars may.

The next problem that we will encounter is a result of the 80Myr jumps. Over this amount of time, the positions of the stars within the selected galaxy will change significantly, dependent upon local gravitational effects and the gravity of the central super-massive black hole. The chaotic movement of the stars will mean that it will be difficult to locate a single specific star within a galaxy, especially if it is a M-class star, which are the most common type of star. It'd be like trying to locate a single, specific piece of hay in a haystack... where vermin regularly nibble on the hay.

So, my answer is that unless a great many more shorter jumps are made, so that a specific star can be tracked with a reasonable expectation of success, the lack of definitive knowledge as to how the spectrum of a star will change over this amount of time will mean that it is unlikely to be possible to use it as a navigational reference... if the star even still exists by the time the ship arrives.

• Maybe with careful selection of the star system the problem of identifying can be mitigated somewhat. Specifically I'm thinking about picking a star in a globular cluster. Both because there's a lot fewer of them in a galaxy, and also because you can rely on the properties of many stars all at once. Sep 18, 2023 at 10:59
• ...and by properties, I of course mean spectra. Sep 18, 2023 at 11:10

I say quite a ton of imagination is required to outline such a journey, but I have gathered enough data over my astronomy reading to say this:

Not possible

First, from 8 billion light-years you do not see individual stars, only galaxies and in rare cases globular clusters of new stars (usually new, as they are the brightest), because of low light intensivity and general angular proximity of various objects contained in a galaxy to each other when viewed from this far. Even if you'd assemble a galaxy-wide interferometer out of several million ships, even assuming they know the distance between them and can use smaller shuttles to carry over information in essential FTL connectivity by pigeons, you won't be able to discern a single star, let alone a planet around it. Assuming that you did that...

Second. After the very first jump you are no longer able to receive interferometry larger than what you've brought with you, thus you would be left with only a general direction to the galaxy with your world, together with any gathered data about its spectre, distance from center (if discernible, assuming yes) and probable orbital period in the galaxy (assumed no, as discerning a star could be done via spectral filtering, but detecting its lateral velocity would require orders more magnitude of sensitivity). So you will no longer see the star. Also you will likely be hitting an issue of changing Hubble constant, so that everything around would get slightly shifted to blue end of spectrum, as the current hypothesis is that the universe is expanding with acceleration. Such an adventure would require exact knowledge of Hubble constant's behavior over the life of the universe.

Next, after three or so jumps of 80 MLY each, you will no longer be able to use galaxies in the local group for exact positioning, as well as directing yourself forward, because they would have been detected at wrong (moved backwards) positions in the local sky sphere, that alone would require recalibration of the entire model of the universe, and using the recorded data won't do much to provide your exact location in the space, since everything would have moved, sometimes unpredictably, as exact motion reversion is impossible due to error accumulation. Yet the journey would still be far from over, as you would still be able to discern the target galaxy among its neighbors, and advance in its general direction.

However, you might have encountered an issue that a close (angularly) galaxy would obscure your target after another jump, so that you might mistake that one for your target and start approaching it instead. Also your target galaxy also moves, and the closer you get, the farther it would be visibly moved from being dead center, thus you would be required to verify if your target is still "there", and the "there" you see is still the galaxy you're about to visit. This would prove sequentially harder, as stars in that galaxy would change over time as fuel in them would burn out, nova/supernova events could cause minor disruptions to the overall galaxy image, etc.

There is another thing you might not have foreseen with that 8BLY galaxy, that is galaxy collision event, like what happened to M32 when it came close to M31. Stars in there were partly dispersed into open space due to galaxies not being uniformly dense, and with initial measurements you won't be able to determine relative speeds of galaxies in the local group of the target, if any, so that another event that would spoil your mission would be another galaxy coming close to the target one, causing some stars to change their course and even be thrown out of both of them. Not just that, but during the collision of two galaxies many stars end up in the active center of either galaxy, essentially getting eradicated into a central black hole, and with the initially scarce information about your world you will be pretty unsure at where to seek the star should this happen.

But, assuming no galaxy-wide destruction has happened to the target galaxy, you will essentially be at a loss at the very last jump, because from 80 MLY away from destination you don't have enough data on the target galaxy's own revolving motion, and given that Milky Way's revolving (at our distance) about 1 full circle over 225 million years, you won't be able to predict where should the star exactly be when you jump into the galaxy (but, you will be able to find it from this distance using some large interference grid of telescopes, if there's still something to find), as 80 million years would translate into abot a quarter to a half of a revolution of the galaxy, if it's similar to Milky Way, and a random but significant number otherwise. Your ship would have to spend some hundred years determining the actual revolution speed of the target star with enough accuracy to not miss the destination with the next jump, or use several smaller jumps gradually reducing the distance to the target in order to increase final jump's precision. (EDIT: 1/4 of a revolution is largely predictable, say doing a jump 79 MLY instead of 80 to leave 1 MLY left, or say leave half a galaxy radius between the ship and the estimated area where the target star would likely be, to properly discern the star from above the galaxy plane - quite doable. So this part of the argument is eliminated, yet there could be troubles when properly employing this part of the plan, so let it still hang in here for analysis.)

But even then... will you ever have something to find out there? 8 billion years is about the entire length of a star's existence in the main sequence, thus if your star was initially anything other than a red/brown/white dwarf, aka something that would remain stable over 8 billion years, it would have evolved either gradually as a main sequence star, or rapidly after being involved in an intra-galaxy star formation event, like getting hit by a close nova, thus its spectre would not be predictable from whatever data you gather over the journey, assuming what you have discerned from the Milky Way was correct and about the target and not a star that was indistinguishably close to the target to mistake them. Also 8 billion years is about two star generations' worth of a timelapse, so the target galaxy should have experienced about two waves of star generation after you've seen it from 8 BLY, rendering it probably unable to get discerned at earlier jumps.

This totals to about 100% of nonexistence of your target where you would arrive with your FTL. So, better search for similar stars in the local group, they are about thousandfold closer and more data is available to gather about them to actually reach their current location rather than ending up at a complete loss in the midst of alien stars.

• 80 million years would translate into 160-640 revolutions of the galaxy, if it's similar to Milky Way I think you got the math wrong. The Milky Way revolves once every 230 million years or so, not thousand. The point still kinda stands. The star's trajectory is pretty perturbed.
– BMF
Sep 4, 2023 at 20:24
• Also, ... because from 80 MLY away from destination you don't have enough data on the target galaxy's own revolving motion... why? We analyze rotation curves of galaxies much farther than 80 Mly.
– BMF
Sep 4, 2023 at 20:26
• @BMF if 230 million years, then yes, this is not enough to completely lose the star if approaching the galaxy from its broadside (outside its plane of revolvement), as a quarter of a rev is definitely not too much. Re rotation curves, we only extrapolate M31/M33 data over a set of elliptical or spiral galaxies, the actual measurements have been performed only against galaxies that are a lot closer than 80 MLY. And a curve is a general averaged parameter of a galaxy, a certain star can move faster or slower due to local gravity field fluctuations. Sep 5, 2023 at 6:29
• -1 Your conclusion relies on items I've excluded from consideration (because I'm only interested in the spectral characteristics of stars over their lifespan and how difficult it would be to re-establish identify as those characteristics change, if they change) and the answer relies on today's scientific limitations despite an advanced enough tech to make the journey. (Galaxy-wide destruction? Who cares? That's not part of the question at all.)
– JBH
Sep 6, 2023 at 6:12
• Then the answer is a "probably yes" if the star is a red dwarf, brown dwarf or white dwarf, AND you are able to discern exactly its spectre from the entire galaxy's worth of light that travelled 8BLY, with adjustments made for expanding universe, local redshift and future movement, AND nothing has happened to the star in those 8 billion years you're seeing it from, and a definite "no" otherwise. Also there are global limits on light detection you are certain to hit with that star and galaxy regardless of tech progress, unless you alter the physics so that for example laser rays don't disperse. Sep 6, 2023 at 7:02

# This would only work for the smallest and dimmest stars

Large bright stars have relatively short lifespans, and change a great deal over the timescales your talking about for your jumps.

However, small stars are very stable and can stay in their main sequence, with relatively stable spectroscopy, for perhaps trillions of years (this is theoretical as the universe isn't even 14 billion years old yet). The problem, of course, is: "How do you see a dim star from 8 billion light years away?". But provided that the star itself is over 8 billion years old, and that, for some reason you can see it from 8 billion light years away (seeing it at this distance does guarantee its age), then you should be able to find that star by its spectrograph, the helium line will brighten as you approach and the hydrogen line will dim, but this should be predictable enough to identify.

As for the Nova bonus question, the stars that we're talking about would be far too small to ever go Nova. They may not be as flashy as their much larger and brighter neighbors, but these are the stars that will still be shining billions of years after these supernova have gone dark.

Stars have predictable life cycles, shifting their core up the periodic table. That doesn't mean that they won't receive input from outside, or that their aging can't be disrupted by gravitational forces. Passing through a nebula could easily change a spectrum enough for it to become less like you'd expect than some of its siblings.

Let's solve this as a Fermi problem. Fermi (of the Fermi Paradox) was notorious for breaking complex problems into sets of discrete variables. The Drake Equation is the most famous of these formulations.

Variables:

V (ly3): The volume of space that the star might pass through over that period of time, based on your ability to predict the motion of all of the stars near it.

D (count/ly3): The density of stars in the vicinity of your star. If your star is in the galactic core, you're going to have issues.

T (0.0 to 1.0): The percentage of stars in the galaxy of your star's general spectral type. Smaller stars have a higher T.

N: (ly3nebula / ly3non-nebula) The percentage of the volume that the star is expected to pass through which is occupied by Nebulae. This determines how much the star's spectrum can be expected to drift.

X: (0.0 to 1.0, where 1 = strongly unique) This is a rating for eccentricity, essentially a statement of how unusual the spectral lines are, and how vibrant they are. Is there a strong chlorine spike in its atmosphere? If so, this will create a recognizable absorption line in the spectrum. Unfortunately, these lines wouldn't be visible from 8 billion ly away because it would be overlaid by all of the dust the light has to pass through on the way there. Maybe they have magitech that allows quantum-pipeline of light from a specific area. That's pretty much the only way they could differentiate a single star from that distance in the first place. Or maybe someone handed over coordinates and a spectral signature for the star?

V * D will give you the locational variability of your star over the 80 million year jumps. Denser stars mean more dancing around.

DTX/N is the difficulty of differentiating your star from the others around it. Thus, the total formula would be (V * D2 * T * X) / N.

All of this needs to be adjusted for scanner technology. If you have the precise spectral signature recorded, and you can simultaneously sample and process the spectral type of every star in the target galaxy, then the only real variables are X / N, which states the probability that a nebula will smear the original spectral type out of recognition.

It sounds like your ship has to use Dead reckoning. When there is too low of visibility, pilots and ships captains will use the knowledge of where they were combined with speed and time to determine where they are.

Since you'd be relying on dead reckoning to determine the position of the destination AND the position of the ship it is absolutely vital that:

1. The measurements of the stars are ABSOLUTELY accurate. Since you're extrapolating over billions of years the tiniest error will get your crew lost in space.

2. The consistency of the warp drives movement. Again, this most be ABSOLUTELY consistent in order for your dead reckoning to be accurate.

In terms of world building this would mean that the hundreds of stops are at closer planets in order to verify that dead reckoning is successful. The Alpha Centauri system is about 4.37 light years away, so it could be used to ensure that the dead reckoning is accurate, from there your crew could just warp exponentially further each time while constantly checking to make sure they're ending up in their expected destinations.

The big problem: It is impossible for us to know how accurate our measurements of stars are Since we won't live long enough to see the results of the experiment, all our hypotheses about the direction and speed of a galaxy are educated guesses that can never be proven or disproven. I can think of some ways around this:

1. Perhaps the ship your crew is on is the first ship to travel long distances and they test the accuracy of the measurements using the process described above.

2. Perhaps dead reckoning has already been tested by other ships or space probes using the warp drive and the process is commonplace in the future.

3. If this is REALLY far in the future you could say that they found our current day star charts and by comparing them to their they could accurately extrapolate out the movement.

Overall it would be a process of trial and error, but given the invention of a warp drive I see the navigation method you're describing as a perfectly valid method of space navigation.

EDIT: It seems like the main question is the precision of the warp engine. If it can instantly get you where you want to go every time you don't need navigation. If it's inconsistent you should explain more about why it is inconsistent and how inconsistent it is.

• I agree with #1. #2 is the problem. There's no such thing. There will never be any such thing. Also, what do you mean by "extrapolating over billions of years?" Each of the 100 jumps brings the ship 80 Mly closer to the star's "present." In that regard, nothing is being extrapolated. Are you talking about predicting the the star's changes? Thanks.
– JBH
Sep 8, 2023 at 15:24
• By number two you mean that the warp drive in inconsistent? The question then is how inconsistent? Can it more or less get you to a destination four or five lightyears away? What I mean by extrapolation is that you're jumping a hundred billion lightyears AKA an insanely vast distance, so even an inaccuracy of 0.1 percent will sink you. Sep 9, 2023 at 5:49
• The problem is that there are so many stars and we identify them by our orbit relative to us, so you don't have anything to go off of if you're the fist one making a trip Sep 9, 2023 at 5:50
• ...But, if you can count on your engine to be consistent for a jump of say 10 light years then you can jump to a star and have a reference point. Sep 9, 2023 at 5:52
• The real question is how new are warp jumps, is your crew the first to ever make one or is it an established field that has know limits? Sep 9, 2023 at 5:53