Globular clusters, those vast ‘cities of stars’ that orbit our galaxy, get a certain amount of traction in SETI circles because of their age, dating back as they do to the earliest days of the Milky Way. But as Henry Cordova explains below, they’re a less promising target in many ways than the younger, looser open clusters which are often home to star formation. Because it turns out that there are a number of open clusters that likewise show considerable age. A Centauri Dreams regular, Henry is a retired map maker and geographer now living in southeastern Florida and an active amateur astronomer. Here he surveys the landscape and points to reasons why older open clusters are possible homes to life and technologies. Yet they’ve received relatively short shrift in the literature exploring SETI possibilities. Is it time for a new look at open clusters?
by Henry Cordova
If you’re looking for signs of extra-terrestrial intelligence in the cosmos, whether it be radio signals or optical beacons or technological residues, doesn’t it make sense to observe an area of sky where large numbers of potential candidates (particularly stars) are concentrated? Galaxies, of course, are large concentrations of stars, but they are so remote that it is doubtful we would be able to detect any artifacts at those distances. Star clusters are concentrations of stars gathered together in a small area of the celestial sphere easily within the field of view of a telescope or radio antenna. These objects also have the advantage that all their members are at the same distance, and of the same age,
Ask any amateur astronomer; “How many kinds of star cluster are there?” and he will answer; “Two, Open Clusters (OCs) and Globular Clusters (GCs)”. The terms “Globular” and “Open” refer to both their general morphology as well as their appearance through the eyepiece. It’s important to keep in mind that both are collections of stars presumably born at the same time and place (and hence, from the same material) but they are nevertheless very different kinds of objects. There does not seem to be a clearly defined transitional or intermediate state between the two. One type does not evolve into the other. Incidentally, the term ‘Galactic Cluster’ is often encountered when researching this field. It is an obsolete term for an OC and should be abandoned. It is too easily misunderstood as meaning a ‘cluster of galaxies’ and can lead to confusion.
GCs are in fact globular. They are collections of thousands, if not hundreds of thousands, of stars forming spheroidal aggregates much more densely packed towards their centers. OCs are amorphous and irregular in shape, random clumps of several hundred to several thousand stars resembling clouds of buckshot flying through space. Their distribution throughout the galaxy is different as well. GCs orbit the galactic center in highly elliptical orbits scattered randomly through space. They are, for the most part, located at great distances from us. OCs, on the other hand, appear to be restricted to mostly circular orbits in the plane of the Milky Way. Due to the obscuring effects of interstellar dust in the plane of the galaxy, most are seen relatively near Earth. although they are scattered liberally throughout the spiral arms.
Image: The NASA/ESA Hubble Space Telescope has captured the best ever image of the globular cluster Messier 15, a gathering of very old stars that orbits the center of the Milky Way. This glittering cluster contains over 100 000 stars, and could also hide a rare type of black hole at its center. The cluster is located some 35 000 light-years away in the constellation of Pegasus (The Winged Horse). It is one of the oldest globular clusters known, with an age of around 12 billion years. Very hot blue stars and cooler golden stars are seen swarming together in this image, becoming more concentrated towards the cluster’s bright center. Messier 15 is also one of the densest globular clusters known, with most of its mass concentrated at its core. Credit: NASA, ESA.
Studies of both types of clusters in nearby galaxies confirm these patterns are general, not a consequence of our Milky Way’s history and architecture, but a feature of galactic structure everywhere. Other galaxies are surrounded by clouds of GCs, and swarms of OCs circle the disks of nearby spirals. It appears that the Milky Way hosts several hundred GCs and several thousand OCs. It is now clear that not only is the distribution and morphology of star clusters divided into two distinct classes but their populations are as well. OCs are often associated with clouds of gas and dust, and are sometimes active regions of star formation. Their stellar populations are often dominated by massive bright, hot stars evolving rapidly to an early death. GCs, on the other hand, are relatively dust and gas free, and the stars there are mostly fainter and cooler, but long-lived. Any massive stars in GCs evolved into supernovae, planetary nebulae or white dwarfs long ago.
It appears that the globulars are very old. They were created during the earliest stages of the galaxy’s evolution. Conditions must have been very different back then; indeed, globulars may be almost as old as the universe itself. GC stars formed during a time when the interstellar medium was predominantly hydrogen and helium and their spectra now reveal large concentrations of heavy elements (“metals”, in astrophysical jargon). The metals have been carried up from the stellar cores by convective processes late in the stars’ life. Any planets formed around this early generation of stars would likely be gas giants, composed primarily of H and He—not the rocky Earth-type worlds we tend to associate with life.
Open Clusters, on the other hand, are relatively new objects. Many of them we can see are still in the process of formation, condensing from molecular clouds well enriched by metals from previous cycles of nucleogenesis and star formation. These clouds have been seeded by supernovae, solar winds and planetary nebulae with fusion products so that subsequent generations of stars will have the higher elements to incorporate in their own retinue of planets.
Image; Some of our galaxy’s most massive, luminous stars burn 8,000 light-years away in the open cluster Trumpler 14. Credit: NASA, ESA, and J. Maíz Apellániz (Institute of Astrophysics of Andalusia, Spain); Acknowledgment: N. Smith (University of Arizona).
Older OCs may have broken up due to galactic tidal stresses but new ones seem to be forming all the time, and there appears to be sufficient material in the galactic plane to ensure a continuous supply of new OCs for the foreseeable future. In general, GCs are extremely old and stable, but not chemically enriched enough to be suitable for life. OCs are young, several million years old, and they usually don’t survive long for life to evolve there. Any intelligent life would probably evolve after the cluster broke up and its stars dispersed. BUT…there are exceptions.
The most important parameter that determines a star’s history is its initial mass. All stars start off as gravitationally collapsing masses of gas, glowing from the release of gravitational potential energy. Eventually, temperatures and pressures in the stars’ cores rise to the point where nuclear fusion reactions start producing light and heat. This energy counteracts gravity and the star settles down to a long period of stability, the main sequence. The terminology arises from a line of stars in the color-magnitude diagram of a star cluster. Main sequence stars stay on this line until they run out of fuel and wander off the main sequence.
All stars follow the same evolutionary pattern, but where on the main sequence they wind up, and how long they stay there, depend on their initial mass. Massive stars evolve quickly, lighter ones tend to stay on the main sequence a long time. Our Sun has been a main sequence star for about 4.6 billion years, and it will remain on the main sequence for about another 5 billion years. When it runs out of nuclear fuel it will wander off the main sequence, getting brighter and cooler as it evolves.
All stars evolve in a similar way, but the amount of time they spend in that stable main sequence state is highly dependent on their mass at birth. Studying the point on the color-magnitude diagram of a cluster’s main sequence where stars start to “peel-off” from the MS allows astrophysicists to determine the age of the cluster. It is not necessary to know the absolute brightness, or distance, of the stars since, by definition, all the stars in a cluster are at the same distance. The color-magnitude (or Hertzprung-Russell) diagram is as important to astronomy as the periodic table is to chemistry. It allows us to visualize stellar evolution using a simple graphic model to interpret the data. It is one of the triumphs of 20th century science.
It is this ability to determine the age of a cluster that allows us to select a set of OCs that meet the criterion of great age needed for biological evolution to take place. Although open clusters tend to quickly lose their stars through gravitational interactions with molecular clouds in the disc of the galaxy, a surprising number seem to have survived long enough for biological, and possibly technologically advanced, species to evolve. Although less massive stars, such as main sequence red dwarfs, tend to be preferentially ejected from OCs due to gravitational tides, more massive F, G, and K stars are more likely to remain.
Sky Catalog 2000.0 (1) lists 32 OCs of ages greater than 1.0 Gyr. A more up-to-date reference, the Wikipedia entry (2), lists others. No doubt, a thorough search of the literature will reveal still more. A few of these OCs are comparable in age to the globulars. They are relics of an ancient time. But many others are comparable to our Sun in age (indeed, our own star, like many others, was born in an open cluster).
Regardless of the observing technique or wavelength utilized, an OC provides the opportunity to examine a large number of stars simultaneously, stars which have been pre-selected as being of a suitable age to support life or a technically advanced civilization. It will also be assured that, as members of an OC, all the stars sampled were formed in a metal-rich environment, and that any planets formed about those stars may be rocky or otherwise Earthlike.
If a technical civilization has arisen on any of those stars, it is possible that they have explored or colonized other stars in the cluster and we have the opportunity to eavesdrop on intra-cluster communications. And from the purely practical point of view, when acquiring scarce funding or telescope time for such a project, it will be possible to piggy-back a SETI program onto non-SETI cluster research. Other than SETI, there are very good reasons to study OCs. They provide a useful laboratory for investigations into stellar evolution.
References
1) Sky Catalog 2000.0, Vol II, Sky Publishing Corp, 1985.
2) https://en.wikipedia.org/wiki/List_of_open_clusters
Suggestions for Additional reading
1. H. Cordova, The SETI Potential of Open Star Clusters, SETIQuest, Vol I No 4, 1995
2. R. De La Fuente Marcos, C. De La Fuente Marcos, SETI in Star Clusters: A Theoretical Approach, Astrophysics and Space Science 284: 1087-1096, 2003
3. M.C. Turnbull, J.C. Tarter, Target Selection for SETI II: Tycho-2 Dwarfs, Old Open Clusters, And the Nearest 100 Stars, ApJ Supp. Series 149: 423-436, 2003
Of the OCs in that Wikipedia list, just 6 are 4 Gy or older – old enough that on Earth gave rise to complex life and phyla that emerged in the Cambrian. A small number that should be easy to focus on: Messier 67, Arp-Madore 2, Collinder 261, NGC 188, NGC 6791, and Gaia 1.
Quanta Magazine has an article on galactic habitable zones: The Best Neighborhoods for Starting a Life in the Galaxy.
Can you place these 6 OCs in the galaxy with reference to the habitability context in that article?
I’ve always thought this idea of ‘suitable galactic neighborhoods’ rather weak. Although it can be argued that some regions of the galaxy might be more suitable than others (for a variety of reasons) it is also true that local conditions in any region could deviate considerably from the mean for that type of region. In addition, these clusters are always in motion, some orbiting docilely down the disk, others intersecting it at various angles. True, some areas may have advantages over others, but there are also many exceptions. I imagine anywhere in the disk is potentially planet/life/civilization friendly while other areas (like the hub or distant halo), might not. But until we know much more about how solar systems form, I don’t think its possible to make any more quantifiable statements than that.
Hi, Alex
You can convert from equatorial coordinates (RA and Dec) to galactic coordinates
(long/lat) at this website
https://ned.ipac.caltech.edu/forms/calculator.html
Keep in mind that although most OCs are at low galactic latitudes, only the nearest are visible due to dust in the galactic plane, Those at higher latitudes are visible at much greater distances, but there are much fewer of them . The upshot is that if you convert all those OC equatorial coordinates to galactic coordinates you will get a very biased sample that misrepresents their distribution about the galaxy..
A more useful exercise is to look at a star atlas and note how many OCs there are along the galactic equator, but if you look them up you will see they are all relatively near us. Those at higher latitudes are fewer, but they can be seen at much greater distances (You’re looking out of the dusty disk).
@Henry,
Thank you. I had to look up what Galactic coordinates meant on Wikipedia. I will try to position the 6 OCs in the galaxy as best I can given the data.
@Henry
Calcs for the 6 OCs with age >= 4000 GY
Object Gal long. horiz (pc) vert (pc) compass dir* [A]bove or [B]elow
M67 216.2 766 487 SE A
Arp-Madore 2 248.4 8828 -864 ESE B
Collinder 261 301.98 2488 -248 NE B
NGC 188 122.99 1533 638 SW A
NGC 6791 70.2 5753 1080 WNW A
Gaia 1 227.6 4552 -663 SE B
* the galactic longitude is the reverse of our terrestrial compass headings, thus 90 degrees is W, not E.
Observations
Most are above or below the thin disc of the galaxy. Only Collinder 261 is within the thin disc. The OCs are split between above and below the center of the disc. Most are situated “ahead” of us in the galaxy’s rotation(?), with the exception of NGC 6791 which is “behind” us.
That these OCs are more in the thick disc than the thin disc conforms to the Quanta Magazine article on habitability with more rocky planets around stars in the thick disc and therefore would seem from this small sample that they are both aged nicely for evolution and in a place where the probability of having habitable rocky worlds is higher, strengthening your argument for this type of cluster.
@Tolley: Time is indeed crucial.
Then again, life on Earth did perhaps suffer a series of extinction events in the first handful of billions of years that periodically reset life from hitting second gear.
Studies of minerals from those eras do at least provide some hints that such events might have occurred.
So what we know from a single example might be misleading. And there might be a rare case out there where life have taken the speed lane.
One interesting detail is that the distances between the stars in one open cluster is that much smaller.
So while the chance of panspermia in our galactic arms is so infinitely small it might not even need to be considered.
In an open cluster, where the meteoritic material will be stirred up as well. Exchange of material between stars might happen, and just perhaps carry some small bug fast enough to a new world before it have had it’s genetic material completely scrambled.
So I agree with the author here, this might be a fertile area in space where we should look both with SETI and also take a good look for biosignatures.
@Andrei
2 very good points. There was a biota (I forget which) which might be complex life that was much older than the Ediacaran life that gave way to the Cambrian life “explosion”. So if that is the case, then it is also possible that even older complex life could have emerged, but left no trace, other than possible clues in the minerals/radioisotope ratios.
Star clusters do make panspermia technically easier, although by how much idk. I would think that this would apply more to GCs than OCs. If so, we might just get that data sometime in the future once we have catalogs of inhabited (vs inhabitable) worlds.
In regard to your earlier question, Alex (How are these OCs distributed across the “habitable” zones of the galaxy?) I would suggest the following approach.
Use the coordinate calculator I linked to above to derive the galactic latitude and longitude of each cluster of interest. The absolute value of the latitude is how far away from the galactic plane the OC appears to us. (Sol is located very close to the galactic plane.)
The absolute value of the latitude is theta, the distance in parsecs to the OC is the hypotenuse, and the opposite side of the right triangle is the length of the perpendicular dropped from the OC to the plane. The distance to the OC multiplied by the sine of the latitude gives the length of the distance of the OC from the galactic plane.
A similar calculation using the longitude will yield the distance ALONG the plane the OC is away from us.
@Tolley
The oldest signs of possible life I remembered offhand was the odd graphite in zircon grains, plus the 3,5 gigayear Apex chert with potential microfossils. Both from Australia. Not to mention the suggestions of RNA life, a hypothesis I either reject or am on the fence about – the reply depending on if I gotten my coffee that day.
I almost posted this as such but asking a quick question to colleague I got a hint: Go look for Nunavut on Wiki. Turned out to be Nuvvuagittuq and 4,28 Gigayear this would definitely be Hadean life if confirmed – but it’s contested.
Lipids and various molecules are indeed what some look into, but do not seem to be preserved from any strata older than the Mesoproterozoic.
But then we got LUCA, and the molecular clock do once again suggest we got a start around 4,5 Gigayears ago, but might have been a self replicating blob without genetic material. And here there’s no doubt in my mind, I’m definitely on this camp and with a firm grip on the trunk of the Phylogenetic tree.
Well no one is able to give a even a good estimate on the chance of panspermia happen, so me mentioning this was just a way of stating it would be less unlikely in an area of space where the distance between the stars are not that great. Yes globular clusters could also be considered, the stars are generally old, but those in the inner parts would be passing very close – stirring up possible planetary orbits and cause wildly changing conditions on those potential worlds. And terrestrial planets will be very rare as those stars are very poor on metals, this while gas giants still might be common.
@Alex Tolley, @Andrei
This is the multicellular (but not complex?) life that is somewhat controversial but with a 1.5 Gy origin – far, far older (gt than 0.8Gy) than the Ediacaran – older than the entire complex life evolution that we know from the Cambrian, starting around 0.55 Gya.
Decimetre-scale multicellular eukaryotes from the 1.56-billion-year-old Gaoyuzhuang Formation in North China
Clusters have often been considered less amenable to life for several reasons. These are a couple of them that I can recall:
1. Higher frequency of close passes by other stars that can disrupt planetary orbits.
2. Increased probability of sterilizing nova events.
These are also reasons that have been put forward for believing that life is less likely near the galactic core.
Its always possible to find reasons why something should (or shouldn’t) happen under certain conditions. But how these effects actually work out statistically is something else. For example, (and for a relatively short time) early in an OCs history massive young stars will be going supernovae, or otherwise evolving catastrophically, effectively sterilizing their immediate neighborhoods. But these evolving stars will also be enriching the nebular material with metals, making the formation of rocky planets easier for subsequent generations of stars. Couldn’t these two counteracting effects operate against each other, with a fortunate resonance of the two resulting? We still know very little about early cluster history and its effect on the statistics of cluster members.
Likewise, we know that in the catastrophic gravitational environment of these clusters that many stars are ejected (another way of saying the cluster dissipates or evaporates). But at the same time, the stars most likely to be ejected are the lightest ones, the M dwarfs, the most populous sub-group. The most massive stars evolve rapidly and blow up or become white dwarfs. The ones in the middle (long-lived F,G,and K subdwarfs) will come to dominate! All these processes work in opposition to one another and we don’t know the details well enough to determine what their combination will result.
We understand OC evolution only in gross, qualitative terms. We still don’t understand the details very well. For example, in all the lists I consulted while researching the article, different authorities were in excellent agreement on which clusters were old and which young. But the actual age estimates in Gyr varied considerable from one researcher to another.
Very interesting to hear technological civilizations could evolve in open clusters. I was already a big fan of OC for SETI because if I was sending out a wave of Von Neumann probes these would be an obvious long term target because of the huge cluster of resources they represent.
In any cluster environment, the close proximity of stars to one another would certainly encourage the development of interstellar travel by any civilizations arising there. No doubt this would result in activity which might be detectable from great distances. I hesitate to speculate any further than that, but that alone suggests yet another reason to concentrate on old open clusters in our SETI research.
Excuse, me. The stars in an OC”s move in the same direction.
They move together like a cloud of buckshot. Its a swarm of stars all moving together in orbit around the galactic nucleus. However, those stars also revolve around the center of mass of the cluster–they all orbit each other! But the cluster itself also is affected by nearby gravitational masses like other clusters or giant, massive molecular clouds, In addition. the cluster may be in an orbit slightly inclined to the galactic plane, meaning it occasionally flies through the plane of the milky way, receiving gravitational pulses from the disk itself. Within the cluster there are near-collisions and close encounters, plus other gravitational interactions and ejections into extracluster space. The globulars are massive and compact enough to stay together as spherical clouds of stars, but the OCs have a tendency to evaporate or dissipate into their surroundings, given enough time.
And even stars that are not lost to the cluster may still occasionally have close encounters with cluster=mates that perturb planetary orbits and drag out showers of Oort-cloud comets and random Houmuamua splinters. Its a chaotic environment, but somehow our own solar system survived it.
“A cloud of buckshot.” Good analogy, Henry!
These OC should be filled to brim with brown dwarfs and many, many rogue planets. Many of these being superearths that may have all the ingredients for advance intelligent life even in the depths of space.
Majority of water hides deep in the interiors of exoplanets.
Abstract: Water is an important component of exoplanets, with its distribution, i.e., whether at the surface or deep inside, fundamentally influencing the planetary properties. The distribution of water in most exoplanets is determined by yet-unknown partitioning coefficients at extreme conditions. Our new ab initio molecular dynamics simulations reveal that water strongly partitions into iron over silicate at high pressures and thus would preferentially stay in a planet’s core. Furthermore, we model planet interiors by considering the effect of water on density, melting temperature, and water partitioning. The results shatter the notion of water worlds as imagined before: the majority of the bulk water budget (even more than 95%) can be stored deep within the core and the mantle, and not at the surface. For planets more massive than ~6 M⨁ and Earth-size planets (of lower mass and small water budgets), the
majority of water resides deep in the cores of planets. Whether water is assumed to be at the surface or at depth can affect the radius by up to 25% for a given mass. This has drastic consequences for the inferred water distribution in exoplanets from mass-radius data.
https://arxiv.org/abs/2401.16394
https://astrobiology.com/2024/01/majority-of-water-hides-deep-in-the-interiors-of-exoplanets.html
If a technogent species has taken root in one of these GC’s they would be able to observe pretty much every part of the universe with all those white dwarfs around. They would also spread very rapidly through the GC as well not only due to the closeness of the neighbouring stars but again the white dwarfs aiding space craft to move faster. They would also offer very long term stable places for stellar empires, as each star ages and goes into a red giant phase that material could be collected to form new M or K type stars and/or planetary objects. It would be a very good place to look for advanced life IMO.
Hi, Michael
You wrote ‘GC’, did you mean ‘OC’? Everything you say is true, globular clusters offer many advantages to spacefaring species and multiple opportunities for SETI researchers. But it is my position that GCs are too metal-poor for it to be likely that they are the abodes of organic life. It is possible to come up with scenarios where rocky planets with a full periodic table arise in GCs but they all seem forced and unlikely to me. In my opinion, its the smaller, more temporary, less stable open cluster that is a better place to look.
Even though OCs tend to quickly disperse their stars into the disk, those few who survive to an advanced age are prime SETI targets. After all, that’s where our own sun was born, even though its birth cluster long ago evaporated.
Either, GC are metal poor but every now and then are enrichment by kilonovas from mergers and even supernovae which would probably affect the close knit star systems badly. But this does not stop more enriched star systems been absorbed or forced in long orbits around the GC. If I were a techsavy species and I was close to one I would make my way to it. Emagine a thousand solar mass BH at the centre of one of these systems, a powerhouse for sure and quite deadly for any aggressive alien races around it for thousands of light years.
Some GC and OGC’s with pictures
https://starwalk.space/en/news/naked-eye-star-clusters-list
You should be able to find them on Aladin, should be able to use their names or cat numbers.
https://aladin.cds.unistra.fr/AladinLite/
M4 cluster appears to have 40 000 white dwarfs in it ! These clusters must move through the disc at some point quite possibly picking up other star systems into their fold.
https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/messier-4
The late Robert Bradbury thought that globular star clusters would be great locations for advanced interstellar ETI. See here for the details:
https://web.archive.org/web/20080820084905/http://www.aeiveos.com:8080/~bradbury/MatrioshkaBrains/GCaA.html
The theme seems to be that ETI had to evolve in such clusters. However, what is to say that a sufficiently sophisticated civilization from elsewhere in the galaxy couldn’t travel to such clusters to take advantage of what they have to offer? If there are no natives there as seems to be the current prognosis, then that would be one less thing for these advanced species to deal with.
Great job, Henry! You are quite literally expanding our range of where to find ETI. BTW, for those who are interested, here is the front cover page of Henry’s original outing of his piece, and how we met:
http://www.coseti.org/sq_v1_n4.htm
Thanks for the kind remarks, Larry. And thanks again for including my first ever article in SETIQuest. Almost 30 years ago….imagine that.
A wink of the cosmic eye, Henry.
I just wanted to say how much I enjoy and admire your contributions Henry. Congratulations on the article. I’m devouring it now.
Hello Henry, Thank you for this very interesting article. Two questions: how do you explain this density of cluster stars in such a concentrated volume of space? Do you think that clusters have a particular role in celestial mechanics when you say that they receive gravitational impulses from the galactic disk? I wouldn’t comment on the idea of an ETI, but I do think that clusters have great beauty. I see them more as a kind of beacon of light in the galaxy, given the multiplicity of their stars…
Fred
First question,
All stars form in molecular clouds. Presumably, this occurs when the amount and density of the star-forming material enters a favorable range due to the cloud’s gravitational collapse. We can actually see this happening in places like the Orion Nebula. At that point, stars condense out of the cloud the way raindrops precipitate out of a thunderstorm. After stars begin to “light up”, their photon pressure and stellar winds (not to mention shock waves from supernovae of the most massive, short-lived stars) greatly increase the turbulence inside the cloud. No doubt this provokes (and interferes with) additional star formation in the cloud, depending on the conditions in different parts of the nebula.
Eventually, the stellar winds, nova shocks and light pressure of new stars in the cloud expel the gas and dust leaving a collection of stars floating in a more-or-less gas-free matrix. This all happens at time scales of millions of years, but there are so many clusters out there we can see them in every stage of their evolution. As an analogy, think of a forest, it evolves through many different stages until we get a “climax forest”, We don’t have to live for centuries to figure this out, we just observe many forests in different stages of development.
Second question
These open clusters usually contain from several hundred to several thousand stars, loosely bound by mutual gravitation. They are several light-years apart. But they are also affected by external gravitational forces that tend to break them up over time. These external forces tend to be other molecular clouds and clusters drifting along with them in the galactic plane. Although the more massive stars in the cluster tend to stay there, the lighter ones tend to leak off, and most clusters tend to break up after a few billion years, scattering their stars into the general population of the spiral arms.
If the cluster’s orbit around the galactic nucleus is slightly inclined to the plane of the galaxy, it will tend to periodically pass through the disk where it is likely to be affected by pulses of disruption from the stellar and nebular material concentrated there. With our short lifetimes, we don’t see this happening, (we only see a snapshot of the process). But there are so many clusters, and the galaxy is so big, we have been able to reconstruct this process over the last century or so.
The globular clusters, on the other hand, were formed early in our galaxy’s history. Conditions must have bee very different back then, because the globulars are much bigger, and much more homogeneous in appearance and content.
And keep in mind, we only understand galactic and cluster evolution in its broad aspect, there is much detail we still don’t know. Our life times are so short, and the time we have had the technical tools to make these studies is so brief, we find it difficult to comprehend that the fixed and stable universe we perceive is (at longer timescales) a bubbling cauldron of violent activity of which we glimpse only
a thin slice. Geologists face the same problem when they study the crust of the earth, or paleontologists experience when they study ancient life forms. We are looking at one instantaneous photograph of a continuous explosion.
Henry,
Thank you for taking the time to reply in such a detailed and instructive way. I’m French, so I don’t have the time to read and translate everything quickly, given the quality of your texts on Centauri dreams, but your reflections are very stimulating. As a side note, I would have liked to have been a cartographer: maps are both fascinating sources of information and a very powerful means of travelling in space – whether geographical, terrestrial or interstellar. I think you’ve already discussed with Paul how to find your way in space outside our galaxy… or for an ETI ;) a fascinating subject…
Fred
This is a good “trade topic”. Whatever the answer turns out to be there are some distinctly different considerations.
Perhaps one way to look at this is that the Earth originated in something like an open cluster of an indeterminate number of stars forming out of a molecular gas cloud ( H2 and enriched) with some stellar relatives of about the same age.
Subsequently the stars dispersed. When in terms of our geological history might be an interesting event in itself. E.g., After the solar system Great Bombardment, was there an “All clear” or was there no connection? But a star like Sirius, say, could not have originated in the same nest 4.5 billion years ago or so as a MS A star. So that would be an argument against that – unless it is the result of smaller binaries merging.
But given these illustrative arguments for where we might be now, it is illustrative of implications, but too narrow a set of illustrative cases to come to a conclusion.
Though whether for open clusters or globular clusters, planet stability in closely packed places still lurks as an issue regarding habitability.
The interstellar medium in a younger cluster would have more enrichment of elements or species beyond hydrogen and helium.
For old clusters of either type, it would be interesting to know some statistics about planets identified: gas giants or smaller.
What with the distance to globulars… have to wonder if transit detection work very well that far. The Gaia mission might be able to extend radial velocity detections, but that would bias results toward large, close-in Jupiters. Its astrometry is an important component of the mission, however, but a considerable problem to detect the equivalent of Earth around a similar sun. And that would have to b a cyclic overlay on a path complicated by all the neighboring stars.
On the other hand, if we can’t get good information about planets, there is still a question about stellar paths in a globular cluster. The outer edges of globular systems seem to concentrate stars much like they are locally; but profiles toward the center increase their packing by orders of magnitude. Reminds me of illustrations of electron clouds – assuming something spherical. But say you have a star out at the perimeter: Is it just taking a breather or can it stay out there indefinitely? If it heads to the center on some sort of elliptic path, than it is likely to be perturbed or have many close calls. Forming circumstellar disks stable enough to form planets closer in might reach an instability region.
So, like Kipling’s “If” poem if you run all these obstacles, you might get a planet, a habitable zone and something habitable? With open clusters, the story has just started, similar to Earth’s history- but with globular cluster stars, the road is longer with the above issues. More time and more stars, and more embedded hazards. …
While the odds are difficult to discern, assuming anyone is home in either, how about which one could be closer to return a long distance call?
Sophisticated searching of deep space
Helena Pozniak
It’s the ultimate astronomers’ question: ‘Is there anybody out there?’ The quest to find out for sure is growing ever more sophisticated.
https://eandt.theiet.org/2024/03/19/sophisticated-searching-deep-space
To quote:
“We think of SETI as Jodie Foster in the New Mexico desert with headphones in the film Contact,” says Lintott. In reality, astronomers are analysing telescope data for anomalies.
A once narrow focus on radio signals has broadened, says Lintott. Astronomers are becoming more creative in how they look and what they look for. “Radio signals have only ever been based on a good guess. There are many other ways you might make intelligent life visible in the cosmos.”