[jamest (OP)]

Thanks to Brian for sending in this one!

[Eternal Student]

Hi.

Possibly.
     The definition of what is a galaxy, or exactly where one ends is vague.   Hence, under one definition of a galaxy, a given star may be in a given galaxy but under another definition it is oustide any galaxy.

One reasonable definition of a galaxy:
    Galaxies consist of stars, planets, and vast clouds of gas and dust, all bound together by gravity.

[Taken from NASA website.     https://science.nasa.gov/universe/galaxies/ ]

    The key here is that we have some understanding of what it means for gravity to be generally keeping a collecton of things (stars, gas, plantets etc.) together.    Gravity follows an inverse square law with distance.   This means that at some distance from the centre of what we would define as the galaxy, gravity will be too weak and the star(s) this far out are not considered to be part of this galaxy any longer.

The definition of a galaxy is then mired by the existance of dwarf galxies apparently inside or just on the periphery of a larger galaxy.   For example, our Galaxy (the Milky Way), has the (Large and Small) Magellanic Clouds as dwarf galaxies.   They can be though of as small galaxies in their own right but they have become trapped in the gravitational well of the Milky Way and now orbit around the periphery of our galaxy.    So, that they are gravitational bound to our galaxy and could reasonably be considered as just a part of one big galaxy instead of being called galaxies on their own.      We can consider ever larger structures (regions of space) until we identify regions we call Clusters or Super-Clusters.   All the mater in these regions is gravitationally bound together, they do not tend to drift apart, but they appear much as many seperate galaxies orbiting around each other in some complicated way (literally "a cluster of  galaxies").    However, since everything in this cluster is gravitationally bound together, it forces us to question whether we should just define the whole cluster as one really big galaxy.

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   So, there probably isn't going to be a definitive answer to the original question.   If there's a star in space somewhere then it is probably inside someones definition of some galaxy.
   None the less we can still try to address some of the spirit of the question:    Stars don't tend to form in complete isolation from other stellar bodies.    Based on our simulations of cosmology and the evolution of the universe,  matter tends to distribute itself into filaments spaning through space.   The overall structure is often referred to as a cosmic web   (you can Google the term Cosmic web to find more info if you wish).    Stars will form only where there is matter for them to be formed from.   So stars tend to form along these filaments rather than in the voids in between them.   Once stars have formed, gravity naturally tends to start attracting other material and vice versa (other material will attract the star), so that stars will usually end up with other stars orbiting around them and/or they are pulled into orbit around some existing cluster of stars.

Exceptions
   Can there be a star that is NOT gravitationally bound to any galaxy?   Yes.    Over enough time, it is likely that every star would eventually be attracted to some some galaxy and orbit in some stable fashion.    However, for a short while (in Astronomical terms that can be a million years), some stars can be "flung out" of a galaxy and fly through the voids for a while.
     Although stars cannot form in the voids between galaxies (since the density of matter is far too low), there are in fact large numbers of intergalactic stars. It has been estimated, for example, that 10 per cent of the mass of the Virgo galaxy cluster is in the form of these stellar interlopers........
     ..... stars can be expelled from their parent galaxy if it collides, merges or passes close to another galaxy.

[Taken from:   BBC Science Focus.    https://www.sciencefocus.com/space/are-there-any-stars-between-galaxies]

Best Wishes.

[evan_au]

The early development of galaxies (and the supermassive black holes around which they seem to form) is a mystery of modern astronomy.
- Images of active galaxies (Quasars) have been observed surprisingly close to the date of the Big Bang, and astronomers don't understand how they grew so fast
- Astronomers hope that the James Webb Space telescope will shed more light on this, with its infra-red capability to view galaxies with extreme red-shifts

Stars can only form where the density of matter is sufficient to draw matter together, despite the effects of temperature and the motion of atoms to keep it apart
- In today's universe, stars form in Giant Molecular Clouds of gas and soot, at temperatures around 10K (-250C). https://en.wikipedia.org/wiki/Star_formation
- It is not clear how such temperatures would occur in the early universe, which was much hotter than today's 2.7K cosmic microwave background radiation - or near active black holes.
- But it is thought that these densities will occur most often in galaxies or globular clusters.

A recent image by the ESA Euclid telescope shows what appears to be around a trillion stars in intergalactic space, within the Perseus Cluster.
- It is thought that these were flung into space by gravitational interactions with the two massive galaxies in the cluster.

Orphan_Stars_Perseus_Cluster_Euclid.png (1021.35 kB . 1029x708 - viewed 512 times)
https://www.esa.int/Science_Exploration/Space_Science/Euclid/Euclid_s_view_of_the_Perseus_cluster_of_galaxies

A similar gravitational interaction (collision) is expected between our Milky Way galaxy, and the Andromeda galaxy, in about 5 billion years.
- The following simulation shows stars being torn off both galaxies and flung into space as isolated stars (the collision starts at about the 30-second mark).

[paul cotter]

Two informative replies, thank you both. I have a quick question for Evan_au: you said stars are formed from gas and soot, where does the soot come from? (intergalactic burning of fossil fuels no doubt, when will they ever learn)

[Halc]

stars are formed from gas and soot, where does the soot come from? (intergalactic burning of fossil fuels no doubt, when will they ever learn)

The 'soot' is from burning of the hydrogen and helium, the virgin gasses. So these later stars are built from the expelled remnants of the earliest starts. Our entire solar system is built from such material.

A common on earlier info:

We can consider ever larger structures (regions of space) until we identify regions we call Clusters or Super-Clusters.   All the mater in these regions is gravitationally bound together, they do not tend to drift apart, but they appear much as many seperate galaxies orbiting around each other in some complicated way (literally "a cluster of  galaxies").

Our local group (Us, Andromeda, all the little guys hovering nearby) are gravitationally bound, and thus, given enough time, will merge into one object, minus the odd stars that get flung away at escape velocity by 3+ body encounters of really massive objects. Yes, as has been pointed out, many of those not-in-galaxy stars have been flung out of galaxies, but they can only last so long before burning out. There is gas out in deep space, and new stars, not part of any galaxy do form out there.

Back to the gravitational binding: Our local group is it. We are not gravitationally bound to anything else, not the Virgo supercluster, the Great Attractor, and other 'nearby' super-high mass areas. We (our local group) are moving at greater than escape velocity from any of them, so we'll never be part of them. Given enough time, they'll disappear over our event horizon and will cease to have further causal effect on us.

There's always the off chance that our solar system will be flung out of the local group not only in the direction of Virgo some day, but fast enough for our system to suddenly be gravitationally bound to it. Better do it quick if you want it to happen before Earth dies. Any mass large enough to do that (*cough* Andromeda *cough*) will likely be here after Earth is gone.

[paul cotter]

Thanks Halc, I am aware of the fusion cycle that ends with carbon but I thought that was restricted to smaller stars which do not explode and thus the carbon remains sequestered in the remnant? Then again, as I have said before, my knowledge of cosmology is primitive.

[evan_au]

where does the soot come from? (intergalactic burning of fossil fuels no doubt, when will they ever learn)

As Halc commented: Stellar burning of fossil hydrogen...

It is thought that the Big Bang produced about 75% Hydrogen, 25% Helium and a tiny trace of Lithium (plus a heap of photons, neutrinos, and whatever Dark Matter is...).

The early universe was hotter than today, but also denser, so how the hypothetical first "Population III" stars formed is a bit of a mystery. Without heavier elements to catalyse fusion through the CNO cycle, it is thought that they might have been monsters of up to 100 times the mass of the Sun, which went supernova in a million years or so of starting fusion.

Today's "Population I" stars form from dust clouds containing lots of carbon, nitrogen, oxygen, iron and other elements from prior supernovas and heavy elements from neutron star collisions.
- To some, the word "dust" conveys the thought of particles the size of sand.
- To astronomers, the typical particle size in these regions is similar to soot from a smokey candle.
- While visible candle soot is mostly hot carbon compounds, interstellar dust clouds are near absolute zero, and have lots of ices of H₂O, C_yH_x, CO₂, NO_x, NH_x, etc as well as SiO_x, FeO_x, as well as the primordial Hydrogen and Helium.
https://en.wikipedia.org/wiki/Stellar_population#Formation_of_the_first_stars
https://en.wikipedia.org/wiki/Nucleosynthesis