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Must a star belong to a galaxy, or could it be completely isolated?

In case it can be isolated (not belong to a galaxy), could it have a planet orbiting around it?

Chris
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cooper
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2 Answers2

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No, stars do not need to be inside a galaxy. It is estimated that about 10% of stars do not belong to a galaxy [1]. While most intergalactic stars formed inside a galaxy and were ejected by gravitational interactions, stars can form outside of galaxies as well [2].

We assume that such stars could have planets, just like stars in a galaxy, although no specific examples have been detected yet.

[1] "Detection of intergalactic red-giant-branch stars in the Virgo cluster", Ferguson et al. Nature 391.6666 (1998): 461.

[2] "Polychromatic view of intergalactic star formation in NGC 5291", M. Boquien et al. A&A, 467 1 (2007) 93-106.

Thorondor
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    I like how this shows both other answers are wrong. I would have thought star formation only happened in galaxies. – user126527 Feb 19 '19 at 10:24
  • Could those planets of extra-galactic origin have metals (i.e. anything else than hydrogen and helium)? I thought all those materials were mostly formed and distributed by supernovae. – Paŭlo Ebermann Feb 19 '19 at 10:36
  • @PaŭloEbermann If stars can be born outside of a galaxy then they can explode outside of a galaxy as well. – castis Feb 19 '19 at 11:54
  • Some further reading... – J... Feb 19 '19 at 12:51
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    If a galaxy would be a prerequisite of star formation, how would the first stars formed before galaxies existed? – mg30rg Feb 19 '19 at 13:36
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    It's worth noting that the example cited, [https://en.wikipedia.org/wiki/NGC_5291](NGC 5291) is a pair of interacting galaxies. During such events large amounts of galactic gas/dust is drawn out of galaxies and compressed via tidal interaction triggering star formation. That can be considered intergalactic in that such clouds of gas and stars formed in them can be given sufficient velocity to never rejoin the merging galaxies; but also isn't in the sense that it's something that only happens in the immediate vicinity of galaxies. – Dan Is Fiddling By Firelight Feb 19 '19 at 15:55
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    I'm not aware of any star formation occurring in deep intergalactic space without any galaxies in the area to serve as a trigger. – Dan Is Fiddling By Firelight Feb 19 '19 at 15:55
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    Possibly relevant: D100 gas tail – Trip Space-Parasite Feb 19 '19 at 21:00
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    @mg30rg I'm purely speculating, but can imagine a model where the first stars form within a mix of spinning, roughly galaxy-equivalent mass, similar to how planets are hypothesized to form within a protoplanetary disk. Which can be sort of thought of as "in galaxies" if we'll allow a loose definition of "galaxy" that doesn't specifically say it contains stars. – aschepler Feb 20 '19 at 01:27
  • As far as I understand, those stars are mostly ones thrown out of galaxies, and they usually lose planets during that. So most of them(say 90%) do not have planets. – Vashu Feb 20 '19 at 03:56
  • @PaŭloEbermann They must have - stars that don't contain metals are not on the main sequence and are very short-lived; I'm sure we would have noticed the discrepancy, assuming we can distinguish if a star has been ejected or formed outside of a galaxy (I'm not sure we can). They might have different proportions, of course, but so do stars in other galaxies (and even stars within one galaxy). That said, "galaxy" is a bit vague term - I'm not sure if there's some precise definition, but the one I know wouldn't technically exclude galaxies with a hundred stars :) – Luaan Feb 20 '19 at 08:15
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They probably have to be formed in a galaxy. But they can escape. There are many rogue planets that are found outside of solar systems. NASA has observed many rogue stars as well. This usually happens when galaxies collide and they throw out some of the stars. And yes, the rogue stars can have planets orbiting them

S.t.r.a.n.g.e.C.h.a.r.m
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    Looking at the other answer (the one with sources), stars probably don't have to be formed in a galaxy. – hyde Feb 19 '19 at 07:23
  • @hyde it goes against Kaiser bias, i.e. the likelihood of passing a threshold for gravitational collapse is enhanced if you are sitting on top of a lower density region (e.g. a proto galaxy). So stars probably form in the vicinity of other stars = a galaxy. – chris Feb 19 '19 at 12:56
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    @chris "Any given star was probably formed inside a galaxy" is a much weaker claim than "Stars probably have to be formed inside a galaxy" – Taemyr Feb 19 '19 at 13:57
  • @Taemyr stars like to form together, and in sufficient number this defines what a galaxy is. So yes stars like to form in galaxies. Doesn't mean its impossible for a star to form alone in the middle of nowhere. Its probably just exponentially rare. Having said that I don't think we know the initial power spectrum properties on stellar scales so this is hypothetical. – chris Feb 19 '19 at 14:01
  • @chris: Exponentially rare doesn't jibe with the 10% figure given elsewhere – Lightness Races in Orbit Feb 19 '19 at 15:44
  • @LightnessRacesinOrbit I would not consider the Virgo cluster to be very empty though :-) – chris Feb 19 '19 at 15:49
  • @LightnessRacesinOrbit the source in Thorondor's answer attributes that 10% to stars being stripped off of their host galaxies, not intergalactic star formation. – asgallant Feb 19 '19 at 22:38
  • @asgallant Mm, fair enough. – Lightness Races in Orbit Feb 19 '19 at 22:43
  • All you need to make a star is sufficient material to aggregate, sufficient time, and a sufficiently dense region of material (or seed). Sure, the process is greatly aided if you have other stars around, providing radiation pressure and collision debris in masse. But you don't need that. You just need a large cloud of hydrogen, a seed, and time for the cloud to coalesce around it. – Drunken Code Monkey Feb 19 '19 at 23:50