<p>The short answer is we don't know. </p>

<p>A longer pedantic answer would point out that use of a term without understanding what you mean by that term is never appropriate. </p>

<p>The term I feel is problematical is "stardust". In our standard picture of big bang nucleosynthesis, we are optimistic (because of the good agreement between calculations and observations of cosmological abundances) that we understand that as the Universe cooled hydrogen, helium, and traces of other elements formed (lithium is the one most often mentioned). It is almost NEVER pointed out that if the Universe is composed of 27% dark matter and only 5% regular matter (mostly hydrogen) then these calculations are an abysmal failure: they don't predict the existence of dark matter! All this is ignored and we congratulate ourselves (or rather the popular media lionize science's "accomplishments"). You can't ignore most of the matter in the Universe and expect that your models will be accurate. </p>

<p>Whether the first star preceded or followed the first galaxy is to some extent a matter of definition. To some extent. It isn't at all clear whether or not any black holes existed prior to galaxy/star formation. </p>

<p>If super black holes existed, then anything within their gravitational range would comprise a galaxy (if you chose to use a reasonable definition of galaxy). What would exist then would be gas (and plasma), dark matter, and the beginning traces of what you'd probably say is stardust: the lithium, boron, etc. which would agglomerate as it collided with similar atoms. The density of this dust would be far too low, I think, to ever form planets without other objects gravity intervening. (Although I probably should never say never). OTOH, if the only black holes existing were sufficiently small, they'd probably evaporate with only transient effects on the particle dynamics. So, we can ignore stardust, most of that was formed IN the first (Generation III) stars. </p>

<p>The question is: which came first; galaxies or stars? And the answer is: what exactly defines a galaxy? We believe there were density fluctuations which would lead to gravitational collapse of the gas surrounding them (and composing them). At what point does this motion qualify as being distinguishable from other motions (other volumes of collapse)? Perhaps you can guess that it will depend on both the dark matter present AND the velocity (temperature) of the gas. A careful analysis would probably involve escape velocities (and as you probably know, that depends on the dark matter distribution) and the point at which "most" gas was orbiting rather than just randomly colliding which would in turn rest on what "most" means (51%, 90%, 99%,...). </p>

<p>So define the term galaxy. If you define it in terms of stars, then the answer is easy: (some) stars came first. If you define it in terms of zones of gravitation, then it is possible that the answer is both. That is, in some cases hydrogen had enough time to collapse into a star before "most" gas was bound to a near by "galaxy" (probably proto-galaxy would be a better term here) and in other places the protogalaxy had defined itself before the first star lit up. We don't know. </p>

<p>It is a mistake to assume that the Universe as we see it today is a reasonable approximation of the Universe as it would appear only 10 or 100 million years after the big bang. </p>

<p>Our models aren't good enough yet (and probably won't be until we understand a lot more about what dark matter is and how it behaves) to be confident that protogalaxies existed before the first stars or that the first stars lit up before the zones I'm calling protogalaxies were separate systems.</p>

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2016-08-07T06:36:56.397Z
Sorry, but this is quite wrong. Most importantly because we _do_ know what came first. We know on which scales the matter that comprise galaxies and stars first started to collapse from the (almost) smooth, primordial soup of particles, and the answer to this _does_ include dark matter. It's true that we don't know what dark matter _is_, but that doesn't mean that we "ignore" its effect.
pela
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2016-08-05T22:15:51.597Z
2016-08-07T01:24:00.503Z
2016-08-07T01:24:00.503Z
Li Zhi
<p>The structure we see in the Universe has formed from the gravitational collapse of the matter that was once an almost smooth density field of gas (&quot;baryons&quot;) and dark matter<span class="math-container">$^1$</span>. The word &quot;almost&quot; is important here, for if it had been completely — or even non-completely but much more — smooth, then the collapse would not have had the time to happen before the expansion of space has diluted the matter enough to prevent any collapse, and we would never had come into existence.</p>
<p>That is, the density field was slightly clumpy, and these clumps — or <em>overdensities</em> — existed on all scales.
But calculating which clump sizes collapse <em>first</em> — stellar-sized, galaxy-sized, cluster-sized, etc. — is far from trivial. Analytical attempts has to make several approximations, but can still make quite meaningful predictions which have subsequently been backed up and refined by numerical simulations, and by now, although many loose ends still exist, we have a rather good picture of structure formation:</p>
<h3>Gravitational collapse</h3>
<p>An overdensity is denoted by
<span class="math-container">$\delta\equiv\rho/\langle\rho\rangle - 1$</span>, where <span class="math-container">$\rho$</span> is the density of the overdensity and <span class="math-container">$\langle\rho\rangle$</span> is the average density.
The evolution of overdensities under the force of gravity can be calculated exactly for <span class="math-container">$\delta \ll 1$</span> using linear perturbation theory, but when <span class="math-container">$\delta$</span> becomes of order unity, the non-linear regime is entered, and severe approximations must be made, so one turns instead to numerical simulations. It turns out that if <span class="math-container">$\delta\gtrsim1.68$</span> (i.e. if a region in space has a density which is 2.68 times the ambient density), it will collapse. The answer to your question is then given by what size of clumps first reach <span class="math-container">$\delta\gtrsim1.68$</span>.</p>
<h3>First clumps</h3>
<p>Primordial quantum fluctuations<span class="math-container">$^2$</span> grew in size during the (admittedly still pretty blurry) inflation, a fraction of a second after Big Bang. In the young Universe dark energy was negligible, and the dynamics of the Universe was dominated by matter. Because dark matter comprises <span class="math-container">$\sim5/6$</span> of the total amount, we can initially neglect the presence of gas, but when the density becomes very high, gas pressure builds up and counteracts the collapse.</p>
<p>The overdensities amplified as matter started to collapse. It turns out that the density fluctuations are larger for smaller scales, so the smaller the clump, the sooner it will collapse. This results in the so-called &quot;bottom-up&quot; formation of structure, which is in contrast to what was originally thought; namely that galaxies formed &quot;top-down&quot; in a monolithic collapse (<a href="http://adsabs.harvard.edu/abs/1962ApJ...136..748E" rel="noreferrer">Eggen, Lynden-Bell, &amp; Sandage 1962</a>).</p>
<p>However, this approach neglects both gas and the motion of dark matter particles (treating it as so-called <em>cold dark matter</em>). Taking into account the effects of this puts a lower threshold for the masses of the structures of <span class="math-container">$\sim10^5$</span>-<span class="math-container">$10^6M_\odot$</span> (e.g. <a href="http://arxiv.org/abs/astro-ph/0604050" rel="noreferrer">Naoz et al. 2006</a>; <a href="http://arxiv.org/abs/0906.4372v1" rel="noreferrer">Yoshida 2009</a>). Hence the first structures that formed are believed to be minihalos roughly the mass of globular clusters.</p>
<h3>Stars, galaxies, and clusters</h3>
<p>Stars consist of collapsed gas and almost no dark matter, and the formation of a star thus needs the theory of hydrodynamics rather than just gravity. In order for matter to collapse to such dense structures as stars, it must get rid of much of its energy. This is not possible for the collisionless dark matter (at least in the normal sense, but <a href="https://astronomy.stackexchange.com/a/14433/5264">this post</a>), but gas, which can collide and cool by radiating, is able to do so<span class="math-container">$^3$</span>.</p>
<p>Through radiative cooling, the minihalos thus fragmented further into first gas clouds and then the first stars when the Universe was a few 100 million years old. Subsequently conglomerations of stars merged into galaxies, which eventually formed sheets, filaments and clusters of galaxies.</p>
<h3>Dust</h3>
<p>The first stars were formed out of pure hydrogen and helium (and small amounts of lithium), and the material for making planets did not exist. But when these stars — which were very massive — exploded as supernovae and polluted the interstellar medium with metals<span class="math-container">$^4$</span> and stardust<span class="math-container">$^5$</span>, the formation of less massive stars became possible<span class="math-container">$^6$</span>.</p>
<h3>Planets</h3>
<p>Dust particles stick together to form pebbles, rocks, and planets. Planet formation is probably not possible for too massive stars<span class="math-container">$^7$</span>, but with the formation of small stars this became possible.</p>
<h3>Timeline</h3>
<p>We can make the following timeline:</p>
<ol>
<li>Minihalos</li>
<li>Gas clouds</li>
<li>Stars</li>
<li>Galaxies</li>
<li>Stardust</li>
<li>Planets</li>
</ol>
<p>However, note that the merging of minihalos to larger &quot;galaxies&quot; (without a significant amount of stars) also may happen at earlier epochs, and in particular that star formation (and dust and planets) is a continuous process which still takes place today (although the bulk of the star formation took place when the Universe approximately 3–6 billion years old (<a href="http://arxiv.org/abs/astro-ph/9708220" rel="noreferrer">Madau et al. 1998</a>)).</p>
<hr />
<p><span class="math-container">$^1$</span><sub>And radiation, which actually dominated the energy density until the Universe was <span class="math-container">$\sim50\,000$</span> years old.</sub></p>
<p><span class="math-container">$^2$</span><sub>The term <em>primordial quantum fluctuations</em> might be the coolest term in physics.</sub></p>
<p><span class="math-container">$^3$</span><sub>There <em>are</em> hypotheses of so-called <a href="https://en.wikipedia.org/wiki/Dark_star_(dark_matter)" rel="noreferrer">dark stars</a>, but that's beyond the scope of this text (primarily because I don't understand them).</sub></p>
<p><span class="math-container">$^4$</span><sub>For an astronomer, the term <em>metal</em> means any element heavier than helium. It's easier this way.</sub></p>
<p><span class="math-container">$^5$</span><sub>Astronomers just call it &quot;dust&quot;, and use only the term <em>star</em>dust when talking to non-astronomers, in order for them not to fall asleep.</sub></p>
<p><span class="math-container">$^6$</span><sub>The reason is that with the many possible electronic transitions of metals, the gas has more ways to cool, and hence may collapse at earlier times, before the proto-star clump reaches high masses.</sub></p>
<p><span class="math-container">$^7$</span><sub>Because the high radiation pressure of very massive stars blows away and/or destroys the dust (I think, but this is not my expertise).</sub></p>

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2016-08-07T20:00:37.620Z
2020-12-17T08:45:07.100Z
2020-12-17T08:45:07.100Z
pela
<p>If a galaxy is <strong>defined</strong> as a collection of planetary systems (and all matter in between), and a planetary system is defined as a collection of planets circling a star (and all matter in between), and a planet (and other smaller bodies, like asteroids, moonlets, moons, etc) is basically just a very very, very large collection of minerals, stardust and gases (which make up the entire universe), <strong>which came first: The planets, planetary systems or galaxies?</strong> <em>I've been given to understand the universe came first...but after that?</em></p>

<p>I thought I understood, at least the layman's basics, of the universe formation. However, today, I’m having trouble <strong>conceptualising the order</strong> in which they would have been formed, <strong>IF</strong> they are <em>defined</em> by the smaller unit.  </p>

<p><strong>Is this just me getting the semantics of classification and the actual chronology of each feature forming, mixed up?</strong> </p>

<hr>

<hr>

<p>VERY Basically,</p>

<ul>
<li>The universe is made up of a complex mix of everything (matter, antimatter, darkmatter, to name just a few), and expanding 'space'.</li>
<li>A very, very, large area of 'space' is covered in very, very, tiny particles: made up of at first gases and later 'stardust'.</li>
<li>A galaxy is formed of gravity-attracted gases, and after the very first stars went supernova and exploded there was also 'stardust', which combines to create 'clumps' of interstellar clouds (cloud nebulae).</li>
<li>The cloud nebulae are the 'nursery' areas for star formation (both initial stars and later generation stars). </li>
<li>The newly created star uses remnants of 'stardust' in the cloud nebulae to form planetary systems and then, a 'short' time later the orbiting planets (and other sized objects). </li>
</ul>

<p>So then the chronological order would be: </p>

<p>Universe (Largest scale) => gases (very small scale) => galaxy => interstellar clouds => <strong>stars => planets (combined to form planetary systems)</strong>. And the tiny particles of gases and 'stardust', are involved in <em>nearly</em> all scales.</p>

<p><a href="https://i.sstatic.net/qzJV2.jpg" rel="noreferrer"><img src="https://i.sstatic.net/qzJV2.jpg" alt="Universe components"></a>
Image taken from <a href="http://www.spaceexplained.com/astronomy-explained/the-solar-system-versus-a-galaxy-versus-the-milky-way-versus-the-universe/" rel="noreferrer">spaceexplained</a> </p>

<h2><hr></h2>

<p>Just writing out this question has helped! I hope you don't mind me posting it anyway, as it might help others and <strong>I still need confirmation that I have</strong>, hopefully, worked myself <em>out of</em> my conundrum (not deeper).</p>

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2016-08-04T22:37:18.173Z
Did you check [Wikipedia](https://en.wikipedia.org/wiki/Galaxy_formation_and_evolution)?
HDE 226868
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2016-08-05T00:53:13.447Z
You need to research galaxy formation, star formation and planet formation. Most stars form within a galaxy, and planets always seem to form along with star formation (usually a few million years later, though). You seem to be confused on a number of other points. *Planetary* systems are what orbit a star — the Solar System only refers to the Sun's planetary system. Planets have also had a specific definition for a few years, so you're criteria is pretty general.
Sir Cumference
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2016-08-05T03:54:33.383Z
I wouldn't say "the universe is made up of matter, antimatter and empty space", there's very little antimatter and there's also things like dark matter, dark energy and (dare I say it) fields which help define forces and give rest mass and all that good stuff.    Saying what the universe is "made up of" is more complicated than we used to think some 50 years ago, but that doesn't really affect your question, just pointing out that I'd re-write that first sentence.
userLTK
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2016-08-05T06:33:10.627Z
@Sir Cumference, thanks for comment but I thought I wrote the order taking that into account. Stars form in the galaxy, planets form around the star (yes a 'short' time later). The planets around the star are together known as solar systems (not THE solar system, I'll edit my question later to correct this to the less specific 'planetary systems'). And yes, planets are not the smallest unit but I was specifically not going into every size scale that 'stardust' can form Eg rocks, asteroids, comets, moons, protoplanets, Neptune sized moons, gas giants, etc. It was a 'very BASIC' general outline.
EveryBitHelps
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2016-08-05T06:37:34.477Z
@userTLK, thanks. yes, I do realise that there is more to the universe than just matter and antimatter. I was just trying to get across the point that there are different things making up the universe. I'll take your advice and fix the first sentence when I edit the question.
EveryBitHelps
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2016-08-05T07:09:29.930Z
The egg, obviously.
Nico
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2016-08-05T07:27:17.270Z
@Nico, finally! Someone mentioned it! When I first got confused that's exactly what I was thinking!
EveryBitHelps
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2016-08-05T07:45:15.780Z
"Stardust" cannot exist before stars!
ProfRob
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2016-08-05T09:26:20.050Z
@EveryBitHelps you could say baryonic matter, which is basically what we think of as stuff.    As for "stardust", Rob Jeffries point makes me laugh but it's true.   How can you have stardust before there's a star.    Stars had a hard time forming in the early universe because it was virtually all hydrogen and helium and they just bounce off each other.   Stars form more easily around a planet sized lump of matter (er, I think).  There were no lumps of matter in the pre-star universe.
userLTK
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2016-08-05T09:31:38.733Z
Just a question to refocus: I've read recently that the first stars formed **before the first galaxies**, not after. Does this disagree with what's being said here? (What I read was low metal stars formed in the reionisation period, after the "dark period", when large scale structure was forming but before galaxies. I don't know if this theory is accepted yet though)
Andy
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2016-08-05T12:11:28.500Z
@EveryBitHelps Most astronomers don't ever use the term "solar systems". The accurate term is "planetary systems". That's just how it is; solar refers to the Sun, and only the Sun.
Sir Cumference
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2016-08-05T12:56:27.307Z
@Rob, +1. Too funny. True, but still funny. I was using stardust as a collective term for all the 'stuff' that is needed to start the processes. I'll leave it for now. If people have a real problem with the inaccurate and incorrect term that *does* get the point across, I'll edit the question again.
EveryBitHelps
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2016-08-06T03:51:02.833Z
@Andy if we define a galaxy as a collection of stars, then stars had to form first.   While the definition of when a star begins is fairly clear cut (when the fusion cycle starts), the definition of when a galaxy begins is not nearly so easily defined.   But I think the microwave background information tells us that galaxies began to coalesce before stars formed, so it's really a matter of what you define as a galaxy.
userLTK
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2016-08-08T11:30:29.190Z
@EveryBitHelps Dust has a very specific meaning and it is incredibly important for the formation of stars and galaxies. Before the first stars there was only gas.
ProfRob
<galaxy><cosmology><star-formation><planetary-formation>
Which came first: Galaxies <=> Stars <=> Planets?
EveryBitHelps
# Which came first: Galaxies <=> Stars <=> Planets?

(Asked by: username_0 on 2016-08-04)

If a galaxy is **defined** as a collection of planetary systems (and all matter in between), and a planetary system is defined as a collection of planets circling a star (and all matter in between), and a planet (and other smaller bodies, like asteroids, moonlets, moons, etc) is basically just a very very, very large collection of minerals, stardust and gases (which make up the entire universe), **which came first: The planets, planetary systems or galaxies?** *I've been given to understand the universe came first...but after that?*

I thought I understood, at least the layman's basics, of the universe formation. However, today, I’m having trouble **conceptualising the order** in which they would have been formed, **IF** they are *defined* by the smaller unit.

**Is this just me getting the semantics of classification and the actual chronology of each feature forming, mixed up?**

---

---

VERY Basically,

* The universe is made up of a complex mix of everything (matter, antimatter, darkmatter, to name just a few), and expanding 'space'.
* A very, very, large area of 'space' is covered in very, very, tiny particles: made up of at first gases and later 'stardust'.
* A galaxy is formed of gravity-attracted gases, and after the very first stars went supernova and exploded there was also 'stardust', which combines to create 'clumps' of interstellar clouds (cloud nebulae).
* The cloud nebulae are the 'nursery' areas for star formation (both initial stars and later generation stars).
* The newly created star uses remnants of 'stardust' in the cloud nebulae to form planetary systems and then, a 'short' time later the orbiting planets (and other sized objects).

So then the chronological order would be:

Universe (Largest scale) => gases (very small scale) => galaxy => interstellar clouds => **stars => planets (combined to form planetary systems)**. And the tiny particles of gases and 'stardust', are involved in *nearly* all scales.

[![Universe components](https://i.sstatic.net/qzJV2.jpg)](https://i.sstatic.net/qzJV2.jpg)
Image taken from [spaceexplained](http://www.spaceexplained.com/astronomy-explained/the-solar-system-versus-a-galaxy-versus-the-milky-way-versus-the-universe/)

## ---

Just writing out this question has helped! I hope you don't mind me posting it anyway, as it might help others and **I still need confirmation that I have**, hopefully, worked myself *out of* my conundrum (not deeper).

**Tags:** galaxy, cosmology, star-formation, planetary-formation
> **Comments:**
> **username_1**: Did you check [Wikipedia](https://en.wikipedia.org/wiki/Galaxy_formation_and_evolution)?  
> **username_2**: You need to research galaxy formation, star formation and planet formation. Most stars form within a galaxy, and planets always seem to form along with star formation (usually a few million years later, though). You seem to be confused on a number of other points. *Planetary* systems are what orbit a star — the Solar System only refers to the Sun's planetary system. Planets have also had a specific definition for a few years, so you're criteria is pretty general.  
> **username_3**: I wouldn't say "the universe is made up of matter, antimatter and empty space", there's very little antimatter and there's also things like dark matter, dark energy and (dare I say it) fields which help define forces and give rest mass and all that good stuff.    Saying what the universe is "made up of" is more complicated than we used to think some 50 years ago, but that doesn't really affect your question, just pointing out that I'd re-write that first sentence.  
> **username_0**: @Sir Cumference, thanks for comment but I thought I wrote the order taking that into account. Stars form in the galaxy, planets form around the star (yes a 'short' time later). The planets around the star are together known as solar systems (not THE solar system, I'll edit my question later to correct this to the less specific 'planetary systems'). And yes, planets are not the smallest unit but I was specifically not going into every size scale that 'stardust' can form Eg rocks, asteroids, comets, moons, protoplanets, Neptune sized moons, gas giants, etc. It was a 'very BASIC' general outline.  
> **username_0**: @userTLK, thanks. yes, I do realise that there is more to the universe than just matter and antimatter. I was just trying to get across the point that there are different things making up the universe. I'll take your advice and fix the first sentence when I edit the question.  
> **username_4**: The egg, obviously.  
> **username_0**: @Nico, finally! Someone mentioned it! When I first got confused that's exactly what I was thinking!  
> **username_5**: "Stardust" cannot exist before stars!  
> **username_3**: @EveryBitHelps you could say baryonic matter, which is basically what we think of as stuff.    As for "stardust", Rob Jeffries point makes me laugh but it's true.   How can you have stardust before there's a star.    Stars had a hard time forming in the early universe because it was virtually all hydrogen and helium and they just bounce off each other.   Stars form more easily around a planet sized lump of matter (er, I think).  There were no lumps of matter in the pre-star universe.  
> **username_6**: Just a question to refocus: I've read recently that the first stars formed **before the first galaxies**, not after. Does this disagree with what's being said here? (What I read was low metal stars formed in the reionisation period, after the "dark period", when large scale structure was forming but before galaxies. I don't know if this theory is accepted yet though)  
> **username_2**: @EveryBitHelps Most astronomers don't ever use the term "solar systems". The accurate term is "planetary systems". That's just how it is; solar refers to the Sun, and only the Sun.  
> **username_0**: @Rob, +1. Too funny. True, but still funny. I was using stardust as a collective term for all the 'stuff' that is needed to start the processes. I'll leave it for now. If people have a real problem with the inaccurate and incorrect term that *does* get the point across, I'll edit the question again.  
> **username_3**: @Andy if we define a galaxy as a collection of stars, then stars had to form first.   While the definition of when a star begins is fairly clear cut (when the fusion cycle starts), the definition of when a galaxy begins is not nearly so easily defined.   But I think the microwave background information tells us that galaxies began to coalesce before stars formed, so it's really a matter of what you define as a galaxy.  
> **username_5**: @EveryBitHelps Dust has a very specific meaning and it is incredibly important for the formation of stars and galaxies. Before the first stars there was only gas.  

---

## 2 answers

### Answer by username_8 on 2016-08-07. Score: 17 | Accepted answer

The structure we see in the Universe has formed from the gravitational collapse of the matter that was once an almost smooth density field of gas ("baryons") and dark matter$^1$. The word "almost" is important here, for if it had been completely — or even non-completely but much more — smooth, then the collapse would not have had the time to happen before the expansion of space has diluted the matter enough to prevent any collapse, and we would never had come into existence.

That is, the density field was slightly clumpy, and these clumps — or *overdensities* — existed on all scales.
But calculating which clump sizes collapse *first* — stellar-sized, galaxy-sized, cluster-sized, etc. — is far from trivial. Analytical attempts has to make several approximations, but can still make quite meaningful predictions which have subsequently been backed up and refined by numerical simulations, and by now, although many loose ends still exist, we have a rather good picture of structure formation:

### Gravitational collapse

An overdensity is denoted by
$\delta\equiv\rho/\langle\rho\rangle - 1$, where $\rho$ is the density of the overdensity and $\langle\rho\rangle$ is the average density.
The evolution of overdensities under the force of gravity can be calculated exactly for $\delta \ll 1$ using linear perturbation theory, but when $\delta$ becomes of order unity, the non-linear regime is entered, and severe approximations must be made, so one turns instead to numerical simulations. It turns out that if $\delta\gtrsim1.68$ (i.e. if a region in space has a density which is 2.68 times the ambient density), it will collapse. The answer to your question is then given by what size of clumps first reach $\delta\gtrsim1.68$.

### First clumps

Primordial quantum fluctuations$^2$ grew in size during the (admittedly still pretty blurry) inflation, a fraction of a second after Big Bang. In the young Universe dark energy was negligible, and the dynamics of the Universe was dominated by matter. Because dark matter comprises $\sim5/6$ of the total amount, we can initially neglect the presence of gas, but when the density becomes very high, gas pressure builds up and counteracts the collapse.

The overdensities amplified as matter started to collapse. It turns out that the density fluctuations are larger for smaller scales, so the smaller the clump, the sooner it will collapse. This results in the so-called "bottom-up" formation of structure, which is in contrast to what was originally thought; namely that galaxies formed "top-down" in a monolithic collapse ([Eggen, Lynden-Bell, & Sandage 1962](http://adsabs.harvard.edu/abs/1962ApJ...136..748E)).

However, this approach neglects both gas and the motion of dark matter particles (treating it as so-called *cold dark matter*). Taking into account the effects of this puts a lower threshold for the masses of the structures of $\sim10^5$-$10^6M\_\odot$ (e.g. [Naoz et al. 2006](http://arxiv.org/abs/astro-ph/0604050); [Yoshida 2009](http://arxiv.org/abs/0906.4372v1)). Hence the first structures that formed are believed to be minihalos roughly the mass of globular clusters.

### Stars, galaxies, and clusters

Stars consist of collapsed gas and almost no dark matter, and the formation of a star thus needs the theory of hydrodynamics rather than just gravity. In order for matter to collapse to such dense structures as stars, it must get rid of much of its energy. This is not possible for the collisionless dark matter (at least in the normal sense, but [this post](https://astronomy.stackexchange.com/a/14433/5264)), but gas, which can collide and cool by radiating, is able to do so$^3$.

Through radiative cooling, the minihalos thus fragmented further into first gas clouds and then the first stars when the Universe was a few 100 million years old. Subsequently conglomerations of stars merged into galaxies, which eventually formed sheets, filaments and clusters of galaxies.

### Dust

The first stars were formed out of pure hydrogen and helium (and small amounts of lithium), and the material for making planets did not exist. But when these stars — which were very massive — exploded as supernovae and polluted the interstellar medium with metals$^4$ and stardust$^5$, the formation of less massive stars became possible$^6$.

### Planets

Dust particles stick together to form pebbles, rocks, and planets. Planet formation is probably not possible for too massive stars$^7$, but with the formation of small stars this became possible.

### Timeline

We can make the following timeline:

1. Minihalos
2. Gas clouds
3. Stars
4. Galaxies
5. Stardust
6. Planets

However, note that the merging of minihalos to larger "galaxies" (without a significant amount of stars) also may happen at earlier epochs, and in particular that star formation (and dust and planets) is a continuous process which still takes place today (although the bulk of the star formation took place when the Universe approximately 3–6 billion years old ([Madau et al. 1998](http://arxiv.org/abs/astro-ph/9708220))).

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$^1$And radiation, which actually dominated the energy density until the Universe was $\sim50\,000$ years old.

$^2$The term *primordial quantum fluctuations* might be the coolest term in physics.

$^3$There *are* hypotheses of so-called [dark stars](https://en.wikipedia.org/wiki/Dark_star_(dark_matter)), but that's beyond the scope of this text (primarily because I don't understand them).

$^4$For an astronomer, the term *metal* means any element heavier than helium. It's easier this way.

$^5$Astronomers just call it "dust", and use only the term *star*dust when talking to non-astronomers, in order for them not to fall asleep.

$^6$The reason is that with the many possible electronic transitions of metals, the gas has more ways to cool, and hence may collapse at earlier times, before the proto-star clump reaches high masses.

$^7$Because the high radiation pressure of very massive stars blows away and/or destroys the dust (I think, but this is not my expertise).


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### Answer by username_7 on 2016-08-05. Score: -3

The short answer is we don't know.

A longer pedantic answer would point out that use of a term without understanding what you mean by that term is never appropriate.

The term I feel is problematical is "stardust". In our standard picture of big bang nucleosynthesis, we are optimistic (because of the good agreement between calculations and observations of cosmological abundances) that we understand that as the Universe cooled hydrogen, helium, and traces of other elements formed (lithium is the one most often mentioned). It is almost NEVER pointed out that if the Universe is composed of 27% dark matter and only 5% regular matter (mostly hydrogen) then these calculations are an abysmal failure: they don't predict the existence of dark matter! All this is ignored and we congratulate ourselves (or rather the popular media lionize science's "accomplishments"). You can't ignore most of the matter in the Universe and expect that your models will be accurate.

Whether the first star preceded or followed the first galaxy is to some extent a matter of definition. To some extent. It isn't at all clear whether or not any black holes existed prior to galaxy/star formation.

If super black holes existed, then anything within their gravitational range would comprise a galaxy (if you chose to use a reasonable definition of galaxy). What would exist then would be gas (and plasma), dark matter, and the beginning traces of what you'd probably say is stardust: the lithium, boron, etc. which would agglomerate as it collided with similar atoms. The density of this dust would be far too low, I think, to ever form planets without other objects gravity intervening. (Although I probably should never say never). OTOH, if the only black holes existing were sufficiently small, they'd probably evaporate with only transient effects on the particle dynamics. So, we can ignore stardust, most of that was formed IN the first (Generation III) stars.

The question is: which came first; galaxies or stars? And the answer is: what exactly defines a galaxy? We believe there were density fluctuations which would lead to gravitational collapse of the gas surrounding them (and composing them). At what point does this motion qualify as being distinguishable from other motions (other volumes of collapse)? Perhaps you can guess that it will depend on both the dark matter present AND the velocity (temperature) of the gas. A careful analysis would probably involve escape velocities (and as you probably know, that depends on the dark matter distribution) and the point at which "most" gas was orbiting rather than just randomly colliding which would in turn rest on what "most" means (51%, 90%, 99%,...).

So define the term galaxy. If you define it in terms of stars, then the answer is easy: (some) stars came first. If you define it in terms of zones of gravitation, then it is possible that the answer is both. That is, in some cases hydrogen had enough time to collapse into a star before "most" gas was bound to a near by "galaxy" (probably proto-galaxy would be a better term here) and in other places the protogalaxy had defined itself before the first star lit up. We don't know.

It is a mistake to assume that the Universe as we see it today is a reasonable approximation of the Universe as it would appear only 10 or 100 million years after the big bang.

Our models aren't good enough yet (and probably won't be until we understand a lot more about what dark matter is and how it behaves) to be confident that protogalaxies existed before the first stars or that the first stars lit up before the zones I'm calling protogalaxies were separate systems.

> **Comments:**
> **username_8**: Sorry, but this is quite wrong. Most importantly because we _do_ know what came first. We know on which scales the matter that comprise galaxies and stars first started to collapse from the (almost) smooth, primordial soup of particles, and the answer to this _does_ include dark matter. It's true that we don't know what dark matter _is_, but that doesn't mean that we "ignore" its effect.  

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