### Which came first: Galaxies <=> Stars <=> Planets?

• 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.

Image taken from spaceexplained

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).

Did you check Wikipedia?

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.

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.

@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.

@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.

The egg, obviously.

@Nico, finally! Someone mentioned it! When I first got confused that's exactly what I was thinking!

"Stardust" cannot exist before stars!

@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.

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)

@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.

@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.

@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.

@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.

6 years ago

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).

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; Yoshida 2009). 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), 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)).

$$^1$$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, 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 stardust 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).