In my class we are studying objects in our solar system and this question seemed to just pop up. And since I cannot answer this, I've been really frustrated for quite a while now and would like some help on understanding this.

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    $\begingroup$ It's not clear what you're asking. What was the actual statement that caused your question? What's the context? $\endgroup$ Apr 22, 2019 at 23:47

1 Answer 1


The start of it all is usually something called a Giant Molecular Cloud, a particular kind of nebula which is denser than usual and cooler than usual. A GMC will typically be 10x to 1000x the mass of the Sun.

That mass is enough to cause the outer layers to fall in on the inner parts and the GMC starts to collapse and shrink. (Diffuse though it is, the mass exerts the same force on the outer edges as it would if the entire mass was concentrated at the center.)

The cloud collapses and frequently fragments into a bunch of smaller collapsing blobs centered on especially dense sections of the cloud. At this point it looks a lot like the Oriion Nebula. The young stars light up and blow away the remaining bits of the GMC, and in the end, you have a cluster of young stars with planetary disks looking something like the Pleiades.

There never was a central mass -- none is needed. The GMC collapses under its own self-attraction.

  • $\begingroup$ The start of it all is where you should've started. Photons colliding to make protons and neutrons, and somehow electrons show up (???), and now there's hydrogen in the universe. Which coalesced itself (because everything except photons have mass), became the first stars, and then the first (type I) supernovas happened, during which heaver elements can form, from which stars that will go type II supernova can form. Then type III. .... And then the dinosaurs came. $\endgroup$
    – Mazura
    Apr 23, 2019 at 5:48
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    $\begingroup$ @Mazura Protons and neutrons are not made by colliding photons. They're made up of quarks and result from from the big bang when it became cool enough for quarks to bond. $\endgroup$ Apr 23, 2019 at 6:50
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    $\begingroup$ The one (over) simplification in the "collaps under gravity" explanations is that orbiting dust and debris, eventually forming a protoplanetary disk, does not simply "collapse" under its own gravity. Not any more than our solar system collapses: All parts are in orbits and there is conservation of energy, after all. Objects accelerate on the way in and thus have exactly the necessary kinetic energy to get out again. What's needed is collision, friction and shedding of kinetic energy by radiation, allowing the orbits to become smaller. At a certain density there must be a runaway effect. $\endgroup$ Apr 23, 2019 at 9:40
  • $\begingroup$ @StephenG -sorry, wrong particles; electrons and positrons. Where did the mass come from that caused gravity? - apparently, quarks. $\endgroup$
    – Mazura
    Apr 23, 2019 at 10:15
  • $\begingroup$ @Mazura You said "Then type III". It's not clear what you're talking about, but whatever it is, I suspect you're wrong. ;) Population III stars formed before population II stars, and although Zwicky devised a type III supernova class, only one object (SN 1961i) was assigned to it, and in modern treatments it would be classed as type II. $\endgroup$
    – PM 2Ring
    Apr 25, 2019 at 5:49

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