system
Given the following post, respond as if you're commenting on Reddit:
human
Before dust clouds collapse into a star, is there a solid core, and if so, what is it made of?
gpt
Absolutely! It depends on what stage it is, but it is generally made out of the same things planets are. This is because it forms from the same cloud, generally! Dust naturally gets drawn to itself through gravity. Dust becomes rocks, rocks become boulders, boulders become...mountain-sized boulders. You get the idea. It's possible they look similar to asteroids. After a while, and due to chance, you'll get a bunch of planet sized ones. These large objects will gravitate towards each other and either combine or destroy each other. After achieving a certain amount of mass compared to its surroundings, it can become spherical...but in a pre-solar system with constant collisions, that's probably not required. The bigger you are, the more dust is attracted to you, and the faster you grow. Sidenote: this is why the larger planets can hold a larger atmosphere and retain more heat.

After enough of this, a HUGE one appears. If it's lucky, and isn't flung out of the dust cloud, it might build up enough mass to reach a turning point and start a nuclear reaction at its core from all the pressure. If it's unlucky (either because of random chance or because there's something bigger than it around), it gets flung out of the accretion area and is known as a brown dwarf. Brown dwarfs can also form along the edge of dust clouds and simply run out of dust to accumulate. One step smaller than a brown dwarf is a gas giant, similar to Jupiter. 

As for what they're made of, it would presumably be the same stuff as brown dwarfs and gas giants:
> Gas giants have some of the characteristics of brown dwarfs. For example, Jupiter and Saturn are both made primarily of hydrogen and helium, like the Sun. Saturn is nearly as large as Jupiter, despite having only 30% the mass. Three of the giant planets in the Solar System (Jupiter, Saturn, and Neptune) emit much more heat than they receive from the Sun.

The younger a celestial body is, the more likely it is to have heavier elements (heavy elements are ONLY made available through the supernova of a star, and the universe is only so old)...but even then, those elements would be scarce.