How are they so densely packed that they end up forming a sphere rather than be a bunch of gas surrounding a core orbiting the sun?
How are they so densely packed that they end up forming a sphere rather than be a bunch of gas surrounding a core orbiting the sun?
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Well, gravity.
One thing that we forget about nebulas is that they are massive. Larger than the largest known stars, and like any other cloud, they’re not uniform.
They form from the remnants of older stars exploding and then over time, the densest parts of those nebulae attract on itself and create a ball of gas. That gas ball sucks in more gas from around itself until the pressure is great enough that the gases fuse together, and a star is born.
The gas giants, like Jupiter, are formations of stellar gas that never ignited, and/or have other impurities that would make it harder to ignite. Sulphur, for example, is harder to fuse than hydrogen.
Remember that Jupiter, as massive as it is, is tiny compared to Sol.
Part of what you may be asking is why do they look like they have a sharp edge, rather than being diffuse. The answer to that has to do with a concept called optical depth, a measure of how much light is scattered away as you look into a substance.
It turns out that when you have a spherical cloud of gas that gets more dense as you get closer to the center and try to calculate the optical depth of a line of sight passing through it at various points, it goes from “little optical depth, you can see through this just fine” to “high optical depth, this is opaque” really fast, so the difference between them looks like a really sharp boundary.