CapitalistImperialistPig

300,000 years after the big bang, after the expansion of the universe had cooled it enough for electrons to recombine (or, actually, combine for the very first time) with hydrogen and helium nuclei, the distribution of baryonic matter (which, in the peculiar argot of astrophysics, means both baryons and electrons) was very uniformly distributed, with density perturbations typically one part in 100,000.  So how do you get that stuff to cluster to the point where density perturbations are 199 parts in 200 to form galaxies or another 10^20 plus to form stars? The first part of the answer seems to require dark matter, which, being impervious to the smoothing effects of photons and electrical interactions, had already gotten rather more concentrated.  It's clumps attracted the now neutral baryonic matter. However, just as expansion of the universe had cooled the overall cosmos, local contraction (to form galaxies and stars) heats it back up, and that heat generates pressure whi...

When stars of moderate mass start to exhaust the amount of hydrogen in their core, the core has to heat up to keep the fusion rate high enough to balance the pressure of the gas above it. As the core begins to exhaust all its hydrogen, burning ignites in a shell above the core of helium ash. This causes the overall luminosity, or brightness, of the star to increase, the overlying volume to expand, and the atmosphere of the star to cool - the star ascends the so called red giant branch, becoming brighter, cooler (and consequently) redder. Many find this intuitive, but there is one strange thing - why should the interior of the star becoming hotter make the exterior cooler? Our friend the virial theorem, which says that 2K+U = 0, where K is the kinetic energy and U is the gravitational potential energy, might have something to say. The interior getting hotter, and the star consequently expanding, increases both K and U, since U becomes less negative. Consequently K for the outer re...

Interstellar molecular clouds of dust and gas exist in a quasi-stable balance between self-gravity and internal pressure. If the mass and density are large enough, and the temperature and pressure low enough, they can collapse to form stars, usually in clusters of a few dozen to a few thousand. Both gaseous and magnetic pressure can be factors. Various factors can trigger collapse, including shocks from collisions with other interstellar clouds, passing through one of the spiral density waves that give spiral galaxies their names, or the shockwave of a nearby supernova. Once a collapse starts, it can become irreversible, as gravity gets stronger as the cloud shrinks. The cloud will heat up as a result of the collapse, but if it contains plenty of dust, it can radiate that heat away in the infrared. Actual stellar formation seems to take place in localized regions of overdensity called cloud cores. Because the molecular clouds are turbulent, individual pieces are likely to have a...