How are stars born?

Stars are born from clouds of interstellar dust, and gas called molecular clouds. Gravity draws the debris from hundreds of exploding stars together over the course of millions of years. These clouds, or nebulae,  are made up of 98% helium and hydrogen, 1.4% hydrogen compounds, and 6% rocks and metals. In order for stars to form the dust grains must block light from other celestial bodies to allow the formation of molecular hydrogen. Furthermore, the MC (molecular cloud) , must exhibit low temperatures, about 10-30K, and high densities of typically 300 molecules per cubic centimeter, otherwise stars cannot form. In low density nebulae, the grains are spread out allowing the passing through of ultra-violet (UV) photons, responsible for the photo-dissociation, meaning that the energy from the photon breaks the molecule apart , thus    

 impeding the formation of molecular hydrogen. 

Debris keep collecting together, and soon the cloud gradually collapses in on itself  as more, and more material clumps together at the center. Molecules, rotating faster at the center, heat up as atoms get packed together because gravity causes the MC to contract (shrink).  At first,the bulk of the cloud is transparent, and heat escapes, but when it becomes opaque, or denser, the temperature rises. When internal pressure halts the collapse of the cloud a protostar* is formed. Eventually the core becomes hot enough to fuse hydrogen into helium, and at this point a star is born!

To understand this fully, we must look at the ongoing battle between gas pressure and gravity. If gravity can overcome the outward push of gas pressure within the gas cloud, which depends on density and temperature, than the creation of a star is possible. When gravity makes  a cloud contract, the internal pressure and temperature increases.  It converts some of it's gravitational potential energy into thermal energy, and quickly rids itself  of any thermal energy build up otherwise thermal pressure would halt the formation of a star, and central temperature would remain fixed. Constant collisions between gas molecules transform thermal energy into photons, and as long as the photons are able to escape then the temperature is kept low.

Nevertheless, there comes a point where the cloud cannot radiate away thermal energy as it becomes smaller, and denser. The energy then becomes trapped within, and the temperature rises thus slowing down the collapse of the cloud. The dense center becomes a protostar with a core not yet hot enough for nuclear fusion.*  It may remain a protostar for 100,000 years or more depending on its mass. The random motion of gas particles, due to the high temperatures, also leads to an overall rotation. This rotation starts out being very slow, but later picks up speed as the  cloud becomes compressed. The rapid rotation makes the gas settle into a rotating disk, and in some cases, newly born protostars shoot out jets in opposite directions along the axis of rotation. The only known explanation for these jets  are magnetic field lines that bec0me twisted by the disk's rotation.

 Although a protostar  might appear like a real star it still lacks the ability to sustain itself.  All the energy it has comes from gravitational contraction, not from nuclear fusion. To ignite fusion it must first contract further to rise the temperature within.  A protostar usually radiates away about half of its energy  while the other half still remains trapped at the core , raising the temperature. Thermal energy is then transported from the center to the surface of the protostar by convection until radiative diffusion-the traveling of photons from the core to the surface- takes over. Eventually when the core exceeds 10 million K, hydrogen fusion can occur, and the protostar becomes a true star!  This star is said to be a main sequence star burning hydrogen in it's core.  Stars can range in sizes  depending on how much matter was gathered during the accreting  process. There are "failed stars" called brown dwarfs*, that do not fuse hydrogen, but are thought to have been created in the same way stars do.

*Protostar: an early stage of the formation of a star when no nuclear fusion taking place at its core.


*nuclear fusion: When temperatures are high enough,  nuclei, that would usually repel eachother,  smash together  due to their high speeds, and form heavier nuclei.


*Brown dwarfs: the reason it fails at becoming a true star is because its mass is so small that when gravity causes it to contract there comes a point when it becomes too packed together that eventually electron degeneracy pressure takes over before the core reaches a sufficiently high temperature to fuse hydrogen into helium.  Electron degeneracy pressure just means that no to electrons can occupy the same quantum state.