Baby stars shine without fusion, powered by gas slamming into them
A newborn star is not yet burning hydrogen at its heart. Instead, a protostar glows because gas falling from its surrounding disk crashes into its surface, releasing radiation. For a star like the Sun, this infant stage lasts roughly half a million years, hidden entirely from ordinary telescopes.
Stars begin inside dense cores, compact knots within molecular clouds. At first each core is balanced, its own gravity squeezing inward while gas pressure and magnetic pressure push out. As it gathers material from the wider cloud, gravity wins and collapse starts. Models of an idealised sphere predict the collapse should begin at the centre and work outward; spectra of starless cores do show contraction, although that outward-spreading pattern has never been caught in action.
Infalling gas first builds a small central object, then a disk circling it. Because spinning material conserves angular momentum, more and more of the incoming gas lands on the disk rather than on the young star. How disk material then spirals inward onto the protostar remains unsolved, part of a wider puzzle about accretion disks that runs through much of astrophysics. The phase ends when the supply of gas runs out, leaving a pre-main-sequence star that keeps shrinking until hydrogen fusion ignites.
Inside, a protostar is cooler than a mature star. Ordinary hydrogen is not fusing at its centre, but theory says deuterium, the heavy isotope, combines with hydrogen to make helium-3, and that heat puffs the object up. Its surface is a turbulent layer of shocked gas quite unlike the calm photosphere of an older star. When Chushiro Hayashi proposed this modern picture in 1966, early models badly overestimated protostar sizes; later calculations and observations showed they are only a little larger than grown stars of equal mass.
Dust in the surrounding core swallows the light and re-emits it at longer wavelengths, so protostars cannot be seen optically or plotted on the Hertzsprung-Russell diagram. Astronomers instead find point-like infrared and millimetre sources inside dark clouds. Those classed as Class 0 or Class I are widely thought to be protostars, though proof is still lacking.
Source: Protostar
