by Joel Hruska, ExtremeTech
Stars are thought to form inside giant molecular clouds of gas, sometimes with diameters hundreds of light years across, with a combined mass equivalent to millions of solar masses. As these clouds collapse, they form fragments. Said fragments are thought to contain the mass that will become both the star and its protoplanetary disc, though there are opportunities for interactions with other nearby clouds. If an aged star near a stellar nursery goes supernova, for example, heavier elements created by the core collapse may form part of the new stars. A nearby supernova can even trigger the formation of new stars when its shock wave sharply compresses the molecular gas cloud.
Planets, in contrast, coalesce out of the grains of ice and dust embedded within the protoplanetary disc surrounding a young star. Stars, in other words, are formed by the compression of gas (mostly hydrogen) in molecular clouds that are often extremely cold, with an average temperature as low as 10K. Planets are formed from grains of ice and dust suspended in the protoplanetary discs of young stars, and begin to accrete into protoplanets and planetesimals as these grains bind together and begin to attract one another. The temperature and physical conditions in the protoplanetary disc are different from those in the molecular clouds that form stellar nurseries, and it’s therefore rather surprising to see a star bulking up, planet-style.