Which is very dim and hence very difficult to find . if it is massive enough though model dependent , the lower mass limit is about 0 . 1 solar mass , the gas in the protostar continues to heat up until the central portion becomes hot and dense enough for , say , the hydrogen nuclei to overcome their mutual electrical repulsion . nuclear fusion will then take place and a star is finally born 如果原行星质量足够大下限约为0 . 1个太阳质量，要注意的是，不同的理论模型会给出不同的下限，原恒星会不断变热，直至核心部分足够热和高压，令氢原子核克服质子间的电排斥力，产生核聚变，成为一颗真正的恒星。
