Star formation. Artwork of the formation of young, hot blue stars. Such starbirth regions arise when clouds of gas and dust are disturbed by shockwaves from star death and formation, or from colliding galaxies. When the density and temperature of the gravitationally-contracting protostars reach high enough levels, ignition of nuclear fusion occurs and stars are born. Younger (population I) stars are blue and hot. Older (population II) stars are cool and yellow, the remnants of an earlier epoch of star formation where the large blue population II stars have long since burned out. The arms of spiral galaxies arise from population I stars that form from the gas and dust of older stars.