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Our Milky Way galaxy is filled with interstellar gas peppered with dust, known together as the interstellar medium. This very low-density material churns with turbulent motion between stars, and most of it can remain in this free-moving form for millions or even billions of years. Where pockets of gas become compressed, sometimes through the pressure from a nearby supernova explosion, the density can rise to the point where the inward gravitational pull of the gas on itself is a stronger influence than the dispersing tendency of turbulence. This interstellar cloud can therefore be gravitationally bound, at least for a while.
Through an interplay of the motions within the interstellar cloud, internal gravitational pull and magnetic fields, the cloud can collapse into unstable, long filaments stretching between dense ‘hubs’ – cloud cores that can further collapse into clusters of dense cores. If a core has enough mass in a small enough volume, the gravitational pull can condense the gas into a hot, spherical ‘protostar’. Material from the surrounding cloud will continue to accrete onto this protostar via an intermediate passage through a circumstellar disc. Eventually enough mass and pressure accumulate to ignite the process of fusion in the central region, creating a fully born star. Stars much more massive than our Sun emit powerful photons that can ionise the remaining gas surrounding the stellar nursery. This brightly coloured gas comprises the colourful nebulae that we now know are not only beautiful, but also signposts of active star formation.