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Giant molecular clouds: star factories of the galaxy

Stars are forming in our galaxy at a rate of between 1 and 4 solar masses of stars per year. In contrast to elliptical galaxies, which are largely devoid of star formation, star formation is still going on in spiral galaxies because of their reservoirs of molecular gas, the fuel for new stars. The discs of spiral galaxies are comprised not only of stars as we clearly see from Earth, but also gas (the interstellar medium, ISM). This is where this gas accumulates into cold, dense, molecular regions known as molecular clouds, in which new stars are formed. Most star formation occurs in massive molecular clouds, known as giant molecular clouds (GMCs). However, while we have a good understanding of how individual stars form, there is less consensus on how their natal clouds of gas accumulate, how long these clouds last, how star formation progresses over their lifetime, and indeed how star formation has progressed over the lifetime of the Milky Way.