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.
