Transcript: The typical environment of the space between stars is a very thin and very cold gas. The interstellar medium typically has about a million particles per cubic meter, and the temperatures are only ten to twenty degrees Kelvin. However there are some regions with densities a thousand or ten thousand times higher than this. In these regions collisions can occur between the atoms and the temperatures are low enough so the atoms will stick together to form molecules. These regions of space are called molecular clouds, and they are the classic birthplaces of stars. Even though the density is higher than pure interstellar space, it’s still an almost perfect vacuum with a density ten to the power fifteen or a thousand trillion times less than the air you are breathing.