Although outer space may seem empty, it is actually full of thinly spread gas and dust called the interstellar medium. Higher density regions form clouds, or nebulas, where star formation takes place. The gas is mostly molecules of hydrogen, so these nebulae are called molecular clouds. The largest molecular clouds have densities of 100 particles per cm3, diameters of 100 light-years (9.5×1014 km), and masses up to 6 million solar masses, and an average interior temperature of 10 K. This is cold and dense compared to elsewhere in space, but would be considered a vacuum on Earth. With low temperatures and high densities, gravity can overcome thermal pressure and start the gravitational collapse that will form a star. 