One way astronomers map star-forming regions is by looking for a specific signature of neutral hydrogen; this tells scientists where these hydrogen clouds are. An electron can be in two energy states, and as it transitions from the higher-energy state to the lower-energy one, it emits radiation with a wavelength of 21 centimeters. Due to the universe’s expansion, light from distant objects has been stretched into lower-energy, and longer wavelength, radiation. This 21cm line thus has been redshifted into the radio regime.Astronomy magazine subscribers can read the full article for free. Just make sure you're registered with the website.