Neutral hydrogen, known as HI, can be detected because of its emissions at the radio wavelength of 21 centimeters. This 21 cm emission
is due to the single electron of hydrogen changing its quantum spin direction. Otherwise, there would be virtually no way to detect neutral
interstellar hydrogen.
If the hydrogen is hot enough, the electron is lost and we observe ionized hydrogen in HII regions. One example of a nearby HII region is
the Orion Nebula.
But there is definitely more than just hydrogen between the stars. Other atoms as well come together to form molecules.
HI and HII regions
Looking for a particular star? Find it here!
