*Direct subsidiary to [[Protostars (Astro)]]!* # Hydrogen Regions Particularly dense concentrations of this loose gas and dust are known as **stellar nurseries**. These 'nurseries' typically consist of ionised hydrogen, or "**H II**", therefore giving them the name of **H II region.** Note that this means we can identify these regions with the generic 656.3nm H-$\alpha$ (H-alpha) line, which correspond to the photons (massless particles of light) from singly-ionised hydrogen. Neutral hydrogen regions - known as **H I regions** - also exist, though detecting them requires the use of a 21 *centimetre* wide spectral line - way past the visible range of our eyes! As a result, these regions are practically invisible to us.