A Sky Full Of Hydrogen . ... Credit Copyright: J. Dickey ( UMn ), F. Lockman ( NRAO ), SkyView Explanation: Interstellar space is filled with extremely tenuous clouds of gas which are mostly Hydrogen. The neutral hydrogen atom (HI in astronomer's shorthand) consists of 1 proton and 1 electron . The proton and electron spin like tops but can have only two orientations; spin axes parallel or anti-parallel. ... NASA Web Site Statements, Warnings, and Disclaimers . NASA Official: Jay Norris. ...