ula e ▁are ▁formed ▁of ▁neutral ▁hydro gen . ▁In ▁the ▁ground ▁state ▁neutral ▁hydro gen ▁has ▁two ▁possible ▁spin ▁states : ▁the ▁electron ▁has ▁either ▁the ▁same ▁spin ▁or ▁the ▁opposite ▁spin ▁of ▁the ▁pro ton . ▁When ▁the ▁atom ▁trans itions ▁between ▁these ▁two ▁states , ▁it ▁releases ▁an ▁emission ▁or ▁absor ption ▁line ▁of ▁ 2 1   cm . ▁This ▁line ▁is ▁within ▁the ▁radio ▁range ▁and ▁allows ▁for ▁very ▁precise ▁measurements : ▁Vel ocity ▁of ▁the ▁cloud ▁can ▁be ▁measured ▁via ▁Do pp ler ▁shift ▁The ▁intensity ▁of ▁the ▁ 2 1   cm ▁line ▁gives ▁the ▁density ▁and ▁number ▁of ▁atoms ▁in ▁the ▁cloud ▁The ▁temperature ▁of ▁the ▁cloud ▁can ▁be ▁calculated ▁Using ▁this ▁information ▁the ▁shape ▁of ▁the ▁Mil ky ▁Way ▁has ▁been ▁determined ▁to ▁be ▁a ▁spir al ▁galax y , ▁though ▁the ▁exact ▁number ▁and ▁position ▁of ▁the ▁spir al ▁arms ▁is ▁the ▁subject ▁of ▁on going ▁research . ▁ ▁Complex ▁mole cules ▁ ▁D ust ▁and ▁mole cules ▁in ▁the ▁inter stell ar ▁medium ▁not ▁only ▁obsc ures ▁phot ometry , ▁but ▁also ▁causes ▁absor ption ▁lines ▁in ▁spect ro sc opy . ▁Their ▁spectral ▁features ▁are ▁generated ▁by ▁trans itions ▁of ▁component ▁electrons ▁between ▁different ▁energy ▁levels , ▁or ▁by ▁rot ational ▁or ▁v ibr ational ▁spect ra . ▁D ete ction ▁usually ▁occurs ▁in ▁radio , ▁mic row ave , ▁or ▁inf ra red ▁port ions ▁of ▁the ▁spectrum . ▁The ▁chemical ▁re actions ▁that ▁form ▁these ▁mole cules ▁can ▁happen