▁par alla x ▁or ▁standard ▁cand les . ▁ ▁Inter stell ar ▁medium ▁ ▁The ▁inter stell ar ▁medium ▁is ▁matter ▁that ▁occup ies ▁the ▁space ▁between ▁star ▁systems ▁in ▁a ▁galax y . ▁ 9 9 % ▁of ▁this ▁matter ▁is ▁g ase ous ▁- ▁hydro gen , ▁hel ium , ▁and ▁smaller ▁quantities ▁of ▁other ▁ion ized ▁elements ▁such ▁as ▁o xygen . ▁The ▁other ▁ 1 % ▁is ▁dust ▁particles , ▁thought ▁to ▁be ▁mainly ▁graph ite , ▁sil ic ates , ▁and ▁ ices . ▁Cloud s ▁of ▁the ▁dust ▁and ▁gas ▁are ▁referred ▁to ▁as ▁ne b ula e . ▁ ▁There ▁are ▁three ▁main ▁types ▁of ▁ne b ula : ▁absor ption , ▁reflection , ▁and ▁emission ▁ne b ula e . ▁Abs or ption ▁( or ▁dark ) ▁ne b ula e ▁are ▁made ▁of ▁dust ▁and ▁gas ▁in ▁such ▁quantities ▁that ▁they ▁obsc ure ▁the ▁star light ▁behind ▁them , ▁making ▁phot ometry ▁difficult . ▁ ▁Ref lection ▁ne b ula e , ▁as ▁their ▁name ▁suggest , ▁reflect ▁the ▁light ▁of ▁nearby ▁stars . ▁Their ▁spect ra ▁are ▁the ▁same ▁as ▁the ▁stars ▁surrounding ▁them , ▁though ▁the ▁light ▁is ▁bl uer ; ▁shorter ▁w avel ength s ▁scatter ▁better ▁than ▁longer ▁w avel ength s . ▁Em ission ▁ne b ula e ▁emit ▁light ▁at ▁specific ▁w avel ength s ▁depending ▁on ▁their ▁chemical ▁composition . ▁ ▁G ase ous ▁emission ▁ne b ula e ▁In ▁the ▁early ▁years ▁of ▁astronom ical ▁spect