▁ 1 9 9 1 ▁or ▁G U ▁Mus ca e . ▁These ▁two ▁orbit ing ▁X - ray ▁teles cop es ▁discovered ▁the ▁system ▁when ▁the ▁system ▁produced ▁an ▁out bur st ▁of ▁X - ray s ▁on ▁January ▁ 9 , ▁ 1 9 9 1 . ▁ ▁Black ▁hole ▁system ▁ ▁It ▁is ▁one ▁of ▁several ▁likely ▁black ▁hole ▁systems ▁that ▁are ▁class ified ▁as ▁X - ray ▁nov ae . ▁Such ▁a ▁nova ▁period ically ▁produces ▁bright ▁out bur st s ▁of ▁X - ray s , ▁along ▁with ▁visible ▁light ▁and ▁other ▁forms ▁of ▁energy . ▁ ▁In ▁such ▁a ▁system , ▁a ▁black ▁hole ▁pull s ▁gas ▁from ▁the ▁surface ▁of ▁a ▁companion ▁star . ▁The ▁gas ▁forms ▁a ▁thin ▁disk ▁around ▁the ▁black ▁hole , ▁known ▁as ▁an ▁acc ret ion ▁disk . ▁In ▁an ▁X - ray ▁nova , ▁the ▁flow ▁of ▁gas ▁is ▁fairly ▁thin ▁and ▁slow , ▁so ▁the ▁acc ret ion ▁disk ▁remains ▁relatively ▁cool , ▁and ▁little ▁gas ▁falls ▁into ▁the ▁black ▁hole . ▁ ▁In ▁the ▁case ▁of ▁G U ▁Mus ca e , ▁the ▁black ▁hole ▁is ▁about ▁seven ▁times ▁as ▁massive ▁as ▁the ▁Sun , ▁while ▁the ▁companion ▁is ▁three - quarters ▁as ▁massive ▁as ▁the ▁Sun . ▁The ▁companion ▁is ▁also ▁cool er ▁than ▁the ▁Sun , ▁so ▁its ▁surface ▁is ▁red der , ▁and ▁the ▁star ' s ▁total ▁lum inos ity ▁is ▁only ▁one - third ▁that ▁of ▁the ▁Sun ' s . ▁Its ▁outer