▁interaction , ▁called ▁charge ▁exchange , ▁results ▁in ▁X - ray s ▁from ▁most ▁com ets ▁when ▁they ▁pass ▁within ▁about ▁three ▁times ▁Earth ' s ▁distance ▁from ▁the ▁Sun . ▁Because ▁L ul in ▁is ▁so ▁active , ▁its ▁atomic ▁cloud ▁is ▁especially ▁dense . ▁As ▁a ▁result , ▁the ▁X - ray - em itting ▁region ▁extends ▁far ▁sun ward ▁of ▁the ▁com et . ▁ ▁See ▁also ▁▁ ▁Bal lo ons ▁for ▁X - ray ▁astronom y ▁ ▁C rab ▁( unit ) ▁ ▁G amma - ray ▁astronom y ▁ ▁History ▁of ▁X - ray ▁astronom y ▁ ▁IR AS ▁ 1 3 2 2 4 - 3 8 0 9 ▁ ▁List ▁of ▁X - ray ▁space ▁teles cop es ▁ ▁Sol ar ▁X - ray ▁astronom y ▁ ▁St ell ar ▁X - ray ▁astronom y ▁ ▁Ult rav io let ▁astronom y ▁ ▁X - ray ▁teles cope ▁ ▁References ▁ ▁S ources ▁The ▁content ▁of ▁this ▁article ▁was ▁adapted ▁and ▁expanded ▁from ▁http :// im agine . gs fc . n asa . gov / ▁( Public ▁Domain ) ▁ ▁External ▁links ▁▁ ▁How ▁Many ▁Kn own ▁X - R ay ▁( and ▁Other ) ▁S ources ▁Are ▁There ? ▁ ▁Is ▁My ▁F avor ite ▁Object ▁an ▁X - ray , ▁G amma - R ay , ▁or ▁EU V ▁Source ? ▁ ▁X - ray ▁all - sky ▁survey ▁on ▁W IK IS K Y ▁ ▁Audio ▁– ▁C ain / G ay