Gaze?upwards?on?a?dark?night,?and?you?will?see?the?'Milky?Way',?a?glowing?ribbon?made?up?of?many?individual?stars.?This?is?our?first?clue?to?the?fact?that?that?stars?are?not?uniformly?distributed?throughout?space,?but?clustered?together?to?form?galaxies.?
Galaxies?can?vary?in?size?between?a?hundred?thousand?and?three?thousand?billion?solar?masses,?and?can?be?broadly?split?into?different?classes,?depending?on?their?shape.?Our?own?Milky?Way?Galaxy?contains?about?two?hundred?billion?stars.?It?is?a?spiral?shape,?which?is?why,?when?viewed?from?our?position?about?two?thirds?of?the?way?along?one?of?the?arms,?it?appears?as?a?band?across?the?sky.?
Gazing?out?into?the?Universe?reveals?thousands?upon?thousands?of?galaxies.?These?galaxies?tend?to?be?grouped?together?in?clusters,?and?these?clusters?in?turn?are?grouped?into?superclusters.?It?appears?the?Universe?has?structure?on?many?different?scales.?
