▁very ▁large ▁peculiar ▁veloc ities ▁of ▁the ▁galaxies ▁in ▁the ▁cluster . ▁ ▁Binary ▁stars ▁ ▁Just ▁as ▁plan ets ▁can ▁be ▁grav itation ally ▁bound ▁to ▁stars , ▁pairs ▁of ▁stars ▁can ▁orbit ▁each ▁other . ▁Some ▁binary ▁stars ▁are ▁visual ▁bin aries , ▁meaning ▁they ▁can ▁be ▁observed ▁orbit ing ▁each ▁other ▁through ▁a ▁teles cope . ▁ ▁Some ▁binary ▁stars , ▁however , ▁are ▁too ▁close ▁together ▁to ▁be ▁resolved . ▁These ▁two ▁stars , ▁when ▁viewed ▁through ▁a ▁spect rom eter , ▁will ▁show ▁a ▁composite ▁spectrum : ▁the ▁spectrum ▁of ▁each ▁star ▁will ▁be ▁added ▁together . ▁This ▁composite ▁spectrum ▁becomes ▁easier ▁to ▁detect ▁when ▁the ▁stars ▁are ▁of ▁similar ▁lum inos ity ▁and ▁of ▁different ▁spectral ▁class . ▁ ▁Spect ro scop ic ▁bin aries ▁can ▁be ▁also ▁detected ▁due ▁to ▁their ▁radial ▁velocity ; ▁as ▁they ▁orbit ▁around ▁each ▁other ▁one ▁star ▁may ▁be ▁moving ▁towards ▁the ▁Earth ▁whilst ▁the ▁other ▁moves ▁away , ▁causing ▁a ▁Do pp ler ▁shift ▁in ▁the ▁composite ▁spectrum . ▁The ▁orb ital ▁plane ▁of ▁the ▁system ▁determ ines ▁the ▁magnitude ▁of ▁the ▁observed ▁shift : ▁if ▁the ▁observer ▁is ▁looking ▁per pend icular ▁to ▁the ▁orb ital ▁plane ▁there ▁will ▁be ▁no ▁observed ▁radial ▁velocity . ▁ ▁For ▁example , ▁if ▁you ▁look ▁at ▁a ▁car ousel ▁from ▁the ▁side , ▁you ▁will ▁see ▁the ▁animals ▁moving ▁toward ▁and ▁away ▁from ▁you , ▁whereas ▁if ▁you ▁look ▁from ▁directly ▁above ▁they ▁will ▁only ▁be ▁moving ▁in ▁the ▁horizontal ▁plane .