in a certain place I found certain data.those are1) the wave length of terrestial alpha line of hydrogen is 656 Nm2)the wave length of alpha line of hydrogen in some distant galaxy is 706 NmMy question is why the difference occurs?

hi,

this difference occurs due to the Doppler,s effect.

the increase in wavelength is also called , red shift because the spectrum seems to be shifted towards red . this happens due to the relative motion between source & receiver. if source  approaches towards the receiver , the observed frequency will increase & if it recedes then the observed freq. will decrease (& so, wavelength would increase ). in this case the galaxy must going away from us  .