# How do we mea­sure dis­tances to space ob­jects?

All About Space - - Harpercollins -

there are many meth­ods to do this. let me cite par­al­lax, be­cause we use it in our Gaia mis­sion. For nearby

– and not-so-nearby – stars we can ex­ploit the fact that their po­si­tion in the sky de­pends on our po­si­tion as an ob­server. Much like near ob­jects ap­pear to move in front of back­ground ob­jects when you as an ob­server move, say, driv­ing by a bunch of peo­ple in front of a land­scape.

in as­trom­e­try we can use the fact that we move with the earth around the Sun, so our ob­ser­va­tion po­si­tion rel­a­tive to the Sun changes by 2aU (ap­prox­i­mately 300 mil­lion kilo­me­tres or 185 mil­lion miles) in six months. a star that moves by one sec­ond of arc (equal to 1/3600 of a de­gree) due to the change in our po­si­tion by 1aU (the earth-Sun dis­tance) has a dis­tance of 1 par­sec (par­al­lax sec­ond). one par­sec is equal to 3.26156 light years, which is the dis­tance light trav­els in 3.26156 years, and is equal to 30.9 bil­lion kilo­me­tres (19.2 bil­lion miles). With the Gaia space­craft, for ex­am­ple, we can ba­si­cally de­ter­mine the dis­tance of half of the Milky Way stars us­ing this method.

For more dis­tant ob­jects, like other gal­ax­ies, we use ‘stan­dard can­dles’. those are vari­able stars, the ab­so­lute bright­ness of which is cor­re­lated with the pe­riod of the vari­a­tion. Know­ing the vari­a­tion, we can de­rive the ab­so­lute bright­ness. Know­ing the ab­so­lute bright­ness and the ap­par­ent, mea­sured bright­ness we can de­ter­mine the dis­tance. even fur­ther ob­jects, like quasars, are mea­sured by look­ing at the red­shift due to the cos­mic ex­pan­sion. the red­shift is di­rectly cor­re­lated to the dis­tance, as

dis­cov­ered by ed­win hub­ble.

Dr Markus Land­graf is a se­nior mis­sion an­a­lyst at ESA