Three meth­ods cap­ture bi­nary stars

Bi­nary stars or­bit the same cen­tre of grav­ity, but of­ten only one star is vis­i­ble. So, as­tronomers use sev­eral meth­ods to iden­tify the cou­ples.

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1 Astro­met­ric bi­nary stars:

Some bi­nary star sys­tems are spot­ted due to ir­reg­u­lar­i­ties of the mo­tions of the bright­est star. One ex­am­ple is Sir­ius and Sir­ius B in the Ca­nis Ma­jor con­stel­la­tion.

2 Eclips­ing bi­nary stars:

In other cases, the or­bits of two stars block out each other's light. One ex­am­ple is Al­gol A in the Perseus con­stel­la­tion, whose bright­ness is clearly re­duced by two thirds ev­ery third day.

3 Spec­tro­scopic bi­nary stars:

Two very close stars must of­ten be spot­ted by means of a spec­tro­scope, that analy­ses the light from the stars. Cas­tor in the Gem­ini con­stel­la­tion was de­tected in this way.

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