kpc away ( Cox-578 ) The disk diameter is about 20 kpc. It is approaching us at 266 km/sec. Credit: T.A.Rector and B.A.Wolpa /NOAO/AURA/NSF . Hubble found that 34 stars were Cepheids by their periodic brightness variation ( No-510 ). Using the known luminosities of Cepheids and the inverse-square law ,