Astronomers made observations of cepheid variable stars [ pictured above ] ? pulsating stars used to measure vast distances ? in the virgo and other clusters to establish the expansion rate and the universe ' s age 天文学家用哈勃观测到仙女星座和其它星群中的造父变星(如图) ，以确定宇宙的膨胀速度和宇宙的年龄。
The longer the period , the brighter the star is . thus , by measuring the period of a cepheid variable , we know its absolute magnitude , hence , we can tell how far it is away by comparing with its apparent magnitude 所以，只要知道某一造父变星的周期，我们便可得知它的绝对星等，只要和它的视星等作比较，便可以求得它和我们的距离。
This usually means that we find out the relation using stars near us , then apply the relation to greater distances . in general , we use accurately determined distance indicators for small distance to calibrate distance indicators for greater distances 事实上，科学家首先找一些离我们较近的造父变星，找出它们的光变周期与光度的关系，然后假设这个关系对遥远的同类变星依然适用。
We have to use some other methods to measure the luminosity or equivalently the absolute magnitude of the cepheid variable . since we can readily measure the apparent magnitude of a cepheid variable , to measure the luminosity means measuring the distance . we use some distance measurement methods , for example , parallax , to determine the relation 要弄清这个关系，我们要知道不同造父变星的光度和光变周期数据，量度光变周期当然不是问题，但要得知一颗恒星的光度，我们首先要得知它的距离，这时便须借助好像视差法等其他量度天体距离的方法。
Except those celestial objects of several hundred light years , astronomers rely primarily on cepheid variables for measuring distances of celestial objects . using ground telescopes to gauge cepheid variables enable us to directly measure celestial objects within 20 to 30 millions light years . for more distant stars , other methods have to be used 除了几百光年内的星体外，天文学家很主要倚靠测量造父变星来计算天体的距离，利用地面望远镜测量造父变星，可直接给出约二三千万光年内星体的距离，而对于更遥远的星体，天文学家便要借助其他方法，但这些方法还是需要利用造父变星来作校正。
