1、下句中,究竟是start还是star；2、as is the number of stars .is so vast

你这个句子有两个错误:1.start 应为stars;
2.and as is the number of stars 中的 is 多余了,应去掉.
因此,正确的句子应为:
But other suns,stars as the astronomers call them,are bound to have planets like our own,
and as the number of stars in the universe is so vast,this possibility becomes virtual certainty.
其句型结构如下:句子主要部分是一个并列句:
But other suns are bound to have planets like our own,and this possibility becomes virtual certainty.
同位语:stars as the astronomers call them,作other suns的同位语,其中:
as the astronomers call them 是定语从句修饰stars .
原因状语从句:as the number of stars in the universe is so vast,是and后分句中的原因状语从句,
说明this possibility becomes virtual certainty的原因.
其中,the number of stars in the universe是原因状语从句的主语,因此谓动用单三形式is.
句意:但是,其他的太阳,即天文学家所说的恒星,肯定会有像我们地球一样的行星.
因为宇宙中恒星的数目极其庞大,所以存在着产生生命星球的这种可能性是肯定无疑的.