''=-4πρGr/3
t=(4πρG/3)^-0.5∫dr/（r0^2-r^2）^0.5 ,(0,r0)
=(3π^0.5/4) ρG^-0.5
这是dimensional analysis（量纲分析）的部分,确定了数量级,前面是系数
数值结果是2.1e+13s = 6.7e+5 yea
美国物理奥林匹克竞赛题A star is born:According to one of the cosmologic
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