亚巨星（subgiant）光谱分类中光度级按照由强到弱顺序分在第四级的恒星，用罗马数字Ⅳ表示。
A66是位于球状星团m 4红巨星支最底部(近亚巨星支顶部)的一颗红星。
A66 is a red star located at the lowest end of the red giant branch (near the top of the sub-giant branch) of the globular cluster M4.
它已经变成了天文学家所谓的‘亚巨星’，也就是说成为已经耗尽了核心中氢燃料的恒星，并且正在变成红巨星。
It has become what astronomers call a subgiant, or a star that has exhausted all of its hydrogen fuel in the core and is on its way to becoming a red giant.
只有2%的红巨星会如此演变，成为B型亚矮星（“B-type subdwarfs”）。
Only about 2% of red giants follow this fate, to become stars known as “B-type subdwarfs”.
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