Glossary term: 亚矮星
Description: 亚矮星是一类比主序列中相同光谱型恒星更暗，但比相同温度的白矮星更亮的恒星。
亚矮星通常分为两类：冷亚矮星和热亚矮星。
大多数亚矮星都属于冷亚矮星，在赫罗图上比主序低一到两个星等，这是因为它们含有的比氦（天文学中称为金属）重的元素非常少。这改变了光和热在它们内部和大气中的传播方式，使它们变得更小、更热、更暗。
在太阳系附近的银河系区域，亚矮星属于银河晕或银河盘的较老部分，形成于银河系的早期阶段。这是因为每一代恒星都会通过核聚变产生更多的金属，并将其中的一部分返回星际介质--恒星形成的气体库。因此，年轻恒星比老恒星拥有更多的金属。因此，金属含量较少的亚矮星通常是较老的恒星。
还有少量光谱型为 O 和 B 的热亚矮星（称为 sdO 星和 sdB 星）。天文学家并不清楚这些天体是如何形成的，但一个可能的原因是巨星由于某种原因失去了外层。外层脱落的原因尚不确定，但其中一个可能的原因是巨星的外层被双星伴星吸积而剥离。
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Term and definition status: The original definition of this term in English have been approved by a research astronomer and a teacher The translation of this term and its definition is still awaiting approval
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