关于恒星光谱的一些基础知识

天文上,恒星的分类主要是根据其表面的温度来划分,由于大多数恒星距离我们遥远,所以只能借助光谱中的吸收谱线来分类。因为温度在一定范围内,会表现出一种特定的吸收谱线。所以我们检测遥远恒星发出的光,基于光谱知识,就可以知道其大致的温度,从而给恒星分类。

 

恒星光谱型:O到M型亮星的光谱照片。图:[北京同好会]>[观测]>[光谱观测]>[恒星光谱型]

恒星光谱概述


在天文学中,恒星分类是根据恒星的光谱特征来对恒星进行分类的。来自恒星的电磁辐射需要通过用棱镜或衍射光栅分成光谱来分析,该光谱表现出散布具有光谱线的彩虹色。每一条线表示一种特定的化学元素或分子,线强度表示为该元素的丰度。不同光谱线的强度主要随光球的温度而变化,不过在某些情况下会存在元素丰度的差异。恒星的光谱类是一个简短的代码,主要解释了电离状态,以及给出光球温度的客观测量。

 

目前大多数恒星都按照摩根-基南(MK)系统来分类,这里会使用字母O、B、A、F、G、K和M,从最热(O型)到最冷(M型)的顺序分类。然后每个字母类又要用数字来细分,其中最热的为0,最冷的为9(例如A8、A9、F0和F1形成从热到冷的序列)。该序列已扩展到其他恒星和类星体的分类中,但这些分类并不适合于经典的系统,如白矮星D类或者碳星的S和C类。

 

在MK系统中,会用罗马数字将光度等级添加到光谱类中。这是基于恒星光谱中某些吸收谱线的宽度而定的,这些吸收谱线会随着大气的密度变化而变化,从而可以将巨恒星与矮恒星区分开来。亮度(光度)级别为0或Ia +的用于超巨星,I类的用于超巨星,II类的用于亮巨星,III类的用于普通巨星,IV类的用于次巨星,V类的用于主序星(矮星),SD(或VI)类的用于次矮星,以及D(或VII)类的用于白矮星。太阳全(完整的)光谱等级为G2V,这表明了主序星的温度约为5800 K。


图片:Spacepotato

 

常规颜色描述


传统的颜色描述只考虑了恒星光谱的峰值。然而,实际上,恒星在光谱中的各个部分都有辐射。因为所有的光谱颜色组合起来都会呈现出白色,所以人眼观察到的实际表观颜色远比传统颜色描述的颜色要轻得多。这种“亮度”的特性表明,可能会在光谱颜色的简化分配中产生误导性。在暗淡光线中排除色彩对比幻觉外,就没有绿色、靛蓝或紫色的恒星了。红矮星是深橙色,而棕矮星实际上并不呈现棕色。但据推测,如果近距离观测,它们会呈现暗灰色。

 

恒星历史分类


在哈佛的分类中,字母排列古怪的原因是缺乏知识,它是从早期的西奇教派演变而来的,随着人类知识系统的提高,逐渐被修改了。

在19世纪60年代和19世纪70年代,开创性的恒星光谱学家安吉洛·西奇为了对观测到的光谱进行分类而创建了西奇分类。到1866年,他已经发展了三类恒星光谱。在19世纪90年代后期,这种分类开始被哈佛分类所取代。

 

恒星现代分类


现代分类系统被称为摩根-基南(MK)分类。每个恒星都被分配了一个以前的哈佛光谱分类的光谱等级和使用罗马数字的光度等级,如下所述,形成恒星的光谱类型。

其他现代恒星分类系统,如UBV系统,是基于颜色指数分类——三个或更多颜色量级的测量差异。这些数字被赋予诸如“U-V”或“B-V”之类的标签,其表示由两个标准滤光器(例如紫外线,蓝色和目视)通过的颜色。

 

恒星死亡残骸


恒星残骸是恒星死亡后有关的物质。该类型中包括白矮星,并且从D类完全不同的分类方案中可以看出来,非恒星的物体难以归纳于MK系统。

MK系统所基于的赫罗图本质上是可以观测的,因此这些残留物不能轻易地绘制在图上,或者根本没有位置可以放置。较为古老的残留物-中子星,因为相对较小而寒冷,所以可以放在图的右边。行星状星云是动态的,并且随着古老的恒星向白矮星分支的过渡转变,随后其亮度会迅速减弱。如图所示,行星状星云将被绘制在图的右上四分之一处的右边。黑洞不会发射它自己的可见光,因此不会出现在图表上。


赫罗图,图:Rursus

 

参考资料

1.  WJ百科-英文版(Stellar classification)

2.  Stellar Classification

 

文章作者:零度星系(天文在线)

审核人员:北极光

最终审核:零度星系

编辑用时2018年09月28日-2018年09月29日

审核用时: 半个小时

最后更新:2018年11月25日星期日


注意:所有信息数据庞大,难免出现错误,还请指出错误所在好加以改之。




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