一条线索来自于主盘内恒星的金属丰度。
One clue comes from the metallicity of the disc's constituent stars.
在这个模型下，一大团气体坍缩并在主盘中心形成大量恒星，它们在那里迅速增加着金属丰度。
In this model, a mass of gas collapses and creates lots of stars at the disc's centre, where the stars quickly boost the metallicity.
因为恒星聚集在星系的中心，其附近的金属丰度最高并沿边缘方向逐渐降低。
Because stars congregate at a galaxy's centre, the metallicity in most nearby spirals is greatest there and drops toward the edge.
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