蓝掉队星（Blue stragglers，BSS），是疏散星团或球状星团中与其他成员有相同的光度，但表面温度较高的蓝色恒星。因此，在星团的赫罗图中，它们的位置有别于星团的其他成员。蓝掉队星看似违背了现行恒星演化的标准理论，因为在同时诞生的恒星在赫罗图上应该很明确的位在同一条曲在线，而由此一位置可以测量出它们原始的质量。因为蓝掉队星偏离了这条曲线，显示它们在恒星演化中有不同的经历。

产生的原因还不是很清楚，但主流的假说是它们原本是双星，经由合并才成为蓝掉队星。两颗恒星的合并只会创造出一颗质量更大的恒星，这颗恒星的表面温度会比同年龄的恒星更热和更亮。如果这种理论是正确的，则蓝掉队星就不会成为恒星演化理论上的问题，合并后的恒星在核心会有更多的氢，使它的行为像较年轻的恒星。有证据支持这种看法，在星团内蓝掉队星的数量明显的与恒星的密集度有关，越密集处的数量越多，特别是在球状星团的核心区域。因为在单位体积内的恒星数目越多，碰撞和密接的机会越高，而星团内确实比其他区域更容易发生。

A blue straggler is a main-sequence star in an open or globular clusters that is more luminous and bluer than stars at the main-sequence turn-off point for the cluster. Blue stragglers were first discovered by Allan Sandage in 1953 while performing photometry of the stars in the globular cluster M3. Standard theories of stellar evolution hold that the position of a star on the Hertzsprung–Russell diagram should be determined almost entirely by the initial mass of the star and its age. In a cluster, stars all formed at approximately the same time, and thus in an H–R diagram for a cluster, all stars should lie along a clearly defined curve set by the age of the cluster, with the positions of individual stars on that curve determined solely by their initial mass. With masses two to three times that of the rest of the main-sequence cluster stars, blue stragglers seem to be exceptions to this rule. The resolution of this problem is likely related to interactions between two or more stars in the dense confines of the clusters in which blue stragglers are found.