附近星系M33中的双星X-射线源X-7，已被证明对试图解释其演化历史的理论工作者是一个挑战。
此前，一直没有可能解释该系统相对于绕快速旋转的黑洞运转的富含氢的大伴星的尺寸来说的大质量组成成分和较小的轨道。现在，Valsecchi等人报告了对演化轨迹所做的模拟，其结果显示，如果M33 X-7在一个2.8-3.1天的轨道中以一个质量为85–99个太阳质量的主星和一个质量为28–32个太阳质量的伴星开始，那么其性质将可以得到一致的解释。
作为对这一新模型的支持，M33 X-7演化历史的不同阶段已在其他双星体系中被观测到。（生物谷Bioon.com）
生物谷推荐英文摘要：
Nature doi:10.1038/nature09463
Formation of the black-hole binary M33 X-7 through mass exchange in a tight massive system
Francesca Valsecchi,francesca@u.northwestern.eduEvert Glebbeek,Will M. Farr,Tassos Fragos,Bart Willems,Jerome A. Orosz,Jifeng Liu& Vassiliki Kalogera
The X-ray source M33 X-7 in the nearby galaxy Messier 33 is among the most massive X-ray binary stellar systems known, hosting a rapidly spinning, 15.65M⊙ black hole orbiting an underluminous, 70M⊙ main-sequence companion in a slightly eccentric 3.45-day orbit1, 2 (M⊙, solar mass). Although post-main-sequence mass transfer explains the masses and tight orbit3, it leaves unexplained the observed X-ray luminosity, the star’s underluminosity, the black hole’s spin and the orbital eccentricity. A common envelope phase1, or rotational mixing4, could explain the orbit, but the former would lead to a merger and the latter to an overluminous companion. A merger would also ensue if mass transfer to the black hole were invoked for its spin-up5. Here we report simulations of evolutionary tracks which reveal that if M33 X-7 started as a primary body of 85M⊙–99M⊙ and a secondary body of 28M⊙–32M⊙, in a 2.8–3.1-d orbit, its observed properties can be consistently explained. In this model, the main-sequence primary transfers part of its envelope to the secondary and loses the rest in a wind; it ends its life as a ~16M⊙ helium star with an iron–nickel core that collapses to a black hole (with or without an accompanying supernova). The release of binding energy, and possibly collapse asymmetries, ‘kick’ the nascent black hole into an eccentric orbit. Wind accretion explains the X-ray luminosity, and the black-hole spin can be natal.
