A cluster of stars smaller than a constellation 星群,星座比星座小的一簇星星
If an object has a lower mass limit over 3 solar masses , occupying a small space not a cluster of stars and not emitting much light , then we can say that it is highly likely to be a black hole ,然后利用开普勒定律,计算出中心物体的质量下限。假若质量超过三个太阳质量,而且它非常细小又漆黑一片,我们便很有理由相信这是黑洞。