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 ,然后利用開普勒定律,計算出中心物體的質量下限。假若質量超過三個太陽質量,而且它非常細小又漆黑一片,我們便很有理由相信這是黑洞。