Scanning the skies for galaxies Canadian astronomer Paul Hickson and colleagues identified some 100 compact groups of galaxies, now appropriately called Hickson Compact Groups (HCGs). With only a few member galaxies per group, HCGs...

How do black holes like this form? The two black holes that spiraled together to produce the gravitational wave event GW190521 were not only the most massive black holes ever seen by LIGO and VIRGO so far, their masses -- 66 and 85 solar masses -- were unprecedented and unexpected.

The many bright, point-like sources in this Chandra Observatory x-ray image lie within NGC 4697, an elliptical galaxy some 40 million light-years away towards Virgo. Like other normal elliptical galaxies, NGC 4697 is a spherical ensemble of mainly older, fainter, low mass stars, with little star forming gas and dust compared to spiral galaxies.