One of the key questions in modern cosmology is determining the precise value of the Hubble parameter (H₀), which describes the linear relationship between an object's recession velocity and its distance.

Recent observations show two objects that are so well aligned with Earth that we see incredible ring shapes.

The Hubble Space Telescope has witnessed an exceedingly rare phenomenon over the last 20 years: the unprecedently rapid dimming of a planetary nebula. The cover image shows the evolution between 1996 and 2016.