This is a Hubble Space Telescope photo of the spiral galaxy NGC 3021. This was one of several hosts of recent Type Ia supernovae observed by astronomers to refine the measure of the universe's expansion rate, called the Hubble constant. Hubble made precise measurements of Cepheid variable stars in the galaxy, highlighted by green circles in the four inset boxes. These stars pulsate at a rate that is matched closely to their intrinsic brightness. This makes them ideal for measuring intergalactic distances. The Cepheids are used to calibrate an even brighter milepost marker that can be used over greater distances, a Type Ia supernova.
NASA, ESA, and A. Riess (STScI/JHU)