NGC 3972 and NGC 1015

These images from the NASA/ESA Hubble Space Telescope showcase 2 of the 19 galaxies analysed in a project to improve the precision of the Universe's expansion rate, a value known as the Hubble constant.

The colour-composite images show NGC 3972 (left) and NGC 1015 (right), located 65 million light-years and 118 million light-years, respectively, from Earth. The yellow circles in each galaxy represent the locations of pulsating stars called Cepheid variables. These stars blink at a rate matched closely by their intrinsic brightness, making them ideal cosmic lighthouses for measuring accurate distances to relatively nearby galaxies.

Another reliable milepost marker is a special class of exploding star called a Type Ia supernova. All of these supernovae peak at the same brightness and are brilliant enough to be seen over relatively longer distances. The small cross-shaped feature in each galaxy denotes the location of a Type Ia supernova.

Astronomers search for Cepheid variables in nearby galaxies containing a Type Ia supernova so they can compare the true brightness of both types of stars. That brightness information is used to calibrate the luminosity of Type Ia supernovae in far-flung galaxies so that astronomers can calculate the galaxies' distances from Earth. Once astronomers know accurate distances to galaxies near and far, they can determine and refine the universe's expansion rate.

The observations for NGC 3972 were taken in 2015; for NGC 1015 in 2013. Both galaxies were observed by Hubble's Wide Field Camera 3.

Links:

• NASA press release
• Three steps to the Hubble constant