Stars in the Andromeda Galaxy's giant stellar stream
This image shows NASA/ESA Hubble Space Telescope images of a small part of the giant stellar stream of the Andromeda Galaxy. The stream is a long structure thought to be the remains of a companion galaxy torn apart by the Andromeda Galaxy's gravity and engulfed in it.
Hubble's position above the distorting effect of the atmosphere, combined with the galaxy's relative proximity, means that this image can be resolved into individual stars, rather than the cloudy white wisps usually seen in observations of galaxies.
These observations were made in order to observe a wide variety of stars in Andromeda, ranging from faint main sequence stars like our own Sun, to the much brighter RR Lyrae stars, which are a type of variable star. With these measurements, astronomers can determine the chemistry and ages of the stars in each part of the Andromeda Galaxy.