This colorful image of the globular star cluster Terzan 12 is a spectacular example of how dust in space affects starlight coming from background objects.
A globular star cluster is a conglomeration of stars, arranged in a spheroidal shape. Stars in globular clusters are bound together by gravity, with a higher concentration of stars towards the center. The Milky Way has about 150 ancient globular clusters at its outskirts. These clusters orbit around the galactic center, but far above and below the pancake-flat plane of our galaxy, like bees buzzing around a hive.
The location of this globular cluster, deep in the Milky Way in the constellation Sagittarius, means that it is shrouded in gas and dust which absorb and alter the starlight emanating from Terzan 12. The cluster is about 15,000 light-years from Earth. This location leaves a lot of room for intervening interstellar dust particles between us and the cluster to scatter blue light, causing only the redder wavelengths to come through to Earth. The interstellar dust clouds are mottled so that different parts of the cluster look redder than other parts along our line of sight.
The brightest red stars in the photo are bloated, aging giants, many times larger than our Sun. They lie between Earth and the cluster. Only a few may actually be members of the cluster. The very brightest hot, blue stars are also along the line of sight and not inside the cluster, which only contains aging stars.
Terzan 12 is one of 11 globular clusters discovered by the Turkish-Armenian astronomer Agop Terzan approximately a half-century ago. With its sharp vision, Hubble has revolutionized the study of globular clusters ever since its launch in 1990. Hubble observations have shed light on the relation between age and composition in the Milky Way galaxy's innermost globular clusters.