The bright globular cluster Terzan 12, a huge, tightly knit collection of stars, fills the frame of this image from the NASA/ESA Hubble Space Telescope.
This globular cluster’s location deep in the Milky Way in the constellation Sagittarius means that it is surrounded by gas and dust that absorb and alter the starlight emitted by Terzan 12.
This star-studded star count stems from a series of observations aimed at systematically exploring the relatively few globular clusters at the center of our galaxy, such as Terzan 12, located about 15,000 light-years from Earth.
Globular star clusters are not uncommon in the Milky Way. About 150 are known, most of them in its outer halo, and Hubble has revolutionized their study since its launch in 1990. However, studying clusters like Terzan 12, which are heavily obscured by interstellar dust, is complicated by the resulting redness of the light, reports ESA.
When starlight passes through an interstellar cloud, it can be absorbed and scattered by dust particles. The intensity of this scattering depends on the wavelength of the light, with the shorter wavelengths being scattered and absorbed the most. This means that the blue wavelengths of starlight are less likely to penetrate a cloud, making stars in the background appear redder than they actually are.
Astronomers (appropriately) call the color change caused by the scattering and absorption of starlight redness, and it is responsible for the vibrant color palette in this image. Relatively clear stars shine brightly in white and blue, while creeping tendrils of gas and dust cover other large parts of Terzan 12, giving the stars an ominous red hue. The more dust there is along our line of sight to the star cluster, the redder the starlight becomes.
A similar effect is responsible for the spectacular pink hues of sunsets here on Earth. The atmosphere prefers to scatter light of shorter wavelengths, which is why the sky appears blue. As the sun sinks into the sky, sunlight has to pass through more parts of the atmosphere, scattering more and more blue light and causing sunlight to take on a distinctive red-gold hue.
Some of the stars in the photo have completely different colors than their close neighbors. The brightest red stars are swollen, aging giants many times larger than our Sun. They are located between the Earth and the star cluster. Only a few can actually be members of the group. The hot, very bright blue stars are also found along the line of sight and not within the star cluster, which only contains aging stars.
The redness of stars often poses problems for astronomers, but the scientists behind this observation of Terzan 12 were able to avoid the effect of gas and dust by comparing the new observations with the clear view of the Advanced Survey Camera and Widefield 3 with existing images . Their observations should shed light on the relationship between age and composition in the Milky Way’s innermost globular clusters, comparable to astronomers’ understanding of star clusters spread throughout the rest of our galaxy.
The Terzan clusters suffer from a kind of astronomical identity crisis: in reality, only 11 clusters have been discovered by Turkish-Armenian astronomer Agop Terzan. The confusion is due to a mistake made by Terzan in 1971, when he rediscovered Terzan 5 (a cluster he had already discovered and reported on in 1968) and named it Terzan 11. Terzan tried to correct his mistake, but the confusion caused remained. Since then, in scientific studies, astronomers have finally agreed on the strange convention that there is no Terzan 11.
Even in our solar system, it is surprisingly common for astronomical objects to be lost and then rediscovered. Smaller planets such as asteroids and dwarf planets are often discovered and then overlooked because their orbits cannot be determined from a small handful of observations, the ESA statement added.