Hubble Observations Offer Insights into Galactic History Using Globular Clusters

According to NASA News Releases, the stellar grouping known as Messier 30 (M30) is named after the French astronomer Charles Messier, who cataloged M30 and other diffuse objects that might have been mistaken for comets during Messier's early telescopic observations. M30 is situated approximately 27,500 light-years away within the constellation Capricornus.

M30 is classified as a globular cluster, which is described as a roughly spherical aggregation of stars containing anywhere from tens of thousands to millions of stars. These clusters are generally found on the peripheries of galaxies, orbiting within the diffuse region known as the halo, similar to moths circling a lightbulb. The Milky Way galaxy possesses at least 150 globular clusters, with the closest one being about 6,000 light-years away. Astronomers estimate that these clusters are remarkably old, with ages ranging from 11.5 to 13.5 billion years, suggesting some globular clusters are nearly as old as the universe itself.

Globular clusters have historically been significant in the study of our galaxy. Over a century ago, astronomer Harlow Shapley utilized observations of a specific type of variable star to determine the locations and distances to numerous globular clusters within the galaxy. Shapley's research indicated that the Sun is not located at the center of the galaxy, and that the galactic center is positioned toward the constellation Sagittarius. However, the report notes that Shapley's distance measurements were inaccurate, leading him to conclude that the galaxy was about twice its actual size.

Present-day astronomers continue to investigate the galaxy by observing globular clusters, as their ancient nature allows them to offer clues regarding the galaxy's early development. Globular clusters that originated within the galaxy can function as time capsules, preserving the original chemical composition of the young galaxy.

Nevertheless, it is possible that not all globular clusters currently orbiting within the Milky Way formed there. Researchers have found that M30 and numerous other globular clusters orbit in a direction opposite to the spin of our galaxy. This finding suggests that these clusters might not have formed within our galaxy at all, and instead, the Milky Way galaxy may have incorporated them from a distant past.

Hubble has observed M30 on multiple occasions. A prior image of M30 incorporated data from Hubble’s Advanced Camera for Surveys (ACS) and was part of an observing program (#10775; PI: Sarajedini) that surveyed dozens of Milky Way globular clusters using red and near-infrared light, revealing stars down to 20% of the mass of the Sun. A more recent initiative, program #13297; PI: Piotto, revisited M30 and other globular clusters in the Milky Way, observing them with Hubble’s Wide Field Camera 3 and employing three filters that captured bluer light. This program gathered data essential for examining exotic objects such as “blue straggler” stars and for investigating the multiple stellar populations that can exist within a single globular cluster.

More recently, Hubble’s ACS has again viewed many of these globular clusters. These observations were part of program #16298, led by PI: Libralato, which examined the movement of different stellar populations in globular clusters over a span of 15 years since Hubble's initial observations—a capability enabled by Hubble's extended operational life and its capacity to resolve extremely minute details in distant objects.

The image mentioned in the source material was constructed using data from all three observing programs and encompasses the broad spectrum of wavelengths visible to Hubble, including ultraviolet, visible, and near-infrared light, with each wavelength aiding the understanding of how these star groups formed and evolved.