An educational article with references listed below.
A collection of stars sparkling in a Hubble Space Telescope image is more than just a beautiful sight. Known as Messier 30, or M30, the object helps astronomers trace the Milky Way’s past. Located about 27,500 light-years away in the constellation Capricornus, the cluster offers clues through the properties of its stars and the direction of its orbit around the galaxy.
A collection of very old stars
M30, also known as NGC 7099, is a globular cluster: a roughly spherical collection of tens of thousands to millions of stars. Such clusters are generally found in the outer regions of galaxies, orbiting within a vast, sparsely populated region called the halo.
The Milky Way has at least 150 globular clusters. Their estimated ages range from 11.5 billion to 13.5 billion years, making some almost as old as the universe. Clusters that formed within the Milky Way can serve as time capsules, preserving traces of the galaxy’s chemical composition when it was young. Studying old stars therefore also means looking for clues about the environments in which they were born.
Orbital direction offers clues to origins
However, being in the Milky Way today does not necessarily mean having formed there. Researchers have found that M30 and many other globular clusters orbit in the opposite direction to our galaxy’s rotation. This difference raises the possibility that some of these clusters originated outside the Milky Way.
Under this explanation, the Milky Way may have captured these clusters in the past. Even so, an orbit in the opposite direction is a clue, not definitive evidence of any individual cluster’s origin. This possibility is precisely what makes observations of globular clusters useful for investigating how the galaxy acquired some of its inhabitants.
Color and motion complement each other
This image of M30 was created using data from three Hubble observing programs, spanning ultraviolet, visible, and near-infrared light. The initial observations with the Advanced Camera for Surveys reached stars with masses just 20 percent that of the Sun. A subsequent program used the Wide Field Camera 3 with three filters that capture bluer light, producing important data for studying exotic stars called blue stragglers and the diverse stellar populations within a single cluster.
Later observations again used the Advanced Camera for Surveys to examine the movement of stellar populations over the 15 years since Hubble’s initial observations. The long observation period and the ability to resolve very fine details make it possible to study these changes in position. The different wavelengths and measurements of motion provide complementary clues about the cluster’s formation and evolution.
Changing our understanding of the Sun’s position
Globular clusters have played a role in understanding the Milky Way for more than a century. Astronomer Harlow Shapley used observations of certain types of variable stars to measure the locations and distances of dozens of globular clusters. His results showed that the Sun is not at the center of the galaxy and that the Milky Way’s center lies in the direction of the constellation Sagittarius.
His distance measurements proved inaccurate, leading him to conclude that the galaxy was about twice its actual size. Nevertheless, his research demonstrated the importance of globular clusters in understanding the galaxy’s structure. Today, Hubble continues that investigation by examining stars and their movements in greater detail.
