A new Hubble image of the M30 cluster hints the Milky Way captured some of its oldest star clusters

Hubble has released a new image of Messier 30, known as M30, a globular cluster about 27,500 light-years away in the constellation Capricornus. NASA published the picture on Nov. 5 alongside a note that M30 and many other globular clusters orbit opposite to the direction the Milky Way spins. The agency says such clusters may have been captured by the galaxy in the distant past rather than formed inside it.

AI, explained every day: The Deep View is the go-to daily newsletter for 600,000+ founders, knowledge workers and professionals who want to stay up to date with artificial intelligence.

Get The Deep View →

We may earn a commission if you sign up.

Astronomers estimate that some globular clusters are 11.5 to 13.5 billion years old, which makes them nearly as old as the universe itself.

M30 and the oldest stars in the galaxy

NASA’s Hubble release describes globular clusters as roughly spherical groupings of tens of thousands to millions of stars that mostly orbit in the galactic halo. The Milky Way has at least 150 of them, and the nearest sits about 6,000 light-years away. M30, also catalogued as NGC 7099, was recorded by Charles Messier among diffuse objects that could be mistaken for comets.

NASA’s Messier catalog entry adds that M30 is about 90 light-years across, formed roughly 13 billion years ago and holds several hundred thousand stars, some of which look younger than they are and are known as blue stragglers.

Messier recorded M30 in 1764 among the fuzzy objects that could be mistaken for comets, and NASA’s catalog entry puts its diameter near 90 light-years. Some of its stars look younger than the cluster, the blue stragglers, and several of the Hubble programs behind the new picture were designed to study them along with the multiple stellar populations found in clusters like this one.

Three Hubble programs in one picture

The image is a composite of data from three observing programs, covering ultraviolet, visible and near-infrared light. Program 10775, led by Ata Sarajedini, used the Advanced Camera for Surveys to survey dozens of clusters in red and near-infrared light; its archive record lists the proposal. Program 13297, led by Giampaolo Piotto, used the Wide Field Camera 3 with three bluer filters, and its archive entry shows the observations that helped study blue stragglers and multiple star populations within clusters.

Program 16298, led by Mattia Libralato, returned to M30 with the Advanced Camera for Surveys to track how its star populations moved over 15 years, and the STScI proposal page carries its details. The credit line names ESA/Hubble and NASA, with Libralato, Piotto and Sarajedini, and the text credit goes to ESA/Hubble. Claire Andreoli of NASA’s Goddard Space Flight Center is the listed media contact for the release.

Stacking ultraviolet, visible and near-infrared exposures matters because each wavelength picks out different stars. Hot blue stars dominate the ultraviolet, cooler red giants stand out in the infrared, and the combined frame shows the whole population at once. A 15-year baseline from program 16298 adds a time dimension no single image can supply, since stars that shift position between visits reveal how the cluster moves.

The numbers give a sense of the spread involved. M30 sits about 27,500 light-years away, while the nearest of the Milky Way’s 150-plus globular clusters is only about 6,000 light-years from Earth, so the family covers a wide range of distances and orbits for astronomers to compare against one another.

Backward orbits as evidence of capture

The argument for capture rests on direction. The Milky Way’s disk rotates one way, and objects that formed with it generally move with that spin. M30 and many other globular clusters travel the other way, according to NASA, and the agency says the pattern suggests they were pulled in from elsewhere. The wording in the release is cautious: the clusters may have been captured, and no specific merger or source galaxy is named.

That caution is fair. Orbital direction is a clue, not a pedigree, and the release does not present measurements that tie M30 to any particular origin. What the three programs supply is a detailed census of the cluster’s stars and their motions, which feeds the broader question of where globular clusters came from.

Shapley, Sagittarius and the history of mapping clusters

NASA’s release also recounts how Harlow Shapley used variable stars to map the distribution of globular clusters, work that showed the Sun is not at the center of the galaxy and that the galactic center lies toward Sagittarius. He overestimated the galaxy’s size by about twice, the release notes. Clusters thus helped define the Milky Way’s layout before anyone asked whether some were immigrants.

NASA lists further background on Hubble’s star cluster observations. Whether the 15-year motion data from program 16298 can pin down where M30 came from is not answered in the release, and NASA does not claim it does. The capture idea stays a hypothesis, in the same register as the release’s own wording about what clusters may have done in the distant past, that future orbit measurements across many clusters would have to confirm or discard.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

AI, explained every day: Get The Deep View →

We may earn a commission if you sign up.