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Hunting alien star clusters and black holes

Six star clusters are outlined in a small section of a vast image of the Andromeda Galaxy, M-31, taken by the Hubble Space Telescope.
NASA, the European Space Agency, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler

Editor’s note: A version of this column has been previously published on the author’s website.

A University of Utah professor and his students are working to produce the first catalog of star clusters in the Triangulum Galaxy, also called Messier 33. They are following the model of a similar catalog he helped produced of a section of the closest large galaxy to our own, Andromeda.

Anil Seth, associate professor in the university’s physics and astronomy department, described the projects on Dec. 11, 2019, during a talk to the Utah Astronomy Club. These meetings are held the second Wednesday of each month in the university’s South Physics Building, 115 S. 1400 East, Salt Lake City, starting at 7 p.m. The public is invited to the free lectures and discussions.

University of Utah associated professor Anil Seth explains a projected image Dec. 11, 2019, in a talk before members of the Utah Astronomy Club.
Joe Bauman

Seth is a knowledgeable, engaging young man of quick movements; generous with his time, willing to listen and explain. Operating at the frontiers of astronomical research, he seems to embody the ideals of scientific inquiry and explication.

Star clusters come in two varieties, open and globular. Globulars, which are far more rare at least in galaxies close enough to observe them, are spherical amalgamations of stars that often are among the oldest in the universe; in our Milky Way galaxy, they orbit the center and are not limited to the galactic plane. Open clusters, hundreds or thousands of times more common, form within galaxies in the plane. The stars of an open cluster compacted from the same clouds of dust and gas, igniting around the same time. They are not necessarily in a sphere and they gradually spread out, eventually dissipating into scattered stars.

The types of stars in clusters indicate their age, which in turn provides a history of processes in the galaxy. “I use data like this to look at how galaxies are built up,” Seth said. Galaxies are a building block that connects the Big Bang to our time, he added. “This galaxy formation step is a real key step in understanding the history of our universe.”

Results of the Andromeda star cluster survey were announced in 2015, following painstaking work to identify clusters that appear in a gigantic mosaic of part of the galaxy made from Hubble Space Telescope images.

According to NASA, this mosaic image (scaled down dramatically so it would fit on a browser) is the largest Hubble Space Telescope photograph ever assembled. It shows a portion of the next-door large galaxy, M-31 or the Andromeda Galaxy, which is 2.5 million light-years away. Composed of 414 photographs, the cropped view stretches across 48,000 light-years. In the version not scaled down, more than 100 million individual stars can be seen, each larger than the Sun.
NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler

The latest news is that the Triangulum Galaxy, M-33, is the subject of its own star cluster catalog, under preparation by Seth and others including University of Utah undergrad students Estephani Torresvillanueva and Tobin Wainer. Like the Andromeda effort, this one is based on Hubble images.

M-33 is about 3 million light-years distant and is the third-largest in our local group, behind Andromeda and the Milky Way. It is “chock-full” of stars and clusters, Seth said. “We’re working on getting out the first publication” of the M-33 star cluster catalog.

To understand the M-33 project, let’s review the way Andromeda star clusters were located and characterized. The survey was dubbed the Panchromatic Hubble Andromeda Treasury or PHAT program. The space telescope surveyed about a third of Andromeda, requiring 828 orbits and using two months of observing time. The mosaic has more than 3 billion pixels, Seth said.

Projecting Hubble’s image of Andromeda, he said the view is a “carpet of individual resolved stars” — stars that are so well-defined that they are separate entities and not just fuzzy masses of objects blurred together. “So we resolved over 100 million stars in this galaxy.” In the two-thirds of Andromeda not photographed, and counting those in the mosaic that are too dim to see, M-31 is thought to have 1,000 to 10,000 times more stars than are in the picture.

Ordinary stars like the sun would not show up, as they’re too small to see from this distance. “These (that are visible) are just the rare bright ones,” he said. Their ages can be calculated by comparing brightness and color. Young stars tend to show up in rings or the galaxy’s spiral arms, which are undergoing star formation. Older stars appear in the central bulge and the extended disk.

Since stars in a cluster all developed about the same time, he said, scientists can “use them as a tool to study where did stars form, how intense was that star formation.” They give clues to the galaxy’s history.

With 3 billion pixels to examine for star clusters, “I spent about two months of my life looking at about one-fifth of that.” Members of the team decided they didn’t want to spend 10 months circling potential star clusters, so they farmed out the work in a citizen science project.

For 25 days, 10,000 volunteers in many countries studied 1.8 million images, parts of the mosaic. Instructed on how to identify a cluster, 80 people would examine each image and circle suspected star clusters. Some found objects that were not really star clusters — such as background galaxies showing through Andromeda — and others missed particular clusters. But when 30 or 50 people would designate a set of objects as a star cluster, the odds were heavily in favor of that being the case.

To check accuracy, the scientists created fake clusters to see whether they were noticed.

When the researchers tested volunteers’ choices against areas that they had themselves examined in the first two months, the results weren’t identical — the confirmation rate was about 90 percent. At times volunteers apparently had false positives and at others they seemed to miss clusters. But after reconsidering their own original work, the scientists realized that much of the difference was because the volunteers had identified “better cluster candidates.” The results were strong.

The project identified 2,753 star clusters, where previously 466 were known. About 50 are globular clusters, most of which were already identified because they are relatively bright. “We detected many, many more faint clusters,” Seth said.

Star clusters could be traced along Andromeda’s spiral arms, Seth said. “There’s a lot of really low-mass clusters,” he noted.

Compared with other galaxies, M-31 had few massive star clusters. It’s not that they were missed, because large clusters are bright, long-lasting and easier to find. They’re simply not abundant.

Other galaxies including M-51, the Whirlpool Galaxy, form stars much faster, according to Seth.

The Whirlpool Galaxy, M-51, is interacting with a smaller galaxy, NGC 5195, to the left. NASA estimates the pair is 31 million light-years away, which is more than a dozen times the distance to Andromeda. (This picture includes a supernova, indicated.) Photo by Joe Bauman, taken June 20, 2011, in Tooele County, Utah
Joe Bauman

”We think that there’s some process here that’s preventing the most massive clusters from being formed” in M31, he said. The Whirlpool, for example, is interacting with a nearby galaxy and maybe that triggers star formation.

Andromeda’s cluster catalog, with its many young star groups, can be “as a tool to study the history of this galaxy,” he said.

Another direction of his research involves galaxies and the central black holes that many contain. Ours and other large examples host supermassive black holes but not all galaxies do.

Two smaller galaxies that have drawn his interest are satellites of Andromeda called M-32 and NGC 205 (also sometimes called M-110, although Messier did not name it when compiling his list). M-32, Seth said, is extremely bright. The greatest surface brightness of any known galaxy in the universe is the center of that little galaxy. “So if you want to take a really nice spectrum of a galaxy, M-32’s the best place to do that.”

It is much smaller than Andromeda’s other best-known satellite, NGC 205, but both have around the same mass. That means M-32’s stars are far more densely concentrated.

M-32 might be “the core of a galaxy that has been torn apart by Andromeda.” Some investigators believe that 2 or 3 billion years ago another large galaxy was captured by Andromeda, which stripped off the outer stars and gas, leaving the core as a satellite. Circumstantial evidence for this is a “giant stellar stream that we see in the halo of the Andromeda Galaxy.” The idea is controversial, as some think M-32 might have formed in a merger of two smaller galaxies.

NGC 205 has lanes, dust clouds and a bright central star cluster. “This is actually a galaxy I’ve been working on recently with a graduate student of mine, Dieu Nguyen. We think this galaxy might host the lowest mass back hole that’s at the center of any galaxy that we know.” However, it’s not certain because determining that depends on timing stars that move, and measurements so far are not quite exact enough.

Star motions at the center of NGC 205 could result from the presence of a black hole 10,000 times the mass of the sun, “the lowest mass of a central black hole that has been measured.” Or the center could be simply a collection of ordinary stellar-mass black holes.

Sometime in 2020, better measurements should be available. “We’ll have an answer if we detect something,” Seth said “— won’t know for sure if we don’t.”

Joe Bauman, a former Deseret News science reporter, writes an astronomy blog at and is an avid amateur astronomer. His email is