When a Galaxy Meets a Cluster

Space is big and even the regions between one galaxy and its neighbor can seem empty once you get “out there.” So, what happens when two objects get close together in space? You get collisions and close approaches.

The birth of our own Moon was likely the result of a collision between the infant Earth and a Mars-sized world called “Theia”. Of course, one galaxy can collide with another. That’s what astronomers predict will happen between the Milky Way and Andromeda Galaxies in a few billion years. The image above shows what it might look like to an observer on a planet inside one of the two galaxies. But, what about other kinds of collisions and near-misses?

Astronomers using the Canada-France-Hawaii Telescope on the Big Island of Hawai’i took a look at a nearby globular cluster called M92. It’s about 27,000 light-years from Earth and can be spotted just at the top of the constellation Hercules. What the astronomers “see” is a stream of stars being pulled out of the cluster. They’re not immediately obvious to the casual observer, but in specific wavelengths of light, they stand right out, as shown here in a plot of data from the observations.

These long, thin lines of stars exist because M92 is too close to the Milky Way galaxy. The immense gravitational pull of the Milky Way is ripping the smaller conglomerations of stars apart. It’s not a fast process. Some of the stellar streams caused by such interactions can last for billions of years. In the case of the stream from M92, it’s been around for about 500 million years. And, that short length of time leads to some interesting questions.

Why Such a Young Stream?

The observations from CFHT and the Pan-STARRS1 survey telescope at Haleakala on Maui provided a lot of data to help astronomers figure out how long the M92 stream has been around. The data also give some idea of the cluster’s origin. It might also help astronomers figure out the distribution of dark matter in the Milky Way and its role in corraling a globular cluster as it passes by.

The M92 cluster itself is about 1.1 billion years old, but the stream is 500 million yeas old. So, something happened ‘recently’ to cause the cluster to lose stars to the stream. That “something” is gravitational interaction with the Milky Way as the cluster passed by. But from where? That raises questions about where M92 originally formed. If it formed elsewhere and only started losing stars as it got closer to the Milky Way, then perhaps astronomers can use the information about its stars to figure out where it came from originally.

More about Globulars

Typically, the globular clusters like M92 orbit the central region of the Milky Way. Such clusters contain stars tightly bound together in a spherical shape. How and where these clusters form is still an unfinished astronomy story. In many clusters, most stars are about the same age. That means they all formed about the same time. However, some have stars of varying ages, which suggests they formed in “waves” of starbirth. If we look at various galaxies that are undergoing starburst activity, it’s easy to see many globulars forming in such areas. And, some of those galaxies are in collisions or close interactions, which spurs the starburst activity.

M92 itself is fairly young, compared to the galaxy. At 1.1 billion years, it’s about 10 billion years younger than the Milky Way. Our galaxy began forming shortly after the birth of the universe, or about 13.4 billion years ago. Could the Milky Way have undergone a large starburst epoch about the time M92 was born? It’s not likely. So, it looks pretty likely that M92 formed somewhere else and got caught up in the gravitational tug of our galaxy and its dark matter halo.

Today, the Milky Way is actively consuming (cannibalizing) several smaller galaxies, including the nearby Large and Small Magellanic Clouds. That’s how the Milky Way has grown over time. It does have other globulars that are much older than M92, so they likely formed along with our galaxy. But, there is little evidence showing that the Milky has interacted with other large galaxies in the past 10 billion years. So, that avenue of globular creation doesn’t seem to be how M92 was created. Perhaps it was born in another collision and migrated here? That would be a fascinating story!

Questions about Our Galaxy and M92

Astronomers want to find out where M92 formed and what the conditions were when it did. What was going on 1.1 billion years ago when it was born? And, what were conditions like when it wandered too close to the core of our galaxy and began losing its stars? Those questions await more observations and answers.

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