The globular cluster M92 is a great example of this species. A roughly spherical ball of hundreds of thousands of stars held together by their mutual gravity is one of about 160 orbiting the Milky Way.
Well, for now. It turns out that our galaxy eats it.
Astronomers have found a stream of stars in front of and behind the cluster as it orbits stellar citizens deprived of the gravity of the Milky Way. This is surprising given M92’s great age – about 11 billion years – which means that something * has recently happened that will change things.
Many of these stellar streams have been discovered in the last few decades. Some are from small galaxies that have passed too close to the Milky Way (which is a large galaxy), and others from globular clusters. Some we are not sure. Half a dozen or so have globulars associated with them, and more than dwarf galaxies, while some do not seem to have a specific source; they probably came from objects that the Milky Way has completely torn apart.
In general, when a small object such as a cluster or a dwarf galaxy approaches, the stars at their outer edges feel more gravity from the Milky Way than from their parent object, and pull away. It̵
These streams have escaped attention for centuries because they are extremely difficult to identify against the billions of other stars in the galaxy. But large studies of the sky, looking at millions or billions of stars, make it possible to notice them. One way is to look only at the motions of the stars; they all seem to be moving together in one direction. If distances can be found, they all fall in an arc, the orbit of the trajectory of the parent object.
In the case of M92, astronomers first examined a pair of huge star studies made using the Canada-France-Hawaii and Pan-STARRS telescopes. They took an area of the sky around M92 and looked for stars with colors corresponding to that of the cluster – knowing the age of the cluster and that the color of the star depends on its mass and age, they could make an incision filtering out stars that did not match the cluster. . They were also looking for stars at about the same distance from the pile, as the stream would have been at that distance.
When they did, the stream popped right out of their data. The total number of stars suggests that the stream has about 30,000 times the mass of the Sun in it, which is now about 10% of the stars in the cluster! So this cluster just throws stars.
Once they had a list of stars in the stream, they turned to Gaia, a satellite that in the last few years has measured positions, color, distance, motion, and more for over a billion stars (yes, a billion). This allowed them to trace the cluster’s orbit back in time and found that it passed through the galactic convexity, the flattened sphere of stars surrounding the galactic center. It also passes right through the bar of the galaxy, a more complex elongated structure at the galactic center.
This is interesting because they can also measure the movement of these stars away from the cluster, giving them guidance on how the flux has changed over time.
Given the distance and speed of the stars in the stream, they found that these stars were removed from M92 only about 500 million years ago, and most of them less only in the last 300 million years. This is very soon compared to the 11 billion age of the cluster.
If it passes through the galactic center of each orbit, then, given the speed of the star’s loss, it must be long gone. This in turn suggests a recently changed orbit. Otherwise, the cluster should not exist.
It is possible that the last passage through the galaxy has changed its orbit, as the gravitational field at the galactic center is complex. If anything else can make this big orbital change, I’m at a loss in terms of what it is. Another cluster could pass and their gravitational interaction affects them both, but the space around the galaxy is very large and the clusters very small. . The chances of a close collision are extremely small.
Astronomers plan to look for more stars in the cluster, perhaps those that have receded, creating a halo around the stream. This can help them understand what is going on. However, the work is diligent, so it may take some time before they receive additional information.
These streams are fascinating to me. They tell us about the distant past of the galaxy, and also about its construction now. As these streams flow, the density of the stars in each part may change due to the changing gravitational field in the galaxy; for example, if they pass near a massive giant molecular cloud of gas and dust, or if the halo of dark matter around our galaxy is not smooth, but instead lumpy. They also tell us about the way globular clusters behave now, how they lose stars, and how this affects their structure.
Quite amazing, considering we didn’t even know about these streams a few decades ago. Advances in technology have given us this chance to look into the past of our galaxy, and – as usual – things are much more complex and much more interesting than we thought.
*Of course, “recently” for an astronomer is different than it is for you, probably. I mean, somewhere in the last 500 million years, it wasn’t long before animals invented solid body parts instead of swimming in the oceans.