The heart of the Milky Way can be a mysterious place. There is a gigantic black hole surrounded by a retinue of stars that astronomers refer to as the Nuclear Star Cluster (NSC). The NPC is one of the densest populations of stars in the universe. There are roughly 20 million stars in the galaxy's innermost 26 light years.
New research shows that about 7% of the stars in the NSC come from a single source: a globular cluster that fell into the Milky Way 3 to 5 billion years ago.
Since the beginning of our species, humans have gazed at the giant star arc of the Milky Way. But it wasn't until the last century that astronomers discovered the galactic center. And only in the last few decades, aided by the development of adaptive optics and other technologies, has the region begun to reveal its secrets, including the existence of a supermassive black hole.
The center of the Milky Way is still a region of intense astronomical study. Astronomers know that galaxies, like the Milky Way, grow by merging with other galaxies and by enveloping smaller galaxies. The Milky Way feeds on two of its satellites, the large and small Magellanic Clouds. Currently, it consumes the gaseous halo that surrounds both dwarf galaxies, but over time it will eventually absorb the stars of the dwarf galaxies as well.
The two Keck telescopes shoot their laser guide stars into the heart of the Milky Way on a beautiful clear night on the summit of Mauna Kea. Photo credit: keckobservatory.org/Ethan Tweedie
It should therefore come as no surprise to find a specific population of stars in the galactic center that was a separate cluster of its own at one time before it was absorbed by the Milky Way.
Researchers at several institutions around the world are working hard to better understand the Milky Way, including how it grows and evolves over long periods of time. Recently, a group of astronomers associated with the European Southern Observatory (ESO) and the Max Planck Institute for Astronomy (MPIA) have made some progress. They observed the galactic center with the Very Large Telescope (VLT) and that data has led to some new discoveries. Three teams of researchers have published new work on various aspects of the galactic center and the newly discovered population of stars that live there.
The three studies are:
The galactic center is difficult to observe because of the avoidance zone (also known as the obscuration zone). It's a region so dusty that it blocks visible light. Special instruments and telescopes that can see in other wavelengths, especially in the infrared range, are required for observation. The VLT in Chile can look into this region.
In “Asymmetrical spatial distribution of sub-solar metallicity stars in the core cluster of the Milky Way”, the research team analyzed the speed, movement and chemical composition of 700 stars in the galactic center.
Populations of stars are not only characterized by how they move together through space. Their compositions also tell the astronomers a lot about their common origins. In particular their metallicity.
This is an image of M80, an old globular cluster. Since these stars formed in the early universe, their metallicity is very low. Image: By NASA, the Hubble Heritage Team, STScI, AURA – Great Images in NASA Description, public domain, https://commons.wikimedia.org/w/index.php?curid=6449278
Stars are mostly made up of hydrogen and some helium. The tiny amounts of elements heavier than these two, which astronomers call metals, reveal a great deal about the star's age and history. Everything is from the early days of the universe.
Astronomers divide stars into three groups: Population III, Population II, and Population I. Pop III stars are the oldest stars to be formed in the universe first. At that time, the universe was 75% hydrogen and about 25% helium with a tiny fraction of heavier elements like lithium and beryllium. The stars formed at that time contain almost nothing heavier than helium. They are called low metallicity stars.
Pop II stars are not that old and form after the first generation of stars. Elements heavier than hygrogen and helium are formed in the hearts of stars and then spread out into space when those stars die. Pop II contains some heavier elements than hydrogen and helium as the previous Pop III stars fused some heavier elements or metals together and gave them to the Pop II stars.
An artist's impression of the universe's first stars, called Population 3 Stars. Pop 3 stars would have been much more massive than most stars today and would have burned hot and blue. Their lifespan would have been much shorter than that of stars like our sun. The metallicity there is also much lower than in younger stars. Photo credit: Public Domain, https://commons.wikimedia.org/w/index.php?curid=1582286
Pop I Stars are the younger stars. Our own sun is one, and since it is made of material created by the previous two generations of stars, the sun and the rest of the Pop I stars have more heavier elements. They have a higher metallicity.
That's a pretty broad overview of how it all works. But it shows that metallicity is extremely important to astronomers.
The key to all of this is that if you find a group of stars moving in a similar way and you find that they share the same metallicity, they are likely from the same cluster and are all formed from the same precursor material . That is what this research found in the center of the Milky Way.
Most of the stars in the center of the Milky Way have the same composition or metallicity. They have a higher metallicity than our sun. However, in new research, astronomers identified a subset of stars with significantly lower metallicity.
This number comes from one of the three new studies. On the left is the full sample of stars viewed with the KMOS spectrograph on the VLT. The center shows the radial velocity of the stars in the NSC with less metallicity than the sun. On the right is the radial velocity of stars with higher metallicity than outside the Sun. The stars above and below solar metallicity appear to have significantly different kinematic characteristics. Photo credit: Do et al., 2020.
About 7% of the stars in the center of the galaxy have this lower metallicity, so they must have a common origin. But the question arises as to how they got there. How did they become part of the same Nuclear Star Cluster (NSC)?
The accepted theory is that these NPCs could be formed, at least in part, from collisions of smaller star clusters over time. These smaller clusters can be grouped within the larger galaxy, held together by mutual gravity, and move through the galaxy. All galaxies have these clusters.
But eventually the gravity of all other matter in the galaxy causes these clusters to slow down, lose their orbits, and slowly migrate towards the galactic center. Several smaller clusters follow this path, which together can form the NSC. So this newly discovered subpopulation of stars in the NSC could be one of those smaller groups.
That sounds nice and neat, doesn't it? But how can this insight be tested? This is where modeling and simulations come into play. In a press release, researcher Manuel Arca Sedda said: "Among other things, our goal was to find out how long it has been since such a star cluster came to the region around the Galactic Center and where it originally came from."
In "About the Origin of a Rotating Metal-Poor Star Population in the Core Cluster of the Milky Way", the research team performed powerful computer simulations that included the NSC, the supermassive black hole in the center of the galaxy, and a massive star cluster with about a million solar masses. The simulation started with the massive star cluster about 160 light years from the galactic center.
As a smaller cluster falls into the galactic center, its cohesiveness gradually decreases over time as it interacts with its surroundings. Some of the stars are ejected from the group. As soon as it reaches the center, this dissolution accelerates and over a relatively short period of time (astronomically speaking) the cluster becomes indistinguishable from the larger NSC to which it belongs.
This number from one of the three new studies shows the velocity vectors of 2% of the simulated stars, or about 21,000 stars within a 5-parsec sphere centered on the galactic center. The left panel is the simulation at 61 million years, while the right panel is at 199 million years. The red lines represent the speeds and vectors of the stars in the nuclear cluster, while the blue lines represent those of the newly identified cluster. Before the dissolution, the SC is clearly visible in the form of a concentration in the upper left area of the NC in the map and remains recognizable for at least 100 myr after the dissolution. Photo credit: Sedda et al., 2020.
The fact that astronomers found this group means that it must have fallen into the galactic center in the not-too-distant past. According to the simulations, it has been in the center and part of the NSC for 3 to 5 billion years.
Next question? Where did it come from According to the researchers, there are a few main options.
It came either from a more distant part of the Milky Way or from outside the galaxy as part of a dwarf galaxy. The simulation examined both possibilities, but the results were against the dwarf galaxy as the source.
“Our results show that an infiltration of a fairly nearby star cluster from the Milky Way itself is more likely,” explains Neumayer. It was likely originally formed around 10,000 to 16,000 light years away.
Fortunately, the researchers were able to test this hypothesis by comparing the group of stars with older globular clusters in the Milky Way and those that entered the Milky Way in dwarf galaxies. The comparisons showed that the newly discovered star cluster had more in common with stars in globular clusters.
Visualization of a simulation that shows the infallion of a globular cluster into the core cluster of the Milky Way. The color scale shows the distribution of star densities along the lines of sight within the Galactic Center. The globular cluster can be recognized as an isolated point that gradually merges with the core cluster over the course of 400 million years and thereby dissolves. Despite the resulting mixing of the two star populations, certain properties of the stars in the globular cluster are retained.
Image: Manuel Arca Sedda et al. (ARI / ZAH) / MPIA
"Although an extragalactic origin of the stars cannot be completely ruled out, this is rather unlikely," concluded Arca Sedda. "This is an additional sign that the central core cluster in the galaxy is at least partially due to the influence of smaller clusters."
Researches like this reveal the true nature of our heavenly house, the Milky Way. For a while the accepted theory is that massive galaxies like ours grow over time through mergers and agglomerations. Over time and through the course of science, we can point to individual groups of stars and see how and when they joined the family.
It's like an excellent family tree.