The core of our Milky Way is buzzing with stars. Recently astronomers reported that it contains at least one ancient star that formed outside our galaxy. Now, an international research team reports finding a grouping of very young ones there, as well. Their presence upends ideas about star birth in that densely packed region of space.
These stellar youngsters live in a dense collection called the “nuclear star cluster” (NSC). It lies about 27,000 light-years away from us and surrounds the area holding the supermassive black hole Sagittarius A*. Observers have long considered the NSC to be one of the more ancient parts of the Galaxy. Proving that requires detailed observations of the stellar populations in the core. But, studying this region is somewhat difficult, at least visually. That’s because of dense clouds of gas and dust that block the view. In addition, the thick population of stars makes it difficult to distinguish their characteristics.
The nuclear star cluster at the Milky Way’s heart, as seen by the Very Large Telescope in Chile. Credit: ESO.
One of those characteristics is age, which is not always easy to determine. Since astronomers want to know more about the lifetimes of stars in the galaxy’s core, the best way to find out how old they are is to look at something called their “metallicity”. As a general rule, old ones tend to be more metal-poor, while younger ones are more metal-rich. Recent observations found several very young stars there with very high metallicities. That’s an indication that things in the core aren’t always what they seem.
This infrared image from NASA’s Spitzer Space Telescope shows hundreds of thousands of stars crowded into the swirling core of our spiral Milky Way galaxy. Infrared light can penetrate the dense clouds of gas and dust and reveal the richness of the stellar population there. Credit: NASA/JPL-Caltech
A team of astronomers at Lund University in Sweden used the Keck II telescope in Hawaii to take infrared spectra of stars comprising the NSC to determine their ages and metallicities. Previous observations hinted that at least three members of the cluster could be unusually young in a region where older ones normally predominate. “We can now confirm this,” said Rebecca Forsberg, a Lund graduate student and member of the research team. “In our study, we have been able to date three of these stars as relatively young, at least as far as astronomers are concerned, with ages of 100 million to about 1 billion years. This can be compared with the Sun, which is 4.6 billion years old.”
The three stars they studied in-depth have interestingly high amounts of heavy elements, including iron. Their metallicity is a measure of how many elements heavier than hydrogen and helium they contain. Metal-poor stars are usually born early in a galaxy’s history (or in the early Universe). Metal-rich ones contain materials shed from older ones when they die—including such heavy elements as calcium and iron. The cluster itself shows a range of metallicities, and the youngest ones have the most abundance of iron.
Astronomers began studying star formation in the NSC back in the mid-1990s. Most of its stars are likely at least five billion years old, or older. Normally, you’d expect that star birth stopped there a long time ago because of the influence of the nearby black hole, and other factors. But, those young ones confuse the picture. They came from recent episodes of star birth. Understanding how those happened gives important clues to the mix of star-forming materials at the Galaxy’s core, and may even tell how the Milky Way itself formed. And, because these types of clusters exist in other galaxies, understanding how ours formed would explain what happened in other places.
There are a couple of theories about the creation of nuclear star clusters. One is that early in a galaxy’s history, globular clusters formed and spiraled into the galactic core. There they became nuclear star clusters. The other idea is that their stellar populations formed “in place” as a galaxy evolved. Either idea could explain our Galaxy’s central cluster. The way to figure it out is to look at the chemical makeup (i.e., the metallicity) of the cluster’s stars since it correlates with when and where they formed.
Omega Centauri is the brightest globular cluster in the night sky. It holds about 10 million stars and is the most massive globular cluster in the Milky Way. Globulars and nuclear star clusters may be related in some way as a galaxy evolves. Image Credit: ESO.
The Lund team took infrared spectra of the cluster members, which revealed the large variation in metallicity between the older and younger members. The three youngest ones with high metallicity indicate that they likely formed in place inside the Milky Way within the last few hundred million years. Interestingly, even these three vary in their metal content from each other. That means they formed in clouds that themselves had different abundances of heavier elements.
Interestingly, the one with the lowest metallicity of the three may be what’s called a “blue straggler”. That’s a star that could be much older but stole mass from a nearby companion. That additional material could make it look younger than it is. Further observations could confirm if this is a new star or an imposter.
The heavy element iron was of particular interest to the team. This element is an important clue to a galaxy’s continuing evolution. Iron is the heaviest element a massive star can “cook up” inside its nuclear furnace. When it explodes as a supernova, the iron and other heavy elements (such as calcium, carbon, nickel, and others) get scattered into space. In time, that material gets mixed into clouds of gas and dust where star birth can take place. So, the oldest, metal-poor stars in any galaxy (and indeed, in the Universe) are ones that formed early on. But, the younger ones that come along later are more abundant in heavy elements because they form in metal-enriched clouds of gas and dust.
That process of stellar birth and death played out in the nuclear star cluster at our Galaxy’s heart. And, it spread heavier elements across the region. “The very wide spread of iron levels [in the cluster] could indicate that the innermost parts of the galaxy are incredibly inhomogeneous, i.e. unmixed,” said Brian Thorsbro, lead researcher and Lund astronomer. “This is something we had not expected and not only says something about how the center of the galaxy appears but also how the early Universe may have looked.”
The Lund University infrared spectral studies of the stars at the heart of the Milky Way will certainly be extended to other stellar populations. Those future observations should provide answers to why astronomers found a wide range of metal abundances in stars in that tightly packed region of the Galaxy.
Astronomers Determine the Age of Three Mysterious Baby Stars at the Heart of the Milky Way
A Wide Metallicity Range for Gyr-old Stars in the Nuclear Star Cluster
