There’s an alien red giant star orbiting in the center of our galaxy. It’s called S0-6 and has chemical fingerprints from its birthplace far outside the Milky Way. This ancient star is spiraling slowly in toward the supermassive black hole Sagittarius A* (Sgr A*) at the heart of the Milky Way. Eventually, it could get drawn into the black hole and destroyed after traveling for tens of thousands of years to get there.
S0-6 isn’t the only alien star circling Sgr A*. It’s part of a population of non-native stars in the region. There aren’t very many “native-born” ones there because the black hole’s intense gravity affects the surrounding environment. The extreme conditions make it hard for stars to form close to the black hole, although there are a few populations in the region. However, the stellar population containing S0-6 appears to come from somewhere else. For S0-6 and its siblings, the trick is to find out where that was, based on their ages, and their chemical makeup (that is, their metallicity). All those are clues to their history.
The central parts of our Galaxy, the Milky Way, as observed in the near-infrared with the NACO instrument on ESO’s Very Large Telescope. The position of Sgr A*,with a mass 4 million times that of the Sun, is marked by the orange cross. The star S2 (S0-2) made a close pass to the region of the black hole in 2018. Courtesy ESO.
How do astronomers know S0-6 formed elsewhere? A team led by Shogo Nishiyama at Miyagi University of Education in Japan used the Subaru Telescope to study it from 2014 to 2021. They used the telescope’s Near Infrared instrument (NIR) to perform high-resolution spectroscopy on light streaming from the star. From that data, they determined its chemical makeup, as well as its radial velocity toward the black hole. At present, S0-6 is about 0.2 parsecs (about 0.04 light-years) away from Sgr A* and getting closer each second.
Based on the metallicity, Nishiyama and colleagues determined that the star itself is rather metal-poor. (That means it does have some elements heavier than hydrogen and helium, but not in great abundance. Its surface temperature and magnitude tell them that S0-6 is a late-type red giant, with an age of no more than 10 billion years (so, about twice as old as the Sun). It also doesn’t travel alone. There are many other stars of similar age and metallicity in a cluster around the Sgr A* region. However, most of them don’t match the metallicities of stars that were born in the Milky Way. Chemically, they’re more similar to those in the Large Magellanic Cloud or other dwarf galaxies near our own.
An example of a dwarf galaxy being shredded (eventually to be cannibalized) by the Milky Way. This image shows the model of the tidally shredded Sagittarius dwarf galaxy wrapping around a 3-D representation of the Milky Way disk (flattened blue spiral). The yellow dot represents the position of the Sun. Stars such as S0-2 and S0-6 likely came from another dwarf galaxy also consumed by the Milky Way in the distant past. Credit: UCLA/D.R. LawNishiyama’s team suggests that this star (and others of its cohort) was born in a nuclear star cluster in a dwarf galaxy entirely separate from the Milky Way. Beyond that, they need more information to decide its origin. “There are still many questions according to Nishiyama, “Did S0-6 really originate outside the Milky Way galaxy? Does it have any companions, or did it travel alone? With further investigation, we hope to unravel the mysteries of stars near the supermassive black hole.”
The star experienced a very different chemical evolution in its home galaxy, compared to those born in our Milky Way. There’s a difference in metallic abundance between the ones born in the Milky Way and those that come from outside. That difference is due to a lot of factors, including the initial abundance of gases and other materials in the galaxy at the time of star formation. Astronomers can tell the difference between “native” Milky Way stellar populations and those from other galaxies that have been cannibalized through collisions and mergers.
Over time, that galaxy encountered the Milky Way. All its components got swept up into our galaxy and the cluster containing S0-6 began a long, slow spiral into the galactic core. Over its 10-billion-year-long lifetime, this star likely traveled 50,000 light-years across the depths of space from its birthplace outside the Milky Way to its current location. Now, it’s subject to the gravitational influence of Sgr A. During the time the research team monitored its motions, they saw its radial velocity accelerate slightly. That tells them that S0-6 is being pulled and influenced by Sgr A.
Stars such as this one are unique in the galaxy. Not only did they come from somewhere else, but they provide information about gravitational conditions surrounding the central supermassive black hole. As they get closer to the black hole, they move faster. The speed of that acceleration will provide clues to the strength of the black hole’s pull at various distances. In that sense, Nishiyama’s team considers them to be virtual “test particles” moving through the gravitational field.
Not only have they studied S0-6, but they spent several years studying the motions of another one, S)-2/S2. It has an orbital period around the black hole of about 16 years. What they’ve learned about its motions also tells them more about the physical characteristics of Sgr A*. For example, they’ve determined its mass to be close to 4.23 million solar masses. Observations of other S-type stars (including S0-6) give a fairly precise mass of the black hole and their continued motions promise more data about conditions around Sgr A*.
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