The oldest of these stars is 1.5 billion years old while the youngest is only 100 million years old, according to a paper published in the Astrophysical Journal Letters.
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This infrared image from Hubble shows Milky Way’s nuclear star cluster, the densest and most massive star cluster in our Galaxy. Image credit: NASA / ESA / Hubble Heritage Team / STScI / AURA / T. Do & A. Ghez, UCLA / V. Bajaj, STScI.
The center of our Milky Way Galaxy, approximately 27,000 light-years away in the constellation of Sagittarius, is a crowded place.
This region is so tightly packed that it is equivalent to having one million stars crammed into the volume of space between the Sun and Alpha Centauri, located 4.3 light-years away.
This nuclear star cluster surrounds Sagittarius A*, the 4.3-million-solar-mass black hole at the center of the Galaxy.
In general, many nuclear star clusters coexist with supermassive black holes, and can be found in more than 70% of galaxies with masses above 100 million – 10 billion solar masses.
“In a previous study, we put forward a hypothesis that these specific stars in the middle of the Milky Way could be unusually young,” said Lund University astronomer Rebecca Forsberg.
“We can now confirm this. 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.”
This panorama shows the central region of the Milky Way Galaxy. It builds on previous surveys from NASA’s Chandra X-ray Observatory and other telescopes, and expands Chandra’s high-energy view farther above and below the plane of the Galaxy than previous imaging campaigns. X-rays from Chandra are orange, green, and purple, showing different X-ray energies, and the radio data from MeerKAT are gray. Image credit: NASA / CXC / UMass / Q.D. Wang / NRF / SARAO / MeerKAT.
In the study, Dr. Forsberg and her colleagues used high-resolution data from the Keck II telescope in Hawaii, one of the world’s largest telescopes with a mirror ten metres in diameter.
For further verification, they then measured how much of the heavy element, iron, the stars contained.
The element is important for tracing the galaxy’s development, as the theories the astronomers have about how stars are formed and galaxies develop indicate that young stars have more of the heavy elements, as heavy elements are formed to an increasing extent over time in the Universe.
To determine the level of iron, the astronomers observed the stars’ spectra in infrared light which, compared with optical light, are parts of the light spectrum that can more easily shine through the densely dust-laden parts of the Milky Way.
According to the reseachers, the iron levels varied considerably.
“The very wide spread of iron levels could indicate that the innermost parts of the Galaxy are incredibly inhomogeneous, i.e. unmixed,” said Dr. Brian Thorsbro, an astronomer at Lund University.
“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.”
“Personally, I think it is very exciting that we can now study the very center of our Galaxy with such a high level of detail,” Dr. Forsberg said.
“These types of measurements have been standard for observations of the galactic disk where we are located, but have been unreachable goal for more faraway and exotic parts of the Galaxy.”
“We can learn a lot about how our home Galaxy was formed and developed from such studies.”
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B. Thorsbro et al. 2023. A Wide Metallicity Range for Gyr-old Stars in the Nuclear Star Cluster. ApJL 958, L18; doi: 10.3847/2041-8213/ad08b1
