Extragalactic Star at the Black Hole

Extragalactic Star at the Black Hole

Astronomers have discovered a star of unusual origin in the innermost center of the Milky Way—it does not originate from our galaxy. The star, designated S0-6, currently orbits closely to the Black Hole at the center of the Milky Way. However, spectroscopic analyses have revealed that it originated more than 50,000 light-years away in a neighboring dwarf galaxy. Indications of this extragalactic origin include its unusual composition and the star’s advanced age of over ten billion years.

The center of our Milky Way is a highly dynamic region. The immense gravitational influence of the supermassive Black Hole, Sagittarius A*, causes the central stars to orbit around its gravitational well at record speeds, measurably altering their light and trajectories. Observations also indicate that amidst this dense congregation of stars, some of the oldest stars in the Milky Way coexist, along with, surprisingly, some protostars—despite the generally adverse conditions.

Examining the Innermost Stars

Astronomers have recently made another discovery in the center of the Milky Way. Shogo Nishiyama and his team from Miyagi University in Sendai tracked and spectroscopically examined some of the so-called S-stars in the Milky Way’s center for over eight years. These stars orbit within 0.13 light-years of the event horizon of the Sagittarius A* black hole and experience a correspondingly strong gravitational influence.

The S-stars provide a unique test case for the effects of strong gravity around supermassive black holes,” explain Nishiyama and his colleagues. Due to the extreme gravity conditions, astronomers have long suspected that most stars in the central cluster probably did not originate on-site but were formed farther out and later drawn near Sagittarius A*. However, it remains unclear exactly where these stars originally came from.

Unusually Old and Metal-Poor

One of the S-stars, designated S0-6, now offers a surprising answer. It orbits only about 0.04 light-years from the central black hole and is one of the few red giants in the center of the Milky Way. Through analyses of the light spectrum, Nishiyama and his team determined that the star S0-6 must be more than ten billion years old, making it one of the oldest stars in its neighborhood.

However, S0-6 has another peculiarity: it contains fewer heavy elements than all stars analyzed in this environment so far, indicating its extreme metal deficiency. This not only confirms its early origin in the cosmos but also makes this star a stellar exotic. “The chemical composition and the high age suggest that S0-6 has undergone a different chemical evolution than the stars in the center of our galaxy,” report the astronomers.

Composition Points to an Extragalactic Origin

What does this mean concretely? To learn more about the origin of the star S0-6, Nishiyama and his team compared the spectral signature of this red giant with stars from various regions of the Milky Way, as well as stars from neighboring dwarf galaxies such as the Large Magellanic Cloud, the Sagittarius Dwarf Galaxy, or the Sculptor Galaxy.

The result is that S0–6 resembles stars in the Magellanic Cloud or the Sagittarius Dwarf Galaxy more than their stellar counterparts in the Milky Way, as determined by astronomers. In their view, this suggests that the star S0-6 was originally of extragalactic origin. Despite its current position in the innermost core of the Milky Way, it must have originated more than ten billion years ago in one of the primordial dwarf galaxies in our cosmic neighborhood.

Traveled 50,000 Light-Years years—at least

This could make S0–6 the first star from the central cluster of the Milky Way for which an unequivocal extragalactic origin has been demonstrated, as reported by astronomers. This implies that over its more than ten billion years of existence, the star S0-6 must have covered a distance of at least 50,000 light-years—probably even more. Typically, stars and star clusters drifting inward follow a spiral route toward the galactic center.

However, the origin of the ancient star S0-6 and the path it took to reach the center of the Milky Way are still open questions. It is also possible that this star is not the only extragalactic immigrant in our black hole. “With further investigation, we hope to unravel the mysteries of stars near the supermassive black hole,” says Nishiyama.

Source: (Proceedings of the Japan Academy, Series B, 2023; doi: 10.2183/pjab.100.007)

Featured Image: The star S0-6 is located just about 0.04 light-years from the central Black Hole of the Milky Way, Sagittarius A*. However, this star likely originated outside our galaxy. © Miyagi University of Education/ NAOJ