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Stars orbiting Sagittarius A* appear 'rejuvenated' after devouring other stars

Our galaxy‘s core is a place of immense activity. A supermassive black hole, Sagittarius A* (Sgr A*), resides at its center, surrounded by a dense cluster of stars. The black hole’s intense gravitational pull accelerates nearby stars to astonishing speeds. In this crowded and chaotic environment, stellar collisions are a common occurrence.

New research from Northwestern University investigates the consequences of these collisions. It uncovers a fascinating phenomenon: stars near the galactic center sometimes merge together or lose mass due to near misses. This dynamic process can lead to surprising changes in stars’ appearances and lifespans.

Stars near Sagittarius A*

“The region around the central black hole is dense with stars moving at extremely high speeds,” explains Sanaea C. Rose, an astrophysicist at Northwestern University who led this study. This unique environment sets the stage for frequent interactions between stars.

Rose and her team explored how these interactions shape stars within the central cluster. Their findings shed light on surprising stellar transformations.

Gravitational encounters in Sagittarius A* stars

The research highlights two primary outcomes for stars near Sagittarius A*:

Close calls lead to stripping

Stars have a layered structure. Their core is the hottest, densest region where nuclear fusion occurs. Surrounding the core are radiative and convective zones where energy travels outward. The outermost layer is the photosphere, the visible “surface” of the star.

When stars have close encounters at extremely high speeds, the intense gravitational interaction primarily targets those outer layers. Think of it like a strong wind stripping away the top layer of sand on a dune, leaving a smaller mound behind.

As a star repeatedly loses its outer layers, its overall mass decreases. This can also expose deeper, hotter parts of the star, potentially altering its appearance and emitted radiation.

Mergers create giants

When stars merge, their cores combine as well. This dramatically increases the central mass and density of the resulting star.

The larger mass allows for stronger gravitational compression within the core, boosting nuclear fusion rates. This gives the newly merged star access to a much more substantial fuel supply (hydrogen) even though it may have originated from an older population of stars.

The increased hydrogen fusion and resulting energy output make the merged star hotter, brighter, and sometimes even larger than before. This excess energy production mimics the characteristics of a newly formed, young star, even though the original stars were much older.

“A few stars win the collision lottery,” notes Rose. “Through collisions and mergers, these stars collect more hydrogen. Although they were formed from an older population, they appear rejuvenated.”

These massive, merged stars have significantly shorter lifespans. They burn through their fuel at an accelerated pace and quickly reach their demise.

Key implications

The study of stars orbiting Sagittarius A*, the supermassive black hole at the center of our Milky Way, carries profound implications for astrophysics and our understanding of the universe. “It’s an environment unlike any other,” Rose emphasizes.

Here are the key implications of this research:

  1. Galactic evolution insights: Understanding the dynamics near Sagittarius A* provides a clearer picture of the processes that drive galactic evolution. By observing how stars interact with the supermassive black hole and each other, scientists can infer how galaxies grow and change over time.
  2. Black hole growth mechanisms: The study sheds light on how supermassive black holes, like Sagittarius A*, accumulate mass. The interactions and mergers of stars in its vicinity offer clues about the mechanisms that feed these colossal entities, influencing their growth and the evolution of their host galaxies.
  3. Stellar population dynamics: The research highlights the violent environment near supermassive black holes, where stellar collisions and mergers are common. This contributes to our understanding of how stellar populations evolve in such extreme conditions, affecting the distribution and characteristics of stars in galactic centers.
  4. Testing theories of gravity: The extreme environment around Sagittarius A* provides a natural laboratory for testing theories of gravity. Observations of star orbits near the black hole can offer insights into the behavior of space and time in strong gravitational fields, testing Einstein’s general theory of relativity and searching for new physics.

Connecting Sagittarius A* star interaction to other galaxies

The recent findings surrounding Sagittarius A* serve as a vital key to unlocking the mysteries of the universe. This research not only illuminates the peculiarities within our own galaxy but also extends its reach to supermassive black holes residing in distant galaxies.

By comparing the dynamics of stars orbiting Sagittarius A* with those around other black holes, astrophysicists can identify universal patterns and behaviors, as well as notable exceptions.

Similarities observed across different galaxies include the high-speed orbits of stars around these colossal entities and the gravitational influence exerted by the black holes, shaping the stellar environment.

Such universal traits underscore the fundamental laws of physics that govern celestial phenomena, irrespective of the galaxy in question.

Moreover, the processes of stellar collisions, mergers, and the resultant formation of “rejuvenated” stars near Sagittarius A* mirror those occurring around other supermassive black holes, suggesting a common evolutionary mechanism for stellar populations in these extreme environments.

However, differences also emerge, primarily due to the varied masses of supermassive black holes and their unique histories. For instance, the rate of star formation and the prevalence of certain types of stars in the vicinity of different black holes can offer clues about the age and development of the host galaxies.

Study significance

Gaining a detailed understanding of our galaxy’s supermassive black hole, Sagittarius A*, is crucial for the advancement of astrophysics. It allows scientists to refine their models of how galaxies and their central black holes evolve over time. By closely examining the behavior of stars and matter around Sagittarius A*, researchers can identify patterns and processes that are likely common to many galaxies.

It provides a baseline for testing hypotheses about how galaxies form, evolve, and interact with their central black holes. Furthermore, it enriches our comprehension of how such massive objects influence cosmic structure, guiding the development of theories that address broader astrophysical questions.

Our galaxy’s heart is a place of constant change, where stars collide, merge, and undergo significant transformations. This research highlights the surprising and often violent processes that shape our cosmic neighborhood.

image Credit: ESO/L. Calçada/

The study is published in The Astrophysical Journal Letters.


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