Chapter 1: The Enigmatic Stars of Sgr A*

What insights do the recently identified stars in close proximity to our galaxy’s central black hole reveal about relativistic phenomena?

A striking illustration shows stars revolving around the supermassive black hole situated at the heart of our galaxy. Credit: Nicholas Hunter.

A recent publication in The Astrophysical Journal by a group of European astrophysicists presents findings on stars orbiting the supermassive black hole known as Sgr A*. Among these, S4711, a cluster of stars recognized for their proximity to the black hole, serves as “ideal subjects to observe gravitational effects” and their impact on stellar orbits, particularly as they navigate rapidly around the black hole (Peißker et al., 2020). Researchers are keen to investigate various phenomena related to these stars, notably how their velocity alters the shape of their orbits. Additionally, they aim to understand how these orbital shapes can be used to test Einstein’s General Theory of Relativity in the context of these high-speed stars.

In August 2017, a paper authored by Andreas Parsa, Vladimir Karas, and their collaborators examined how the orbits of these stars have been influenced over time. They measured the stars' velocities relative to the speed of light, termed the relativistic parameter. Intriguingly, their findings indicated a correlation, suggesting that the orbits of stars located extremely close to Sgr A* undergo shifts influenced by their varying speeds during their orbital paths. The study also noted that “the effects may differ from one orbital cycle to another,” but emphasized that “a substantial population of massive objects (each with several tens of solar masses) impacts the orbit of S2” (Parsa et al., 2017, p. 18). Further research is necessary to solidify these conclusions.

One major factor affecting the structure of these stellar orbits appears to be their velocities approaching that of light as they travel around the black hole. The elliptical nature of these orbits makes them ideal candidates for testing relativistic effects such as mass dilation, particularly as they reach speeds close to light. However, accurately determining the mass of these stars, known as squeezars, has proven challenging. Over several years, observations have indicated fluctuations in the mass of these stars as their speeds escalate, suggesting the feasibility of studying mass dilation—a phenomenon predicted by Einstein’s Relativity, where an object's mass varies based on its velocity.

In June 2003, a pivotal study titled “Squeezars: Tidally Powered Stars Orbiting a Supermassive Black Hole” explored the interactions between these stars and Sgr A*. The researchers noted that these stars exhibit characteristics that set them apart from ordinary stars, such as their heightened brightness due to tidal forces from the black hole. By analyzing the thermal energy emitted by these stars, they estimated the evolution of the squeezars’ orbits, temperature, and luminosity. They proposed the existence of potentially 120 stars in close proximity to Sgr A* (Alexander & Morris, 2003, p. 28), laying the groundwork for future research.

Recent studies have shifted focus from S2, the star initially thought to be closest to Sgr A*, to identifying other stars with similar orbital characteristics. Research indicates that while S2 is indeed near the black hole, several other stars might occasionally travel closer. In a June 2020 publication, Florian Perissker and colleagues highlighted how these stars form an 'X'-shaped configuration, noting that “no significant disturbance of the system occurred that could have led to a random arrangement of the stellar bodies” (Ali et al., 2020, p. 17). This indicates that the stars have been drawn into their orbits rather than forming around Sgr A*.

However, uncertainties remain as earlier observations mistakenly identified S2 with another star in the cluster, S62. Recent insights from Peißker's team revealed that S2’s orbit could shift by as much as 10 degrees, with the next adjustment anticipated by late 2022. After refining previously held data, they determined that S2 is approaching the black hole at a distance of 5.2 astronomical units, roughly five times the distance between Earth and the Sun. S62 is expected to be a focus of future observations, but caution is needed to differentiate between stars that might be overlapping in their orbits.

Understanding the distance of these stars from Sgr A* and their velocities is crucial for testing relativistic effects. While S2 has been found to move at 3 percent of the speed of light, other stars have been observed orbiting the black hole at speeds reaching 6.7 percent of the speed of light. Accurate measurements of these stars' masses over time will provide vital information regarding mass dilation as they near light speed. Currently, Peißker’s team has approximated the mass of S62 when it is closest to Sgr A*, and future observations will allow for comparisons that could further illustrate mass dilation effects during their orbits.

In 1937, Albert Einstein, alongside Leopold Infeld and Banesh Hoffmann, published a paper asserting that “the only equations of gravitation which follow without ambiguity from the fundamental assumptions of… relativity (are those) for empty space” (Einstein et al., 1937, p. 65). This paper focused on the mass and time dilation of orbiting objects. The degree to which a star's speed approaches that of light and how its mass and orbital dynamics are affected remains a critical area for astronomers to empirically explore. The data collected from these stars in close proximity to the supermassive black hole Sgr A* in such extreme conditions will likely be pivotal as further investigations into Einstein's theories unfold.

Chapter 2: Videos on Stellar Orbits

The first video, "Fastest Star Found Orbits Sgr A* Black Hole," delves into the discovery of stars with remarkable speeds orbiting the Milky Way's central black hole, providing insights into relativistic effects.

The second video, "See stars orbit Milky Way's black hole Sagittarius A* in this zoom in," offers a closer look at how stars navigate the gravitational pull of Sgr A*, enhancing our understanding of stellar dynamics.