The Earth, Moon, and Sun form a stable 3-body system, but there is no simple equation one can apply to 3-body systems to predict their orbits. This 3-body problem becomes increasingly complicated as more objects are added to the system for analysis. Simulations show that 3-or-more-body systems are chaotic and highly dependent on the initial conditions to predict their ultimate outcomes. Given that gravity only attracts objects together, one surprising but not uncommon outcome of these simulations is that one or more objects get flung out far from the others, or ejected.
A team of astronomers looked at a very complicated multi-body system to investigate how stars can be ejected from the center of the Milky Way Galaxy, where they are densely packed. They suspect that pairs of stars orbiting each other may pass by the supermassive black hole at the center of the Milky Way, known as Sgr A*, then one of the companions gets captured by the black hole, and the uncaptured companion gets flung away from the Galactic center. This process is known as the Hills mechanism, and can cause the ejected star to move at a breakneck 3,500 kilometers per second. That’s almost 13 million kilometers per hour (km/hr) or 8 million miles per hour (mph)!
Such fast-moving stars have already been documented. Instead, the team looked for stars slower than 100 kilometers per second, which is 360,000 km/hr or 220,000 mph. If these stars exist, they could get stuck in distant regions of the Galaxy, skewing the results from studies of stars that have always been there.
First, they conducted simulations to check if they could, in principle, find slow-ejected stars in the exterior of the Galaxy. They started with a 2-dimensional grid filled with stars at different densities to represent the Milky Way, then they used a program called Agama to calculate how the orbits of the stars around the center of the Milky Way would change over 5 billion years.
Finally, they applied the programs MIST and PyGaia to calculate how bright the stars would appear and how uncertain their distances would appear to an Earth observer. Their overall result from this part of the study suggested a very low, difficult to observe ejection rate of 1 every 10,000 years for slow stars. They noted that this estimate could be considerably lower than the real figure for the Milky Way because their simulations don’t take into account events like black hole mergers at the Galactic center.
For the second part of the study, the team tried to find evidence of slow-ejected stars in the actual Milky Way Galaxy. Stars from this region that have been flung out are distinct from those in the exterior of the Galaxy, containing more metals and following different orbital paths. The team examined 2 prior surveys of data about stars in the Milky Way, utilizing the unique information from each to identify stars with these characteristics.
One of these surveys was the Milky Way Survey from the Dark Energy Spectroscopic Instrument or DESI, a device fitted onto the 4-meter Telescope located at Kitt Peak National Observatory. This survey’s data allowed them to determine how much metal a star contained and what its velocity is relative to Earth for around 5 million stars. The other survey came from the Gaia satellite, which precisely maps the positions of almost 2 billion stars. This data allowed the team to accurately calculate the DESI stars’ actual motion in the sky.
The team didn’t conclusively find any ejected stars in the data they gathered. They noted that this doesn’t necessarily mean there are no ejected stars in the Milky Way’s exterior, considering the limitations of the data, potential biases in their selection, and possible flaws in their detection process. However, they did claim that their inability to find ejected stars in overlapping datasets from DESI and Gaia suggests that the real rate of ejection is low, only 1 every 360 years at most, and that this limit has held for the last 5 billion years. They also suggested that future astronomers use their results to estimate the timeline for events in the Milky Way’s history, especially the mysterious past of Sgr A*’s mergers with other black holes, of which little is known.
