The Milky Way and Andromeda Galaxy Are Already Exchanging Stars

High-velocity stars escaping at over 700 km/s are part of the Milky Way-Andromeda star swap.

The Andromeda Galaxy is a spiral galaxy and is the closest galaxy to the Milky Way. It is also the largest galaxy in the Local Group. Last research has unveiled that these two galaxies are already in a slow-motion collision course despite their being about 2.5 million light-years away from each other. The full merger of the two galaxies into a single elliptical galaxy will require billions of years, but the initial indication of this cosmic collision has been acquired by the European Space Agency’s Gaia telescope, which shows that stars have begun migrating from the Milky Way to Andromeda and vice versa.

High-Velocity Stars and Galactic Escape

Andromeda Galaxy (M31) is the distant spiral galaxy from the Milky Way, but the first in the vast system with the incredible surface brightness. According to recent findings, the two galaxies, respectively, are 2.5 million light-years apart and are on the verge of a slow-motion collision. Even though into a single elliptical galaxy this merger is scheduled billions of years later, flyby effect records of the European Space Agency’s Gaia telescope confirm that the first phase of the cosmic interaction has begun, with stars being exchanged between The Milky Way and Andromeda.
Usually, Milky Way stars follow circular orbits with the Sun taking about 220 million years to cover one round at a speed of 250 km/s, which is less than the necessary speed to allow the Sun to escape from the galaxy’s gravity. However, in 2005, a group of scientists of the Harvard-Smithsonian Center for Astrophysics identified a very unusual event occurring – one of the stars was moving away from the galactic center of the Milky Way with the velocity of more than 700 km/s, which is fast enough to get out of the black hole’s grip. These stars called “high-velocity stars” (HVS) have been seen in more significant numbers now.
The disruption of one of the binary star systems in the Milky Way’s core area in interaction with central supermassive black holes is a probable cause of the existence of these high-velocity stars. In the event of a capture of one star by the black hole, the companion had been subject to ejection, and, therefore, the speed carried by this star could be enormous. There is another explanation for the origin of high-speed stars, namely, explosion of a supernova in a binary system may lead to the last star being expelled with an enormous force.

The Andromeda Connection

One more theory is that several of these high-speed stars are not actually Milky Way natives but have traveled from Andromeda. In order to confirm this assumption, a group of researchers from Karlsruhe Institute of Technology (KIT), headed by Lukas Gulzow, investigated the phenomenon using data captured by the Gaia space telescope. With the Gaia data, the researchers tracked around 18 million stars of high velocity comparing them to the dynamics model of the Local Group.

Analysis can also show definitively that some of these stars had originated from Andromeda. In the same way, it is quite possible that certain high-speed Milky Way stars are currently unidirectionally traveling to Andromeda. This movement of stars suggests that the process of gravitational forces is already taking place between the two galaxies even though the merger is millions of years away.

Future of High-Velocity Stars in the Milky Way

Andromeda and the Milky Way are coming together at a rate of 85 km/s. The Milky Way’s gravitational force as it increases these rogue stars accelerates them, a phenomenon referred to as “hyperbolic excess of speed”. This energy gives them a push, and most of the time, these stars go on interstellar journeys that end up with them leaving the Milky Way. En route, interactions with the stars of the galaxy can slow down some of them, if it happens several times, which will make them “permanent residents” eventually.
Even though Andromeda and the Milky Way are still apart, the stars moving between them already mark the start of their future long-awaited merger. The event portrayed here unveils the magnitude of the processes in which galaxies are engaged and demonstrates that they can even communicate over distances spanning millions of light years, thus laying out the groundwork for the eventual cosmic unification.

According to Universe Today