Free Astronomy Magazine September-October 2025

SEPTEMBER-OCTOBER 2025 T his selection of images of external galaxies illustrates three encounter sce- narios between our Milky Way and the neighboring Andromeda galaxy. In the panel number 1, a wide-field DSS image showing galaxies M81 and M82 serves as an example of the Milky Way and Andromeda passing each other at large distances. The panel number 2 shows NGC 6786, a pair of interacting galax- ies displaying the telltale signs of tidal disturbances after a close encounter. The panel number 3 shows NGC 520, a cosmic train wreck as two galaxies are ac- tively merging together. [NASA, ESA, STScI, Till Sawala (University of Helsinki), DSS, J. DePasquale (STScI)] His team includes researchers at Durham University, United King- dom; the University of Toulouse, France; and the University of West- ern Australia. They found that there is approximately a 50-50 chance of the two galaxies colliding within the next 10 billion years. They based this conclusion on computer simulations using the latest observa- tional data. Sawala emphasized that predicting the long-term future of galaxy interac- tions is highly uncertain, but the new findings chal- lenge the previous con- sensus and suggest the fate of the Milky Way re- mains an open question. “Even using the latest and most precise observational data available, the future of the Local Group of several dozen galax- ies is uncertain. Intriguingly, we find an almost equal probability for the widely publicized merger scenario, or, conversely, an alternative one where the Milky Way and Androm- eda survive unscathed,” said Sawala. The collision of the two galaxies had seemed much more likely in 2012, when astronomers Roeland van der Marel and Tony Sohn of the Space Telescope Science Institute in Baltimore, Maryland published a detailed analysis of Hubble observa- tions over a five-to-seven-year pe- riod, indicating a direct impact in no more than 5 billion years. “It’s somewhat ironic that, despite the addition of more precise Hubble data taken in recent years, we are now less certain about the outcome of a po- tential collision. That’s be- cause of the more complex analysis and because we consider a more complete system. But the only way to get to a new prediction about the eventual fate of the Milky Way will be with even better data,” said Sawala. Astronomers considered 22 different variables that could affect the potential collision between our gal- axy and our neighbor, and ran 100,000 simulations U sing data from the Hubble Space Telescope, scientists have developed this simulation of the head-on collision of our Milky Way galaxy and the Andromeda galaxy. Actu- ally, there is only a 50-50 chance of the two galaxies collid- ing within the next 10 billion years. [NASA/ESA, space.com]