Milky Way and Andromeda: A 90% Likely Merger express
The Milky Way and the Andromeda Galaxy are like two vast flocks of birds drawing together in the celestial expanse. New data from the Gaia telescope shows: Andromeda barely deviates sideways. The chance of their merger is a whopping 90%. Even the gentlest crosswind could steer their paths apart, but calculations say: it’s almost nonexistent.
Their trajectory is also influenced by smaller satellites — the Large Magellanic Cloud and the Triangulum Galaxy. Through their gravity, they subtly nudge the main participants, much like birds from neighboring flocks unintentionally adjusting the overall course. Gravity here isn’t magic but the manifestation of spacetime curvature, akin to a dip in stretched fabric under the weight of balls. And invisible dark matter adds mass to that fabric, steering the motion.
In 6.5 billion years, the galaxies will merge into a single system. The Earth’s sky will change, but the collision itself is safe: stars are so sparsely scattered that they easily pass by one another.
Original abstract
as written by the authors arXiv:2603.22863
It has long been predicted that the Milky Way (MW) will eventually merge with Andromeda (M31), a view reinforced by \textit{HST} measurements indicating a small M31 transverse velocity. However, using updated \textit{Gaia}-based proper motions (PMs) and including the dynamical influence of the Large Magellanic Cloud (LMC) and M33, Sawala et al. reported an MW--M31 merger probability of $$\sim$$50\% within 10 Gyr, leaving the fate of the Local Group (LG) uncertain. Adopting their semi-analytic framework, we revisit this problem with the latest and most precise \textit{Gaia}-based PMs for M31 and M33, corrected for systematic offsets in \textit{Gaia} astrometry. In our fiducial model, the MW--M31 merger probability rises to 90\%, with a median merger time of $$6.5_{-1.5}^{+1.3}$$ Gyr, broadly restoring the classical picture. A sensitivity analysis shows that the merger probability depends strongly on the adopted M31 PM through two channels: a direct effect via the radial-tangential balance of the MW-M31 orbit, and a satellite-mediated effect, where the M31 PM fixes the orbital plane and determines how satellite-induced barycentric reflex motions project onto it, either promoting or suppressing a merger. Given this sensitivity, current measurements, while favoring a high merger probability, remain inconclusive, spanning from 64.7\% to 100\% across the 2$$\sigma$$ PM region. Future PM measurements with uncertainty of $$\lesssim2\,\upmu\mathrm{as\,yr^{-1}}$$ will be required to reach a firm conclusion, i.e., to constrain the probability range within 10\% at the 2$$\sigma$$ level.
Galaxy mergers don’t destroy: they create: millions of new stars are born from gas and dust, like a fireworks display lasting billions of years.
In science fiction, such as the series ‘Star Trek’, such mergers are sometimes depicted as arenas of battle. In reality, it’s not a collision but a slow blending, like two immense bird flocks merging into one.