The Milky Way and Andromeda have appeared locked on a destructive course for decades. Now, thousands of simulations suggest their collision is not destiny, but a contest between gravity, uncertain measurements and two smaller neighboring galaxies.<\/p>\n
Astronomers have known for a century that Andromeda is moving toward the Milky Way<\/a>. Because the two spiral galaxies dominate the Local Group, which contains about 100 smaller galaxies, their approach became one of astronomy\u2019s most familiar forecasts.<\/p>\n The standard account held that the pair would collide in five billion years. Gravity would distort both galaxies and eventually combine them into a large elliptical system sometimes nicknamed \u201cMilkomeda.\u201d<\/p>\n Effects of M33 and the LMC on the trajectory of the MW. Panels in the top row are projected in the orbital plane defined by the initial MW and M31 positions and velocities, whereas those in the bottom row are projected perpendicular to this orbital plane. (CREDIT: Nature Astronomy) A forecast built from 50,000 possible futures<\/p>\n A team led by Till Sawala of the University of Helsinki<\/a> modeled the Milky Way, Andromeda, the Triangulum galaxy M33 and the Large Magellanic Cloud.<\/p>\n The team used measurements from the Hubble Space Telescope and the European Space Agency\u2019s Gaia mission. The observations track galactic positions and motion across the sky.<\/p>\n Such sideways motion is exceptionally difficult to measure at galactic distances. Even small errors can produce very different paths when projected billions of years ahead.<\/p>\n Instead of selecting only the most likely value for each measurement, the team sampled a broad range of plausible positions, velocities and masses. Its main model ran 50,000 possible realizations.<\/p>\n The Milky Way\u2019s mass was set near one trillion times the Sun\u2019s mass. Andromeda was estimated at about 1.3 trillion solar masses. M33<\/a> and the Large Magellanic Cloud were smaller but still massive enough to alter the larger galaxies\u2019 motion.<\/p>\n The simulations followed the systems for 10 billion years or until a merger. They included dynamical friction, which causes galactic orbits to lose energy and decay.<\/p>\n Distance between the MW and M31. On each panel, we show 100 realizations of our fiducial Monte Carlo model and also state the probability for a MW-M31 merger within 10\u2009Gyr. (CREDIT: Nature Astronomy) Two smaller galaxies shift the outcome<\/p>\n M33 and the Large Magellanic Cloud pulled the forecast in opposite directions.<\/p>\n M33 increased the chance of a Milky Way-Andromeda merger. Its gravity reduced Andromeda\u2019s sideways motion relative to the Milky Way, encouraging a closer encounter.<\/p>\n The Large Magellanic Cloud had the opposite effect. Although its mass is only about 15% of the Milky Way\u2019s, its pull changes the Milky Way\u2019s motion and adds movement outside the original orbital plane.<\/p>\n \u201cIn earlier studies, researchers often focused only on the most likely measurements,\u201d Sawala said. \u201cWe’ve simply been able to explore a much larger space of possibilities, thanks to better data.\u201d<\/p>\n A model containing only the Milky Way and Andromeda<\/a> produced mergers in slightly fewer than half of the simulations. Adding M33 raised the probability to about two-thirds. Adding only the Large Magellanic Cloud lowered it to slightly more than one-third.<\/p>\n When all four galaxies were included, the chance of the Milky Way and Andromeda merging within 10 billion years stood just above 50%.<\/p>\n Effects of M33 and the LMC on the trajectory of M31. Panels in the top row are projected in the orbital plane defined by the initial MW and M31 positions and velocities, whereas those in the bottom row are projected perpendicular to this orbital plane. (CREDIT: Nature Astronomy) <\/p>\n The timing also shifted. Among simulations that ended in a merger, the median collision occurred after 7.6 billion years, much later than the familiar five-billion-year prediction.<\/p>\n Only about 2% merged during the next five billion years.<\/p>\n The Large Magellanic Cloud<\/a> always merged with the Milky Way in the calculations. M33 had an 86% chance of joining Andromeda first.<\/p>\n A close pass could preserve both spirals<\/p>\n The simulations produced two distinct outcomes.<\/p>\n In roughly half, the Milky Way and Andromeda passed close enough for their orbits to decay. They then merged, usually after an initial encounter, creating an intermediate-mass elliptical galaxy.<\/p>\n In most remaining cases, the galaxies never approached within about 200 kiloparsecs, or roughly 650,000 light-years. Without a close pass, dynamical friction could not pull them together.<\/p>\n \u201cThese results significantly alter our understanding of our galaxy\u2019s fate,\u201d said Alis Deason of Durham University<\/a>\u2019s Institute for Computational Cosmology. \u201cWe once thought we\u2019d merge to form a colossal galaxy nicknamed \u2018Milkomeda.\u2019 Now, there’s a real chance we might avoid this entirely.\u201d<\/p>\n Possible future MW-M31 orbits. Colored lines show probability densities for the positions of the MW (bright yellow to blue) and M31 (bright orange to purple) in 100 Monte Carlo samples of the fiducial model, integrated over 10\u2009Gyr or until the merger. (CREDIT: Nature Astronomy) <\/p>\n Earlier calculations were not necessarily wrong. Using the same restricted assumptions, the team recovered similar paths and timing.<\/p>\n \u201cWhen we used the same initial assumptions as earlier studies, we found the same results,\u201d Sawala said. \u201cThe difference now is we have a broader set of data to explore, making our predictions more nuanced.\u201d<\/p>\n The outcome proved especially sensitive to Andromeda\u2019s proper motion and the masses of all four galaxies<\/a>. Within the allowed range of Andromeda measurements, some combinations produced a merger probability above 90%, while others brought it close to zero.<\/p>\n More precise observations will narrow those possibilities, but Andromeda alone will not settle the question. The distances, motions and masses of M33, the Large Magellanic Cloud and the two major galaxies also matter.<\/p>\n The limits of predicting a distant future<\/p>\n They treat the galaxies as smooth, spherical dark-matter haloes with fixed masses and concentrations. They do not fully include substructures, changing mass, gas behavior or the wider cosmic surroundings.<\/p>\n About a quarter of the Local Group\u2019s bound mass may lie outside the two main haloes, adding uncertainty over billions of years.<\/p>\n<\/p>\n The team tested different merger distances and treatments of dynamical friction. Those changes did not erase the central result: current observations support both survival and collision at nearly equal rates.<\/p>\n