The conventional narrative is that the Milky Wan and Andromeda galaxies are falling towards each other, fated towards a collision in five billion years, followed by a merger that will take place over another two billion years. The two spiral galaxies will then coalesce into a single elliptical galaxy. This has been the scientific understanding for decades, but as we understand more about the universe, the story gets more complex, more uncertain, and more fascinating. The two galaxies are separated by a distance of 2.5 million years, and are the largest galaxies in the Local Group, consisting of Milky Way, Andromeda and dozens of dwarf galaxies. We do not know how many of these dwarf galaxies are gravitationally bound to the Local Group, and how many are merely zipping through the neighborhood.
Our most refined observations suggest that the two spiral giants are hurtling towards each other at 110 kilometers per second. There are unknowns in the timing, and even the certainty of the rendezvous. The two galaxies need not necessarily merge into a single, colossal entity, and may merely graze each other like dancers in a fleeting pas de deux. There is roughly a 50 per cent chance that no merger will take place in the next 10 billion years. The sideways motion of Andromeda in relation to the Milky Way is notoriously hard to measure. In simulation where this parameter is adjusted shows merger times varying between 6.7 billion years to 544.6 billion years, an unimaginably vast period of time that dwarfs the present age of the universe.
The third-largest member of the Local Group is the Triangulum Galaxy (M33), that is gravitationally influencing the path of Andromeda. The maths to model the gravitational interactions of the Milky Way, Andromeda and the Triangulum Galaxy simply does not exist, and is known as the Three Body Problem. Triangulum is largely nudging the two galaxies towards a merger, but another resident of the Local Group, the Large Magellanic Cloud pulls the interaction in the other direction, making a collision less likely. The latest resolution of the timing of the merger pushes it to between 7.6 and 8.0 billion years.
Most star systems would be unaffected by the merger, despite its scale. The vastness of space and just how tiny stars are in comparison reduces the odds of two star systems colliding to vanishingly small. The Solar System, located at a distance of 27,000 lightyears from the core of the galaxy would require a change in address, flung towards the outer regions, doubling its distance from the galaxy core by two or three times. There is a slight chance that the Sun would be slung clean out of the galaxy, becoming a wandering star. Even this extreme fate would not affect the Solar System, though the aging Sun would be on the verge of transforming into a Red Giant.
Milkdromeda Rises
While most stars would miss each other, gas clouds would slam together, triggering new phases of star formation. The voids of space would be lit up by brilliant stellar nurseries, shining in the light of thousands of energetic newborn stars. The galaxies would be distorted during the merger, with the two cores becoming bright blobs spiraling inwards towards each other. The spirals of both the galaxies would lose their distinct shapes, blending into a single elliptical galaxy, that appears as a glowing orb of stars stretching across hundreds of thousands of lightyears. The dwarf galaxies in the local group would fall in, over the next 150 billion years, completing the evolution of the local group. The rest of the distant galaxies in the sky would fade over billions of years, their light lost forever.
Both the Milky Way and Andromeda are falling towards the barycenter of the Local Group, the gravitational midpoint of all the galaxies in the neighbourhood. The gas, dust and star systems are merely following a natural path down the gravitational wells created by the mass densities. At the heart of the Milky Way is Sagittarius A*, a black hole containing 4.3 million solar masses. At the core of Andromeda is a much more massive black hole, estimated to contain as much mass as a 100 million Suns. The two black holes are expected to spiral towards each other, forming a close binary system as the two galaxies merge.
Life might emerge in the planets assembled from the material leftover from the newborn stars. Humanity, if it endures, might watch the chaotic skies birth new stars from distant outposts. Only remote cameras can capture the views simulated above, on an inhospitable Earth. The Sun will be a fading ember, most of its nuclear fuel spent. The two galaxies may miss in the next close approach, brushing past each other exchanging some star gas, ice, dust and a few stray stars. The spiral arms of the Milky Way may remain intact for another epoch. The Milkdromeda merger over the next few billion years remains a hypothetical, but there is more certainty regarding the ultimate fate of the Local Group.
Image Sources:
Milky Way/Andromeda collision (artist’s impression): James Gitlin
Collision Scenario: NASA; ESA; A. Feild and R. van der Marel, STScI
Collision Sequence: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas, and A. Mellinger
Sources:
Kinematics of the Local Group gas and galaxies in the hestia simulations
The M31 Velocity Vector. I. Hubble Space Telescope proper-motion measurements.
Andromeda on collision course with the Milky Way
Apocalypse When? No Certainty of a Milky Way — Andromeda Collision
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