Edited by Maryam Kamal.
The Andromeda galaxy is now hurtling towards us at a rapid 110 kilometres per second. Andromeda is roughly 2.5 million light-years away and astronomers calculate that an inevitable collision with our galaxy, The Milky Way, will take place 4 billion years in the future. The collision will ultimately result in a merger between the two most massive galaxies in the Local Group, Andromeda and The Milky Way,¹ thus leaving lasting consequences on the surrounding galaxies. The resulting elliptical galaxy has often been nicknamed Milkdromeda.²
From 2002 to 2010, astronomers used data from the Hubble Space Telescope to track the motion of stars in Andromeda relative to background galaxies. This is to determine Andromeda’s lateral velocity relative to the Milky Way, which was found to be smaller than its speed of approach, showing scientists that a merger will occur in the future. An inevitable collision was confirmed due to the gravitational attraction between the galaxies being greater than the repulsive force of dark energy. This means that the force pulling the galaxies together is stronger than the force pushing them apart. Furthermore, astronomers have also predicted that the Triangulum galaxy, the third-largest galaxy in the group, will be involved in the collision. This is due to the chance that it will merge with the new supermassive elliptical galaxy even further into the future. However, “there is a 7% probability that M33 [Triangulum] gets ejected from the Local Group”,³ sending it off into the depths of space.
The Milky Way and Andromeda contain supermassive black holes at their centres. As the galaxies merge, through dynamical friction with surrounding stars, the gravitational interactions would cause a fall in momentum and kinetic energy of the black holes. Eventually, there will be a black hole merger at the new galactic core. Meanwhile, the stars with gained momentum and kinetic energy may be ‘slingshotted’ into the outer reaches of the new galaxy. A few unlucky stars will be completely expelled from the galaxy, left to wander intergalactic space until they find a new home. One of them could be our Sun.
Scientists have calculated that there is a 12% chance the Sun will be ejected - along with the Earth and other planets.⁴ However, scientists have stated a more likely prediction whereby there is a 50% chance the Solar System will remain in the new galaxy, albeit further away from the galactic centre.⁵ Either way, there will be minimal adverse effects on the Solar System.
In fact, most stars in both galaxies will be left undisturbed due to the vast distances between the stars in either galaxy.⁶ During the collision of two spiral galaxies, massive amounts of hydrogen usually end up being compressed, creating a hotbed of star formation. An example of the phenomena is in the collision between the Antennae galaxies as shown in Image 2, where large amounts of colliding gas can be seen lighting up the vicinity in a brilliant display.
However, astronomers doubt that such a spectacular show will occur in the Andromeda–Milky Way collision due to the little amount of hydrogen gas left in both galaxies. Therefore, it is unlikely to facilitate the explosive star formation present in the Antennae galaxies, which contain sufficient hydrogen gas.⁷ The resulting shape of Milkdromeda (or Milkomeda) is yet to be accurately determined, but it will most likely be a giant elliptical galaxy. A lenticular galaxy could form if sufficient gas is left in both galaxies.⁸
From the moment of the first contact between the galaxies, it will take billions of years before the collision properly concludes. Image 3 below is a render of what the collision may look like from Earth.
First Row, Left: Present day
First Row, Right: In 2 billion years
Second Row, Left: In 3.75 billion years
Second Row, Right: In 3.85 billion years
Third Row, Left: In 3.9 billion years
Third Row, Right: In 4 billion years
Fourth Row, Left: In 5.1 billion years
Fourth Row, Right: In 7 billion years
Credits: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas, and A. Mellinger
Glossary:
Local group: a group of gravitationally-bound galaxies in a sphere roughly 10 million light-years in diameter
Lateral velocity: component of a celestial body's velocity that is at 90° to its line-of-sight velocity
Lenticular galaxy: a type of galaxy that has characteristics reminiscent in both spiral and elliptical galaxies
References:
Cowen, R. (2012, May 31). Andromeda on collision course with the milky way. Nature News. https://www.nature.com/articles/nature.2012.10765
Behold our dazzling night sky when the milky way collides with Andromeda in 4 billion years. kottke.org. (n.d.). https://kottke.org/19/10/behold-our-dazzling-night-sky-when-the-milky-way-collides-with-andromeda-in-4-billion-years
Marel1, R. P. van der, Besla2, G., Cox3, T. J., Sohn1, S. T., & Anderson1, J. (2012, June 8). IOPscience. The Astrophysical Journal. https://iopscience.iop.org/article/10.1088/0004-637X/753/1/9
Cain, F. (2015, December 26). When our galaxy smashes into Andromeda, what happens to the sun? Universe Today. https://www.universetoday.com/1604/when-our-galaxy-smashes-into-andromeda-what-happens-to-the-sun/
Muir, H. (2007, May 14). Galactic merger to 'evict' sun and Earth. New Scientist. https://www.newscientist.com/article/dn11852-galactic-merger-to-evict-sun-and-earth/?ignored=irrelevant#.VDVdDvl_uVB
Brian Dunbar. (n.d.). NASA - NASA's Hubble shows milky way is destined for head-on collision. NASA - NASA's Hubble Shows Milky Way is Destined for Head-On Collision. https://web.archive.org/web/20140701085917/http://www.nasa.gov/mission_pages/hubble/science/milky-way-collide.html
Cox, T. J., & Loeb, A. (2008). The collision between the milky way and Andromeda. Monthly Notices of the Royal Astronomical Society, 386(1), 461–474. https://doi.org/10.1111/j.1365-2966.2008.13048.x
Ueda, J., Iono, D., Yun, M. S., Crocker, A. F., Narayanan, D., Komugi, S., Espada, D., Hatsukade, B., Kaneko, H., Matsuda, Y., Tamura, Y., Wilner, D. J., Kawabe, R., & Pan, H.-A. (2014). Cold molecular gas in merger remnants. I. Formation of Molecular Gas Disks. The Astrophysical Journal Supplement Series, 214(1), 1. https://doi.org/10.1088/0067-0049/214/1/1
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