A meteor collision with planet Earth is the forefront of science fiction narratives. However, the crash of multiple galaxies is the reality often ignored. These collisions are a seldom, yet reoccurring effect, notwithstanding the age or rate of expansion of the universe as in the vacuum created new galaxies form and clash, thus the probability of these crashes remains almost constant. The expansion of the universe was proven by the red shift of light and means that galaxies are moving away from each other, thus the probability of a collision between two particular galaxies theoretically decreases. Although more common between large and small galaxies, the Andromeda-Milky Way collision, or Milky Way vs. Andromeda as NASA prefers to call it, is no such case; it is a galaxy collision which will take in approximately 4 billion years time between the two largest galaxies in the Local Group.

The Andromeda galaxy, our closest galactic neighbour, is located ~2.5 million light-years (one light year is approximately 10 trillion kilometres) away from the Milky Way. However, it is moving towards the Milky Way at a speed of 402,336 kilometres per hour. This speed would allow travel from the Earth to the moon in under an hour; the measurements were taken using the Doppler Effect, the observation of a change of frequency and wavelength of a wave relative to a stationary observer. Such attraction is being caused by the mutual pull of gravity between two galaxies and the invisible dark matter that encompasses both galaxies.

After a century of speculation and much debate about the fate of the collision scientists have come to the conclusion that there will be a definite clash. However, the exact fate of our solar system is yet to be discovered. Two scientists working with the Harvard-Smithsonian Centre for Astrophysics stated that when, and whether, the two galaxies will collide depends solely on the transverse velocity of Andromeda. Current calculations also suggest a 50% chance that in a merged galaxy, our solar system would be swept away almost three times further away from the galactic core than it is presently located. The evidence was never definite as astronomers could only measure the velocity of Andromeda along the line of sight to the Earth because they could not observe sideways motion of the galaxy. A definitive prediction was made after repeatedly observing regions of the galaxy over an eight year period using the Hubble Space Telescope by a group of NASA astronomers at the Space Telescope Science Institute. The motion of stars, relative to the background galaxy, was thus possible to be observed over the time period, additionally informing about the galaxys motion. Experts estimated the complete mergence of the two into a single elliptical galaxy at circa 6 billion years. Furthermore, a second galaxy, Triangulum, is also predicted to collide with the Milky Way. However, this clash would be far smaller in size in comparison and is most likely to be a collision after the clash of Andromeda and Milky Way. Nonetheless, a 9% chance is existent that Triangulum will collide before Andromeda.

Unless the human race manages to achieve a multi-planet stature, the collision is of generally low concern. It is expected that in circa 1.4 billion years, as the Sun develops into a Red Giant (followed by a dense White Dwarf), the temperature on Earth will become too hot to sustain liquid water, leading to a possible extinction of the human race and Earth as we know it. Further information will follow as three papers on the Hubble observations and the consequences of the mergence are reported in an upcoming issue of the Astrophysical Journal. Astronomers are currently beginning to record Andromedas sideways motion differently, as water masses have been discovered in the galaxy. These are regions of radio-bright emissions associated with star formation. In approximately two years even precise sideway motion may be obtained by Hubble. We at last have a clear picture of how events will unfold over the coming billions of years says Sangmo Tony Sohn, Baltimore, Maryland-based institute and we certainly hope so.

Hazim Zia

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