In 4.5 billion years from now, our galaxy will undergo the most significant upheaval in its entire history. This will occur when it collides with the Andromeda Galaxy.
From hundreds of millions of years prior, if our civilization still exists in the Solar System or somewhere else in the galaxy, the many generations that follow will witness a spectacular sight, even though it prepares for one of the most violent events that can occur in the universe.
The Earth and our entire Solar System exist within a colossal structure known as the Milky Way galaxy, which contains at least over 100 billion stars. This galaxy is part of a larger assembly called the Local Group—a cluster of more than 50 spiral galaxies located about 2.5 million light-years away from us. On clear, moonless nights, you can see it even with the naked eye as a small, dim streak in the region of the Andromeda constellation.
Andromeda is a fascinating target for amateur astronomers. Since it can be seen even with the naked eye, it becomes even clearer when viewed through telescopes.
For astronomers, observing this galaxy plays a significant role in understanding the evolution of galaxies in general and our own galaxy in particular.
Being relatively close and the largest galaxy in the Local Group, with a spiral structure similar to the Milky Way, Andromeda serves as an excellent model to observe and test existing theories about the structure and evolution of galaxies. This helps refine our understanding of galaxies in the universe, especially regarding our own galaxy.
The Approach of Andromeda
Astronomers’ observations indicate a blue shift in the spectrum obtained from the Andromeda Galaxy. Simply put, a blue shift is the phenomenon where the light received corresponds to one or more elements emitted from a galaxy or star with shorter wavelengths than its normal wavelengths.
Since each element emits its own characteristic wavelength, obtaining a shorter wavelength than usual indicates that the source (here, the galaxy or star being observed) is moving closer.
This phenomenon is known as the Doppler effect. Conversely, if the observed galaxy or star is moving away, astronomers will see the received wavelength longer than the original wavelength, known as redshift.
Today, astronomers know that the universe is expanding in all directions. Clear evidence of this is observed through the redshift measured in the spectra of all distant galaxies.
This was first discovered in 1929 through observations made by Edwin Hubble. This may lead you to wonder why we are observing a blue shift in the Andromeda Galaxy if the universe is continuously expanding?
The answer lies in the distance. Hubble’s observations showed that the farther away a galaxy is, the faster it is moving away from us. He generalized this into Hubble’s Law, which states that the speed at which a galaxy moves away is proportional to its distance from us.
This means that at shorter distances, the expansion of the universe is less pronounced, or in other words, something causing the universe to expand (which scientists today temporarily refer to as dark energy) has a very weak effect at close distances. Meanwhile, all massive objects in the universe exert gravitational influence on each other’s motion.
As Newton’s famous formula tells us, the strength of gravitational force is inversely proportional to the square of the distance, meaning that the closer the objects, the stronger the gravitational pull.
The contradiction between these two forces (dark energy and gravity) causes galaxies to move away from each other distinctly at large distances (strong expansion and weak gravity), but the opposite at close distances. The 2.5 million light-year gap between the Milky Way and Andromeda is quite small when considering the scale of galaxy distances.
At that distance, the immense mass of these two galaxies causes them to exert a much more pronounced gravitational influence than the effect of the universe’s expansion.
In other words, it’s not that the universe isn’t expanding at close distances; rather, at this distance, the stretching is much weaker compared to the motion caused by gravity—much like walking on a treadmill while moving slower than the treadmill itself. You still make progress, but overall, you are pulled back.
Currently, the measured blue shift indicates that the Andromeda Galaxy is moving towards our galaxy at a speed of about 110 km/s. However, this speed is not fixed and will increase over time as the distance between the two galaxies continues to shrink. Astronomers estimate that the collision will begin in about 4.5 billion years.
What will happen, and what fate awaits both our galaxy and planetary system?
Galaxy Mergers
Based on theoretical calculations and detailed computer models, most astronomers agree that the result of the collision between these two massive galaxies in the future will lead to the formation of a new galaxy created from the merger of these two galaxies. Such galaxy mergers in the universe are not too rare.
Both Andromeda and the Milky Way are thought to have previously merged—or swallowed—several smaller galaxies (known as dwarf galaxies) before they grew to their current sizes.
This approach and merger process will undoubtedly have significant consequences for both galaxies, as well as the stars and planets within them. However, it may not be as dramatic as most people imagine.
First of all, the collision between stars and planets, similar to scenarios in many science fiction movies, will essentially not occur. Although astronomical images may show galaxies (such as Andromeda) as chaotic and filled with stars, the truth is that the density of stars in these galaxies is quite low.
The simplest example in our neighborhood is that the nearest star to the Sun is 4 light-years away, while the distance from Earth to the Sun is 8 light-minutes, and from Earth to the Moon is just over 1 light-second.
This means that the space between stars in a galaxy is much larger than the size of the stars and planets. The probability of two distant stars colliding is much, much lower than you and another person throwing two very small pebbles from opposite ends of a stadium toward each other and hoping they will touch mid-air.
Thus, it can be said that it is almost impossible for another star from Andromeda or even from the Milky Way to crash into our Solar System. But that doesn’t mean the Solar System will be completely safe.
In fact, the influence of Andromeda does not wait until the collision begins. The gradually diminishing distance increases the gravitational force between the two galaxies, directly impacting the orbits of stars in both galaxies. Long before the collision starts, the orbit of the Sun begins to be affected.
Although the density and precise orbits of stars in Andromeda have not been observed in enough detail to draw definitive conclusions, it is quite likely that stars located near the edge of the Milky Way, like the Sun, will be pushed out of the galaxy as the merger process occurs. This pushing out process will also change the structure of the planetary system to some extent. Earth and the other planets will not have the same orbits as they do now.
The Fate of Earth and Humanity
Being pushed out of the current orbit will obviously have a significant impact on our planet. Imagine if right now the distance from Earth to the Sun increased or decreased by just 1.5 times; what would happen? The temperature would certainly be very different from now (much colder if moving further away and hotter if moving closer to the Sun); the yearly cycle (and along with it, the weather cycles) would change completely, and tides, earthquakes, tsunamis… would occur on different cycles (which could be more dangerous or not) along with many other consequences.
Of course, if the orbital change is not too drastic—specifically remaining farther than Venus and closer than Mars (from the Sun)—then the millions of years process will be enough for many species to evolve and adapt.
The issue is that we currently do not know for sure whether this change will remain within that range, or if Earth will be pushed to the edge of the Solar System, or conversely, pushed into the current orbit of Mercury or even closer to the Sun—in either case, hope for life would be lost.
In reality, you don’t need to worry about the aforementioned scenario. According to astronomers’ calculations, the Sun’s evolution will cause its radiation output to increase significantly in about 3.75 billion years.
This increase in radiation will heat the Earth to the point where all liquid water will evaporate from its surface, and thus, by that time, life on Earth will essentially no longer exist.
In the span of over 3 billion years, if future generations manage to survive and advance in science and technology, they will likely have exhausted Earth’s resources and will have had ample time to develop means for traveling to other planetary systems in the galaxy.
Even if our species does not face extinction, after such a lengthy period, the process of evolution would certainly transform our descendants into beings with few similarities to modern humans. It’s important to remember that dinosaurs went extinct about 65 million years ago, at a time when mammals had yet to develop any physical traits resembling those of humans or today’s primates.
If civilization were to persist and evolve for another 3 billion years, Earth’s intelligent species would possess the capability to navigate between planets and survive the upheavals caused by the merging of two galaxies.
In over 3 billion years, any living organism in the Milky Way will witness a spectacular sight as a colossal galaxy looms in a vast area of the night sky.
Further down the line, the galaxies in the Local Group will continue to merge and may eventually become a single massive galaxy. However, that scenario is at least 150 billion years away, by which time most stars in the galaxy will have extinguished, and star formation will nearly cease.
The enormous galaxy formed at that time will be an old elliptical galaxy, with the brightest area being the central region, which houses a supermassive black hole even larger than the one currently at the center of the Milky Way, continuing to accrete gas and dust from its surroundings.
