In nature, large organisms preying on smaller organisms as food for survival is not unheard of. But how astounding would it be if a similar phenomenon happened in galactic and intergalactic space? As a matter of fact, it does! Some galaxies can voraciously gobble millions and millions of stars to satisfy their hunger. Stars and galaxies look enchanting as long as we are oblivious of their trait of ‘hunting down’ humble celestial objects. So far, we have been lucky enough to not become their next meal. But, for how long?
Our galaxy, the Milky Way, is constantly hurtling towards our nearest spiral neighbor, Andromeda, which will one day swallow our entire existence. As a matter of fact, the Milky Way itself has been incubated by eating several tenuous star systems and galaxies for the past millions of years. There is no doubt, but fear, to say that our galaxy is on its way to be gobbled up by Andromeda, the order has been placed and will reach its destination any time after 4.5 billion years. The Milky Way has feasted on other galaxies for years but the tables seem to be turning now!
Although the idea of being eaten up by some giant space predators is intimidating, this phenomenon has opened numerous gateways to unlock one of the greatest mysteries of the universe: the evolution of galaxies. In an awe-striking evolutionary process called the Galactic Cannibalism, large galaxies eat small ones by slowly stripping off their stellar mass. The larger galaxies merge with their companion as a consequence of the interaction between their gravitational fields, thus resulting in larger, irregular galaxies. Throughout the event, old stars die, and several new stars are formed. These cannibal galaxies devour everything that comes in their way – be it stellar dust, stars, dark matter, or even other galaxies. A cannibal galaxy can take millions of years to finish its meal before starting its next meal.
To understand why these interactions take place, think about the last time you played with a pair of magnets. If you bring two pieces of magnets close enough, a point reaches when their magnetic fields interact and you feel a force that attracts both the pieces together. Letting off both the pieces free when they are within one another’s field will allow them to collide. Galactic collisions work in the same way.
We know space is vast yet highly dynamic. Therefore, celestial objects may encounter interactions and collisions with other celestial objects throughout the course of their journey. The gravitational field exists around every object here as well as in space and its influence is imparted on any entity that comes within its extent. So, when two objects in space get close enough, they both are under the influence of another’s gravitational field. Gradually yet perpetually, they are attracted towards each other and a collision occurs.
Collisions can be within two objects of either the same or different sizes. It can be endured if incident objects are of nearly the same size; objects restoring their shapes being resilient to the change. But if one object is mightier than the other, odds are, the smaller galaxy is unable to survive the strike, and instead becomes a part of a heavier galaxy – this is where galactic cannibalism comes into play. Imagine a vacuum cleaner sucking all the material that comes within its course. The more material it sucks, the bigger the vacuum bag gets. Cannibal galaxies suck the matter out of smaller galaxies akin to a vacuum cleaner, hence evolving into larger galaxies.
Galactic cannibalism has opened new avenues to understand the evolution of galaxies which has mystified scientists for centuries. For many years, scientists believed that galaxies formed as a consequence of the Big Bang — nearby stars clumped together to form galaxies with gravity as an agent tying them in groups. This would imply that all the stars within a galaxy are of the same age since they were created at the same time. But scientists have been finding stars of all different ages in many galaxies. For instance, the oldest star in our home galaxy, the Milky Way, is HE 1523-0901 which is about 13.2 billion years old. On the other hand, we also see young stars that have formed recently. Scientists are unsure as to when it all began; galactic cannibalism is an age-old phenomenon, perhaps as old as the universe itself.
It was the American astronomer, Edwin Hubble, who first noticed this extraordinary occurrence. On one of the starriest nights in 1910, Hubble was busy stargazing in Mount Wilson Observatory, California, when he noticed an unusual motion of stars around the Andromeda galaxy. While most of the stars and galaxies were moving away from the point of observation, there happened to be a cluster of stars surrounding Andromeda that exhibited discrepant motion: it was being pulled towards Andromeda. What was causing these stars to behave differently? Subsequent studies found thousands of stars in the stellar halo of Andromeda that are older than the age of the Galaxy itself. We now know that these ancient stars are remnants of all those galaxies that were once cannibalized by Andromeda billions of years ago. Old stars are the reflection of the early universe. Therefore, digging information about them has greatly helped scientists in understanding how the early universe evolved over the years as compared to its current state.
Numerous galaxies have evolved this way. The galaxy we inhabit is also a cannibal. Andromeda is pulling us with the speed of 75 miles per second. A collision in an estimated 4.5 billion years is inevitable. This is one of the ways through which the world, as we know it, will end. The galactic cannibalism of our Milky Way by Andromeda could mark the end of human existence, but who knows for sure? Galaxies are quite enigmatic; for all we know, it could mark the beginning of another epoch.