Will our Galaxy merge with another Galaxy in the future? What are the potential consequences of this event?

 The Andromeda Galaxy is hurtling toward the Milky Way at 110 kilometers per second. Yet when they collide, virtually not a single star will touch another.

Long modeled as an inevitable head-on impact in four to five billion years, recent astrometric data from the Gaia space telescope shows the encounter might be a glancing blow or delayed, though the two giants will eventually coalesce. Even earlier, the Milky Way is expected to absorb smaller neighbors, including the Large Magellanic Cloud in roughly 2.4 billion years.

Galaxies, after all, are mostly empty space. If the Sun were scaled down to the size of a ping-pong ball, the nearest neighboring star would be another ball roughly a thousand kilometers away. Because stellar distances are so vast compared to stellar diameters, the two stellar populations will pass through each other like ghostly swarms without their individual suns crashing together.

The consequences will instead be gravitational and atmospheric. Interstellar gas clouds, unlike stars, are diffuse and sprawling; when they collide, the resulting friction and compression trigger a period of rapid star birth known as a starburst. Over several passes spanning hundreds of millions of years, mutual gravitational tides will strip away the familiar spiral arms of both galaxies, hurling streamers of stars across intergalactic space. The Solar System has about a fifty percent chance of being flung into the outer tidal halo, but the planets themselves will remain firmly tethered in their orbits around the Sun.

Eventually, the two central supermassive black holes—Sagittarius A* and Andromeda's far larger monster—will sink toward the combined galactic core through dynamical friction. After orbiting each other for millions of years, they will merge into an even larger gravitational anchor, releasing gravitational waves.

The final result will be a single, smooth elliptical galaxy, often dubbed "Milkomeda." Any distant observers watching the event would see the night sky transform from a quiet ribbon of light into a chaotic blaze of young blue stars, before fading into an ambient, featureless glow. By that point, however, Earth will be long uninhabitable, as the Sun's steadily increasing luminosity will have boiled off the oceans billions of years earlier.

An artist impression of the night sky as the Andromeda Galaxy and the Milky Way begin their merger. Source: Wikimedia Commons.

What are the sizes of galaxies nearest to our Galaxy?

 The Milky Way's nearest galactic neighbors range from massive spirals twice its size to a tiny, 5,000-light-year dwarf that it is actively eating alive.

These neighbors belong to the Local Group, a gravitationally bound collection of more than 50 galaxies spanning roughly 10 million light-years.

The Local Group is dominated by three major galaxies. The Andromeda Galaxy (M31) is the absolute heavyweight. Located 2.5 million light-years away, it spans an estimated 152,000 to 220,000 light-years in diameter—making it up to twice the size of the Milky Way, which measures roughly 100,000 light-years across. The Triangulum Galaxy (M33), the third-largest in the Local Group, is a more modest spiral measuring about 60,000 light-years in diameter.

However, the galaxies physically nearest to the Milky Way are its satellite galaxies. The most famous are the Magellanic Clouds, visible to the naked eye from the Southern Hemisphere. The Large Magellanic Cloud (LMC) measures about 14,000 light-years across, while the Small Magellanic Cloud (SMC) spans roughly 7,000 light-years. Despite their smaller stature compared to the Milky Way, they are complex systems packed with intense star-forming regions.

The title of nearest galaxy belongs to a pair of dwarf galaxies that are currently being consumed by the Milky Way. The Sagittarius Dwarf Spheroidal Galaxy sits just 70,000 light-years from Earth and measures roughly 10,000 light-years in diameter. Even closer is the Canis Major Dwarf Overdensity, located a mere 25,000 light-years away and spanning about 5,000 light-years. This structure has been stretched so severely by the Milky Way's tidal forces that astronomers continue to debate whether to classify it as a true dwarf galaxy or simply a long ribbon of stellar debris being woven into the Milky Way's galactic disk.

A schematic of the Local Group featuring the Milky Way, Andromeda, Triangulum, and dozens of satellite dwarf galaxies. Photo by Pablo Carlos Budassi is licensed under CC BY 4.0.

How does gravity work in empty space? Does it attract mass over vast distances?

 Right now, the gravity of a galaxy billions of light-years away is influencing Earth. The secret to this infinite reach? Space isn't actually empty, and gravity isn't a pull.

Under Isaac Newton's classical mechanics, gravity was thought to be an invisible tether pulling objects instantly across a void. Albert Einstein's General Relativity overhauled this model, proving that space is not a featureless vacuum, but a dynamic, four-dimensional fabric known as spacetime.

Mass and energy warp this fabric. The classic analogy is a bowling ball resting on a trampoline, which stretches the material downward into a depression. If you flick a marble across that trampoline, the marble's path will curve toward the bowling ball. The bowling ball isn't "pulling" the marble with an invisible force; the marble is simply traveling in a straight line across a curved surface.

When we ask how gravity attracts objects over vast distances, the answer is that it isn't an attraction in the traditional sense. Earth orbits the Sun because the Sun curves the spacetime around it into a massive bowl, and Earth rolls along the rim. Every object in the universe is just following the natural contours of the space it inhabits.

This curvature does not stop at the edge of a solar system; it extends infinitely. Gravitational ripples propagate outward at the speed of light. While gravity's strength diminishes over distance following the inverse-square law—meaning if you double the distance, the gravitational influence drops to one-quarter—it never reaches absolute zero.

We can physically observe gravity's vast reach through a phenomenon called gravitational lensing. When light from a distant star travels across the universe, it must follow the warped spacetime created by massive objects in its path. A black hole or galaxy cluster bends the space around it, acting as a cosmic magnifying glass that warps and redirects the ancient starlight before it reaches our telescopes.

A central black hole acts as a gravitational lens, bending the spacetime around it and warping the trajectory of light from a background star. Source: Wikimedia Commons.

What is the fate of stars in a Galaxy after a merger occurs?

 Despite the apocalyptic visuals of a cosmic merger, individual stars almost never crash. Instead, billions are violently scrambled or flung into deep space.

The distances between stars are so vast that a galaxy is overwhelmingly empty space. If the Sun were the size of a ping-pong ball, Proxima Centauri would be another ping-pong ball sitting roughly 700 miles (1,100 km) away. A galaxy collision is less like a demolition derby and more like two sparse flocks of birds flying through each other. The stars themselves sail past one another untouched.

While the stars survive the encounter, their surrounding architecture is completely destroyed by gravity. As the two immense masses pass through one another, their combined pull radically rewrites stellar orbits.

In a typical merger between two spiral galaxies, the neat, rotating disks of stars are completely disrupted. Many stars are flung into chaotic, highly elongated orbits that crisscross the new galactic center in random directions. Over hundreds of millions of years, this scattered swarm settles into a giant, featureless ball of stars known as an elliptical galaxy.

Not every star makes it into the new structure. The intense gravitational whiplash during the initial approach often tears long ribbons of stars away from the outer edges of the galaxies. These "tidal tails" can stretch for hundreds of thousands of light-years, permanently ejecting billions of stars into the lonely void of intergalactic space.

While the established stars avoid direct impact, the sprawling clouds of cold hydrogen gas between them do not. The gas clouds smash into each other, compressing violently and triggering a massive, furious wave of new star formation known as a starburst. This sudden ignition produces millions of huge, blue stars that quickly burn through their fuel and detonate as supernovae.

Once the gas is entirely consumed by this explosive birth cycle or blown away by the resulting stellar winds, the merged galaxy loses its ability to make new stars. The massive blue stars die out, leaving behind only the older, cooler, long-lived red stars, slowly orbiting in a quiet, dead elliptical galaxy.

The Antennae Galaxies are in the midst of a violent merger. While individual stars sail past each other untouched, colliding gas clouds ignite brilliant pink clusters of new star formation, and gravitational forces rip long tails of older stars out into deep space. Photo by ESA/Hubble is licensed under CC BY 4.0.

What is the largest known galaxy in the universe, and where is it located?

The name of the largest known galaxy is IC 1101 .

It has a radius of 1.9569 million light-years, which dwarfs the radius of our Milky Way galaxy, which is only 52,850 light-years. And it is approximately 12.31 billion years old.

This enormous elliptical galaxy is 320 megaparsecs (1.05 billion light-years) from Earth. It is also incredibly massive (no wonder), as it has a mass equivalent to about 100 trillion stars.

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If our Galaxy was shrunk down to the size of a pea, how big would the universe be?

 It would be (roughly) five kilometers in diameter.

The Milky Way is 100,000 light years in diameter (1 followed by five zeros, or 1e5). The Observable Universe is (roughly) 100 billion (100,000,000,000) light years in diameter (1 followed by eleven zeros, or 1e11). Divide the Universe diameter by the Milky Way diameter to get the proportion. 1e11/1e5 = 1e6. So the Universe has 1,000,000 (a million) times the diameter of the Milky Way.

If a pea is 5 millimeters in diameter, multiply by a million to get 5,000,000 mm. There are a million mm in a kilometer, so… five kilometers in diameter! — about three miles.

Amazingly small, when you think of it.

In our local group of galaxies, galaxies are (roughly) 25 times their diameters distant from each other. So the nearest galaxies to this pea-galaxy would be 5 x 25 = 125mm (five inches) away. For the universe as a whole, the average distance would be around 400mm (16 inches).

Picture: Structure of the Universe. Every bright pixel is a galaxy. This image is one billion light years across (1/100 of the diameter of the Observable Universe) Created by the Max Planck Institute in Germany

Millennium Simulation Project

How big is the Milky Way?

We thought that our Milky Way Galaxy was more than 100,000 light-years across. We were wrong.

The analysis of 100,000 larger stars of our galaxy revealed that the youngest population of stars, on average, is in the middle of the flat spiral disk. They get older towards the middle, where the central bulge is located, and older towards the outer edge.

New stars only form up to around 40,000 light-years from the center, making our Milky Way Galaxy around 80,000 light-years across

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. Beyond this boundary, only old stars exist that didn’t form there but migrated over billions of years after forming closer to the center. Therefore, only very old stars exist beyond that point.

The inner flat disk is a bit older than the center of the spiral disk because it started forming stars first, when our galaxy was young.

Our Sun is also part of this huge star migration. It didn’t form 26,000 light-years from the center where it is now located. Its composition indicates it formed either in a Wolf-Rayet nebula or in nebulae enriched by winds from such stars. They are more common near the galactic bulge. Therefore, in the 4.5 billion years since our system formed, our Sun may have moved up to 10,000 light-years.

It might have moved because the central bulge of the Milky Way flattened as it became more bar-like, making our galaxy a barred spiral galaxy. Stars additionally migrate outward due to radial migration of galactic dynamics.

Alternatively, there is a theory that star formation drops at around 40,000 light-years because our galaxy is warped at that distance, probably due to interactions with the Dwarf Sagitarius Galaxy, which, on its vertical orbit, pierces the disk of our galaxy and emerges on the other side. This warp rotates roughly every 700 million years, which is almost three times as slowly as one full orbit of the Sun around the center, 230 million years.

The size of the Milky Way spiral disk doesn’t, for now, change the estimates of our galaxy’s star count, and it’s still 200 to 400 billion stars. We also still think that our galaxy is more massive than Andromeda, even though Andromeda contains 400 to 800 billion stars. This is because the Milky Way contains more dark matter.

Could a star be so far away that the light from it is too weak for us to see?

 Easily.

A calculated gamble was taken to use the Hubble Space Telescope — pointing it at a known, very dark area of sky. The total exposure time was of 11 days.

They got this:

That is the Hubble Deep Field and those aren't stars but $3,000$ entire galaxies of stars invisible to the naked eye.

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It takes a few photons of light hitting your retina in a short space of time for you to perceive something is there. While there are a massive number of photons emitted by a star, the number per unit area drops off by the inverse square law.

A certain amount of photons passing through an area at a given distance passes through $2^2=4$ times the area at $2$ times the distance, $3^2=9$ times the area at $3$ times the distance, $4^2=16$ times at $4$ times etc. The number of photons per unit area drops off quickly and so much so that the photons from the galaxies in the Hubble Deep Field arrive far too far apart in time to trigger a human retina and let you know anything is there.

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That image is one twelfth the width of the Moon. Looking away from our galaxy's disc (where there would be significant dust to get in the way) you should expect a similar number of galaxies would be found over most of the sky and doing something similar $3$ years later in the southern sky produced something similar.

But as with the original almost all of the galaxies that are actually in the night sky are invisible to the naked eye.

How big is the observable universe if the distance to the Andromeda Galaxy is one foot?

If the distance from Earth / Milky Way Galaxy to the Andromeda Galaxy were a foot when the real distance is 2.5 million light years, then the 150,000 light year diameter Andromeda Galaxy would be the size of a dime and the Milky Way about 2/3 that size.

There are about 100 mostly small and few big galaxies within 2.5 million light years of us. You can see the Andromeda galaxy in a very dark sky with the naked eye a few times bigger than the moon’s apparent size.

The radius of the observable universe accounting for expansion of space isn’t 13.5 billion light years since the big bang but 46 billion light years. That’s only about 18,400 times the Andromeda distance, so it would be 3.5 miles in radius or 6.8 miles in radius about the size of a city center. That’s a big number but nowhere near infinity at that scale.

The volume of the sphere would be 26 trillion cubic feet, which you could compare with high estimates of the number of galaxies as 20 trillion.

The actual universe is larger than what is observable limited by the amount of space light can travel since the big bang, and no one knows if it is infinite or just really really big.

What is the biggest solar system in our galaxy?

 We know definitively what the biggest solar system in our galaxy is. It’s ours! Our sun and planets.

Because it is the only one called the “solar system”, since “solar” comes from the word “sol” or “sun”. So it’s literally the system of bodies around the sun.

But when we correct that to “star system”, we really have no idea what the largest one is. As strange as it sounds, we know orders of magnitude more galaxies than star systems. So the odds are overwhelming we have not found the largest star system in the galaxy.

However, all jokes aside, it is possible that our own solar system may be up there. The largest star system we have found to date is Kepler-90, with 8 known planets. That happens to be the same number of planets in our solar system. However, smaller planets are difficult to find in other star systems so Kepler-90 very well could have more hiding in there.

If Andromeda is the closest galaxy to us, why have we not started exploring it yet?

 The nearest star beyond our own Sun is about four light-years away - or about 25 trillion miles. The farthest any man-made object has traveled from Earth (Voyager I) is currently 15 billion miles away. That is, Voyager I, launched in 1977, is about one sixteen hundredth of the way to the nearest star outside our Solar System … after nearly fifty years traveling in space. The Andromeda Galaxy is over 600,000 times as far away as that star. So that galaxy is about a billion times farther away than the most distant thing we have launched into space.

See all those stars in the photo below? They are all in our little section of the Milky Way galaxy. Every single one of those is many times farther away than Proxima Centauri (the nearest star I mentioned). That arrow locates the Andromeda Galaxy, which is about twice the size of our own galaxy, the Milky Way.

So maybe that’s the reason.

If our solar system somehow got close to another during the Andromeda collision, what would the effects be?

What do you see in the above simulated collision between Andromeda and Milky Way?

Do you see any star smashing into each other? No.

Do you see explosions or physical carnage at the level of individual stars? No.

But, Do you see stars getting flung out? Yes.

See, galactic collisions are galactic level gravitational events, instead of stellar collisions. So mostly stars do not collide (because of unimaginable space in between), but they surely get gravitationally influenced.

If a star could come near to the Sun by around 50,000 AU, it will start flexing its gravitational muscle on. Things would get serious when a star comes closer than 1,000 AU (although very unlikely, but surely possible). That’s when things start to make a carnage a solar system level.

Some so much so that some planets will flung out of galaxy. (Notice how blue object gets effected by other gravitational bodies)

But which ones will be effected, or whether they will be can not be predicted because Galactic collisions are utterly complex processes.

Unimaginable number of highspeed astronautical dynamics can bring on any new threat at any time to the people living in the galaxies (assuming if there are beyond Earth). And by ‘any time’ I don’t mean in matter of days or weeks, but sometimes centuries and millenniums also.

Our Earth has faced a few violent phases in last 4.5 Billions Years. But most of time, it has stayed calm which has given life many chances to evolve. During collisions, this won’t change much, but the number and duration of calm phases are expected to reduce.

What do we mean by calm phase?

Something closer to what we see today, only a beautiful sky.

What do we mean by violent phase?

Quite Possibly .

Earth being flung out would be less likely though, unless extremely unlucky.

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But while thinking about this and discussing this, we mostly forget one important part.

Any possible life, (if there would be) may have shifted to further planets because of increased Solar activity (as sun would also be about to die by that time around.)

Secondly, life would have evolved a great deal by that time. May be it would have started multiple times over by then, so somewhere else in the galaxy or in another galaxy altogether.

We humans take pride in calling ourselves intelligent in mere few 100 thousands years of evolution. By the time this collision would unfold, 3Bn + years would pass on.

How would earth be affected if the super massive black hole at the center of the galaxy turned into a quasar?

Without a warning, Earth was enveloped by a beam of intense light from the center of the galaxy. The pole of the supermassive black hole Sagittarius A* was pointing at our planet. It was the end of human civilization.

People who were outdoors were evaporated immediately, and only a faint shadow of their existence was imprinted on the ground where they stood. Much more horrifying fates awaited people inside the buildings. The hot air evaporated the glass in the windows and entered their rooms. Their skin, exposed to extremely high temperatures, melted to flesh as they experienced enormous pain for a fraction of a second. Fortunately, it didn’t last long; they died quickly. Or maybe not….

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We have recently discovered that the axis of rotation of Sagittarius A*, the supermassive black hole at the center of the Milky Way, is oriented in such a way that we are facing its pole. This is surprising because the planes of the spiral disks of galaxies would tend to align with the equatorial regions of the event horizons of supermassive black holes at their centers.

When these monsters become active by feeding on vast amounts of matter, they can evolve into active galactic nuclei or quasars. A beam of intense radiation can then emerge from their poles that can seriously damage everything in its way. We recently observed one causing nova explosions in the giant elliptical galaxy M87.

Nevertheless, the supermassive black hole at the center of our galaxy is 4.3 million times the mass of the Sun and is relatively modest in size compared to the mass of our galaxy and other supermassive black holes elsewhere. We are located 26,000 light-years away from the center, and a significant amount of gas and dust along the way would block some of the radiation. The above scenario could happen in other galaxies, which contain supermassive black holes hundreds of times or more massive than the one we have in the Milky Way.

At most, the Earth’s atmosphere would get seriously damaged, and dark clouds would appear that would block the energy of the sun for years, causing nuclear war-like conditions and famine that could dent the human population to a large extent.

We think that our supermassive black hole has been in its active galactic nucleus phase or quasar many times in the past. In other galaxies, they last millions to even up to a billion years, but their length depends on the sources of matter the supermassive black hole can feed on. It can be from a recent galactic merger. Our galaxy has not collided with a huge one for 8 to 11 billion years since the Gaia-Encaladus-Sausage galaxy increased the number of stars in the Milky Way by about 50 billion.

Perhaps the next quasar phase will occur after the merger with the Andromeda Galaxy and its supermassive black hole, which is 20 to 30 times more massive than our own. This might be in 4.5 billion years or more. This behemoth of a unison between two supermassive black holes could do far more damage, and its galaxy would supply theirs, stir up and move our nebulae that can feed this voracious source of destruction for hundreds of millions or even a billion years.

The question was: How would earth be affected if the super massive black hole at the center of the galaxy turned into a quasar?