# Why do galaxies collide?

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If the universe is expanding outward, what is the processes for one galaxy to get off track enough to collide with another?

Say, the Andromeda Galaxy and the Milky Way.

The universe is expanding on a large scale. But locally things are always messy.

Locally, galaxies are not set in stone, they move relative to each other, and the directions are random. If they're moving towards each other fast enough, then they will collide.

Also, there's gravity. Some galaxies are bound to each other by gravity, and that will tend to pull them together.

As to why galaxies move at all, relative to each other - well, things in this universe have kinetic energy, and it's distributed randomly. Being distributed randomly, all kinds of scenarios are possible - things running away from each other, zooming past each other, bumping into each other, etc.

It's a messy and random universe, and the order of expansion becomes apparent only on the largest scale.

The galaxies don't really get "off track" - it's not impossible, but that kind of thing probably doesn't happen anymore (as space continues to expand). What actually happens is that galaxies form gravitationally bound clusters - within the cluster, the acceleration due to gravity is larger than the equivalent expansion of space between the galaxies, so rather than growing more distant, the galaxies in question actually get closer together over time. Eventually, this results in a collision and a merger.

If the expansion remains roughly constant, there will come a point where we'll no longer be able to see any galaxies outside of our own cluster. But for those close enough, this has little effect - just like the expansion of space doesn't cause atoms, planets, solar systems or galaxies to get bigger.

I'm not sure that anyone has answered the question asked. The root cause is indeed that gravitationally bound structures with freefall timescales that are much shorter than the age of the universe are not greatly affected by the general expansion of the universe (NB: Structures with freefall timescales longer than this are not going to be the source of many galaxy collisions). That is, locally, the expansion within such structures is negligible. However, that does not necessarily lead to collisions on a a timescale shorter than the age of the universe.

The first reason for galaxy collisions is that galaxy clusters have a very large number density - that is, the spacing between galaxies is not vastly larger than the "size" of a galaxy, where here, "size" means the effective interaction cross-sectional radius. As a result of these high densities, the freefall dynamical timescales in rich clusters (and even smaller groups of galaxies) are of order billions of years and so there is plenty of time for the galaxies to interact. As a contrast, think about how you might construct a scale model of stars in the local neighbourhood and compare the sizes of stars with their separations. It would in fact be difficult to make such a scale model with any meaningfully sized stars. On the other hand, you can make a scale model of say the local group of galaxies because their separations are only $$sim 10$$ times their sizes.

The second reason is that many galaxies contain gas and that gas can easily dissipate kinetic energy and also transfer angular momentum. Another factor is that massive clusters of galaxies contain intracluster gas that can also serve to dissipate kinetic energy. In a gravitationally bound system, then objects that are in orbit around each other or around a common centre of mass need ways in which kinetic energy and angular momentum can be lost in order for a collision to take place. Even without gas, the fact that galaxies exist in groups and clusters means that n-body interactions can serve to dissipate energy and angular momentum to make a collision happen.

I think it's also worth noting that cosmological expansion is measurable only on the very largest of scales. Hubble's law tells us that the farther away an object is, the faster it's being pulled away by the expansion. That rate is approximately $$70 km.s^{-1}Mpc^{-1}$$ -- in other words, for every megaparsec (Mpc -- about 3 million light years) that you look farther out in space, an additional $$70 km.s^{-1}$$ is tacked onto its recessional speed. For a relatively close galaxy, its actual velocity through space can be much larger than this, and in the specific case of the Andromeda Galaxy, which is only about 2.5 million light years (0.77 MPc) away, it is actually approaching us at around $$110 km.s^{-1}$$. For much, much farther galaxies, billions of light years away, all of those $$70 km.s^{-1}$$ add-ons stack up to a much, much higher recessional speed.

Galaxies don't get "off track" - to see how collisions happen, we need to go right back to galaxy formation early on.

So, Big Bang happens. Space starts to expand - dramatically and to a huge extent. That's space itself expanding, not galaxies moving within space, by the way - distances themselves change. (Which is why it's called a "metric" expansion, metric being a term for distance-measures, and also why cosmologists say the Big Bang happened "everywhere").

At some tiny fraction of a second, the massive expansion winds down. Space continues to expand, but at a much slower rate. The last of the fundamental forces breaks away, and the cosmos is left as an insanely hot dense mix, so hot that even basic particles like protons, neutrons and electrons can't yet exist - although quarks can.

But there are some very subtle things going on. Even though expansion left us with an incredibly uniform, homogenous universe, the density does sliiightly vary between places. As things cool down, and particles start to condense (and annihilate, and other things), the universe is left with what cosmologists call acoustic waves - basically standing waves. And if you've ever seen videos of a tray of sand being vibrated, you'll know that one effect is that it leaves some places with more sand, some with less, due to interference patterns. So our universe ends up, as it expands, with some areas denser, some less dense.

A second effect comes into play. You'll know (or have heard of) dark matter. We don't know what it is made of, but we know it exists (galaxies couldn't form without it, they'd fly apart or take longer than the age of the universe to form), and we know a lot about how it behaves - what forces it responds to, and what forces it doesn't. Interact via gravity - yes, very weakly. Interact via electromagnetic force - no, not at all. That latter bit is crucial.

When "ordinary" matter collapses, it heats up. That's how we get stars, for example. The radiation released during collapse also acts as a kind of pressure, opposing collapse, slowing it down. That's why stars like our sun are stable for so long. Dark matter doesn't interact electromagnetically (as far as we know) so it can't experience or create electromagnetic radiation. So when it collapses, it doesn't get hot, it doesn't release radiation… I think you can see where this is going. There's no radiation released during collapse to resist further collapse, so it can collapse much faster than ordinary matter. As an aside, because it can't release radiation, it also can't jettison the energy that must be got rid of to allow dense objects to form. So it ends up quickly collapsing to a hazy diffuse "halo", but then can't collapse much more. And no surprise, it collapses in those places where the universe was fractionally denser. So you get what cosmologists call "filaments" and "halos" of dark matter, a bit like a sponge or a swiss cheese, with comparative "voids" separating them. Ordinary matter is more strongly attracted to these already-existing dark matter filaments and halos. It collapses towards them. The self-gravity of the ordinary matter is enhanced by the gravity due to the concentrations of dark matter there - and ordinary matter can lose energy by radiation, so it collapses more than dark matter does, to form the galaxies and their contents which we can see today.

Gravity can do this, because the expansion of the universe has by now slowed down so much from its "heyday", that gravity can pull some of the matter together within space faster than the expansion can add space between them. Over cosmic distances, gravity is much much weaker, and expansion dominates, so clusters and superclusters still move apart, but within clusters, the galaxies and groups of galaxies are accelerated by gravity enough that they mostly stay in their groups and clusters, and move around or orbit within them.

So we end up with a universe that, on a cosmic scale, we see expansion "winning" as gravity is weak, so we see superclusters moving apart. But within clusters and galaxy groups, we see gravity "winning" because it's stronger over smaller distances, so clusters and gravitationally bound entities like galaxies stay together.

What this in turn means, is that galaxies and galaxy groups are bound by gravity more than they are separated by expansion. So they remain moving within their clusters and groups, despite universal expansion. And, occasionally, because motion of 3 or more separate bodies under gravity is chaotic (and clusters can contain billions or trillions of galaxies), entire galaxies will get ejected, or collide, or do whatever galaxies do. And that's how it happens.

(Although you didn't ask, it's a natural question to wonder what happens next. We believe that the rate of expansion has slowly sped up. That means that in the far future (tens and hundreds of billions of years), that galaxies will have to be even closer together, for gravity to dominate expansion. So clusters that are stable now, might break apart in the far future. If expansion accelerates enough then even smaller bodies could ultimately break up, perhaps galaxies themselves, or even stars and atoms. But that's something nobody knows.)

Although the Universe is expanding and, in general, the further away a galaxy is from us the faster it appears to be moving away from us. This does not apply to the galaxies in the Local Group. which is a gravitationally bound structure. The Andromeda galaxy is moving towards the Milky Way at about 400,000 km/h and the Milky Way and Andromeda are expected to collide in about 4 billion years time. When this happens, a large new single galaxy will be formed. The new galaxy which will be formed by the merger is sometimes given the name Milkomeda . For more details see my recent blog post on this topic.

Over billions of years, Milkomeda will gradually absorb the other Local Group members.

In general any gravitationally bound structure such as: stellar systems (e.g. solar system ) our galaxy and groups and clusters of galaxies will not get larger as the Universe expands)

## When galaxies collide, you get… a heron?

Galaxies are collections of billions (or, for big ones, hundreds of billions) of stars, sometimes with lots of gas and dust, surrounded by a halo of invisible dark matter. On the whole they’re pretty big, tens or hundreds of thousands of light years across.

That size becomes interesting when you consider that many galaxies come in groups or clusters, where they may be dozens or hundreds or thousands of galaxies. They all move around, and are separated by a few million light years. That’s a lot, sure, but the size of any individual galaxy is a pretty decent fraction of that. That means collisions are frequent.

On top of that, galaxies have a lot of mass, which means a lot of gravity. They don’t have to physically collide for them to affect each other a near pass will do. The closer the pass the greater the effect, especially when you have a smallish galaxy passing a larger one.

Then things get interesting.

So have a look at NGC 5394 and NGC 5395, two galaxies that are passing in the night:

The galaxies NGC 5394 and 5395, undergoing a very close pass that’s affecting both of them. Credit: NSF’s National Optical-Infrared Astronomy Research Laboratory/Gemini Observatory/AURA

Wow. Did I mention I love me some galactic collisions? You can see why. They're gorgeous!

The pair is about 160 million light years from Earth, close enough on a cosmic scale for us to get a pretty good look, especially when we’re looking with the mammoth Gemini North 8-meter telescope (to wit: The primary mirror is 8 meters across, the size of a very large living room or the width of a tennis court). Incredibly, this image, which used multiple observations in different filters to create a color composite, had a total exposure time of just 42 minutes. Holy cow.

The little one, NGC 5394, is passing the bigger one, NGC 5395. The gravity of the encounter does interesting things to both galaxies. For example, a star in the outer disk of the little one feels a pull toward the galaxy's center, but as it passes the other galaxy the gravity of it pulls on the star as well. This draws out a long streamer from the little galaxy, called a tidal tail. The smaller galaxy was probably already a spiral, but the arms got pulled out like taffy and exaggerated during the encounter.

The bigger galaxy is affected, too you can see distortions in the arms. Look at the bottom right of the galaxy: The dark dust lanes and pink gas clouds don’t align with blueish arms (where massive, hot, blue stars form). I have a suspicion that dust lane can be traced clockwise past that bright star to the left and back all the way around the galaxy to a different spiral arm, which is why it appears above the blue arm. I also can see faint blue trails on both the left and right of the big galaxy, too. What a mess!

A wide-angle view of the colliding galaxies NGC 5394 and 5395. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona

Both galaxies are undergoing a burst of star formation (the pink gas clouds are where stars are busily being born, lighting up the gas around them). That’s common in collisions. Interestingly, one paper I read about them (and mentioned in the Gemini press release) says that there’s a ring structure in the big galaxy, difficult to see here but clearer in other images, that indicates there was a previous encounter between these two! Rings can occur when there’s a direct collision, and a small galaxy passes through the disk of the bigger one. If that’s the case, this may be the second time these two have interacted. In turn, that implies the smaller one is gravitationally bound to the bigger one, and that means its fate is sealed: Merger. The two will collide again, coalescing into one, bigger galaxy.

I mentioned earlier that galaxies are big compared to the distances between them. That’s not the case for stars. A big star might be a few million kilometers across, but they tend to be many trillions of kilometers apart. That makes a physical collision between stars unlikely, at least in the outer parts of the galaxy. In the inner part, near the center, distances are much smaller. Also, so many stars drop down into the center that a collision becomes possible. Still, it’s rare compared to the sheer number of stars in a galaxy, very much the exception and not the rule.

Gas clouds, on the other hand, can be many light years across, so collisions between them are not only common in collisions but inevitable. That’s why so many galaxy mergers are also undergoing a burst of star formation: The clouds collide, collapse, and churn out lots of stars.

One more thing. In the press release, as well as in a few other places, I found the nickname for this pair: The Heron Galaxy. At first that struck me as weird they don’t look like a heron to me. Then I realized I needed to rotate the image:

Oh. That’s better! The other shot is a Great Blue Heron I spotted here in Colorado by a pond. The resemblance is actually pretty good.

But not for long. In a few million years this pair will look very different. The size difference is enough that I think the bigger galaxy will retain its overall disk structure, if not the spiral. I guess astronomers in that far future will have a better sense of that.

Incidentally, this will happen to our own galaxy as well, when we collide with Andromeda. But breathe easy: You have about 4.6 billion years before that happens. Plenty of time to prepare. And in the meantime we can look to collisions like the Heron Galaxy to learn more about our own distant fate.

## The first simulation

As soon as astronomers realized that there are objects that are separate from our own Milky Way, they observed some galaxies that appeared to be much messier than normal. But it wasn't immediately obvious that galaxies actually do anything as interesting as merge.

After all, this fantastic physical process takes hundreds of millions of years to play out, so the few short decades that we've been observing them simply isn't enough time to watch the drama unfold in real time. For quite a while, astronomers didn't know if galaxies were actively merging or if a fraction of galaxies just looked all weird and gangly and that's the way the universe worked.

Simulations ultimately unraveled the mystery of tangled galaxies. But surprisingly, it was not digital simulations that did so these simulations didn't use a computer.

The year was 1941, and proto-computational-scientist Erik Holmberg wanted to examine the behavior of merging clusters of stars. But he couldn't just manufacture a bunch of stars in the lab and watch them interact gravitationally over millions of years.

So Holmberg got clever. To represent a galaxy, he arranged a couple dozen lamps. Each lamp stood in for trillions of solar masses of stars, gas, dust and other assorted members of the galactic milieu. Then, he related the brightness of each lamp to the gravitational attraction of that galaxy chunk the more massive the chunk, the brighter the lamp.

Holmberg proceeded to measure the total amount of light falling on each lamp from all of the others. This was proportional to the gravitational force from the other parts of the galaxy. This trick worked because both light and gravity follow the same inverse-square relationship: If the distance from a source doubles, both the strength of gravity and the intensity of light drop to a quarter of the original. Holmberg could then rearrange each lamp step-by-step based on the measured "gravitational pull" of the surrounding lamps.

Image: An infrared observation (by NASA’s WISE and Spitzer missions) of the giant galaxy cluster MOO J1142+1527, located 8.5 billion light-years away. Note the red old galaxies in its core. [ESO/M. Kornmesser]

This article was originally published in the Summer 2019 (vol. 48, no. 3) issue of Mercury magazine, an ASP members-only quarterly publication.

Let’s visualize together for a moment. We live on Earth—the third planet from the Sun. The Sun is a pretty average star, not very large, and living its best middle-aged life. Though it’s special to us, our solar system isn’t so unique either there are about 300 billion other stars in the Milky Way galaxy, most of which also have planets.

If we zoom out a little more, we can see that the Milky Way is dancing with roughly 50 other galaxies … which also have hundreds of billions of stars with planets. This little collection of galaxies is known as the Local Group, and even that’s not very exceptional. That’s because some galaxies are in huge groups with hundreds to thousands of other galaxies, all gravitationally bound to one another. These structures are called galaxy clusters, and they are the most massive objects in the Universe. Let me repeat that: the MOST massive.

If we were in one of these clusters, our night sky would look very different. For one thing, it would be brighter, as our neighborhood would be quite crowded. The stars in our own galaxy would appear red or yellow instead of blue, as blue colors signify youth in a galaxy, emitting from areas where new stars are born—but our galaxy would likely never birth a new star again. In fact, almost every galaxy in our neighborhood would look like a dim, orange blob of light because nearly all galaxies in clusters are no longer forming stars—a conundrum that really bugs astronomers.

How do we figure out what killed galaxies in clusters if we can only take pictures of their carcasses?

Let me paint a picture for you: if we think of galaxies as our grandparents, galaxies in cluster environments would be the ones you know lived a hard life just by looking at them. They’re always tired maybe they had too many kids all at once, or they just partied a little too hard in their youth, and now they live in a home with other senior citizens who went through something similar. However, galaxies living in small groups or isolation, like our Milky Way, would be those grandparents that still go salsa dancing once a week and can do more push-ups than you can. Really, they will probably outlive us all. These two types of galaxies were born at the exact same time. But the less crowded ones are still gently forming stars and hanging onto that blue-colored youth, essentially the equivalent of enjoying a really slow retirement.

So, what exactly happened to cluster galaxies that caused them to age so rapidly? To get to the bottom of this, astronomers have created simulations to understand the possible births and lives of cluster galaxies based on data from hundreds of real senior galaxy clusters nearby. This is essentially the equivalent of taking thousands of photos of senior citizens and using physics to model what these citizens might have looked like when they were babies.

As you can imagine, a lot of assumptions and theories go into these models, so we need pictures of middle-aged, teenage, and baby clusters to really test our theories. Thankfully, in the last decade, we’ve started filling in our evolutionary photo album with more middle-aged and teenage pictures . but we’re seriously lacking baby photos.

Galaxy cluster “babies,” also known as protoclusters, can be difficult to identify because they don’t have the same qualities as their descendants. The galaxies inside a protocluster are likely blue, instead of red, as they are still growing and forming stars. They are also more spread out across the sky as they are still traveling across vast distances to eventually fall into the final cluster formation. And they are hard to see because they live in earlier times in the Universe and are therefore farther away from us.

Due to these difficulties, we must seize the opportunity to study a protocluster deeply whenever we do find one. In the past two years, international teams working with the Subaru Telescope and archival data from the Herschel Space Observatory have identified some very big, very distant protocluster candidates. One of those protoclusters is already proving important to filling in our evolutionary photo album.

The Distant Red Core (DRC) is a massive galaxy protocluster caught in the act of formation in the early Universe. Our current measurements show that every galaxy in this cluster is simultaneously forming stars at extreme rates—I’m talking thousands of times faster than the Milky Way galaxy. These galaxies are also violently merging with their neighboring galaxies, a mechanism that’s believed to trigger extreme bursts of star formation as huge reservoirs of gas collide. According to detailed simulations, these processes are believed to accelerate galaxy evolution, bringing the senior lifestyle to the cluster galaxies at a much more rapid pace than their isolated peers.

Soon, we will determine exactly how many galaxies are expected to collapse into this protocluster—is it tens or is it hundreds? Then, we will predict what the cluster might look like today after evolving for 11 billion years.

Over the next decade, new powerful telescopes will help us discover and characterize more infant galaxy clusters so we can get a more complete story. Even more enticing, since galaxy clusters are likely some of the first objects to form in the Universe, we will be able to use these protocluster studies to map out the fingerprint leftover from the Universe’s own birth—the Big Bang.

Galaxies come in a range of colours, but more distant galaxies appear redder than nearby galaxies.

First, a bit of background on how galaxy colours are measured. We take a picture of the galaxy through a filter that only allows a specific colour of light through. There are a bunch, suppose we use the 'g' filter (g for green). This tells us the total magnitude (sort of like brightness) in the g-band. Then we take another picture of the galaxy, suppose we use the 'r' filter (r for red), giving the magnitude in the r-band. Then the two magnitudes are subtracted, giving what's called the (g-r) colour. Because of the way magnitudes are defined, higher values of (g-r) correspond to redder colour, which is a bit counter-intuitive. So now we know what colour the galaxy appears to be, but this may not correspond to the colour that the galaxy actually is (as it would appear to someone near the galaxy).

To determine the actual colour, the spectrum of the galaxy is observed. A spectrum measures the intensity of light across a range of frequencies. Different elements/chemicals absorb or emit light at specific frequencies, which appear as bright or dark lines in the spectrum. By measuring the space between lines, we can figure out which lines correspond to specific transitions of particular elements/chemicals. We know what frequency the lines should occur at because we can measure them in a lab on Earth, but for a distant galaxy the lines are all moved to lower frequencies (i.e. they are redder in colour). In this way the amount of reddening, or redshift, can be quantified.

Redshift can occur for a few different reasons, one of which is the Doppler effect. The idea is the same as when a siren on a vehicle moving toward you sounds higher pitched than when it is moving away from you. The light is reddened, so we conclude that distant galaxies are moving away from us.

Edwin Hubble noticed that the further away a galaxy is, the redder it is, and so the faster it is moving away, according to the relation:

This is a key piece of evidence telling us that the Universe is expanding.

## This Is What It Looks Like When Galaxies Collide

The Hubble Space Telescope has been responsible for some pretty spectacular photos since it was launched into space 25 years ago, but a new image (above) of a celestial object astronomers call NGC 3597 is definitely one of its standouts.

NGC 3597 is essentially a cosmic mash-up of two "good-sized" galaxies that have collided and are on their way to becoming a giant elliptical galaxy, according to NASA. The object lies about 150 million light-years from Earth in the constellation Crater, aka The Cup.

The collisions of two sizable galaxies typically takes a few hundred million years, said Dr. Meg Urry, director of the Yale Center for Astronomy & Astrophysics.

"We know this because we can 'watch' galaxy collisions in computer simulations," she said. "Because the time scales involved in the evolution of our universe are so huge, we do astronomy with snapshots like this image."

"By piecing together the many mergers we see throughout the universe, in very different stages -- and by modeling them in our computers -- we have managed to figure out how things work over most of cosmic time," Urry added. "Pretty amazing, really."

The combined gas and dust of NGC 3597's constituent galaxies has formed dozens of so-called proto-globular clusters, which appear in the image and will go on to become fully fledged globular clusters. Such clusters can be described as "mothball-shaped sets of tens of thousands of stars" that orbit the centers of galaxies, said Dr. Jay Pasachoff, an astronomer at Williams College in Massachusetts and co-author of The Cosmos.

Why do the contents of merging galaxies form clusters instead of violently smashing into each other?

"There is so much empty space in galaxies that the galaxies pass through each other with almost no chance of any stars actually hitting each other," Pasachoff said. "We see a number of such galaxy collisions around, with the galaxies pulling out long 'tails' from each other by their gravity."

The new photo, he said, "typifies the beauty that we expect to see from Hubble."

We put Rich's question to astronomer Carolin Crawford.

Carolin - This is a very good question. It's one that comes up quite often because as you say, we know the whole universe is expanding. What you have to be clear about here is that space is expanding and it's pulling the galaxies along for the ride. However, if two galaxies are close enough to each other, that gravity that they feel - they feel each other's gravity - that overrides the motion of the expanding space so they can move through the space towards each other. So for example our galaxy and the Andromeda galaxy, which is our nearest neighbouring major galaxy, are so close that they're only about 2.5 million light-years apart.

Kat - Hardly any distance at all.

Carolin - Well, believe me it isn't in space terms. That's near enough that they're feeling each other's gravity and they're getting pulled towards each other. In about 6 billion years, we're going to collide. So, if galaxies are close enough that their mutual gravity dominates over the expansion of space, that's when you get them colliding.

One interesting fact is that there are some revolving structures in space that aren't mostly flat - they're known as elliptical galaxies. And the difference here is that elliptical galaxies usually don't have much gas or dust in them. Interestingly enough, the orbits of objects in the inner solar system also tend to be coplanar, whereas the orbits of the minor planets in the outer solar system tend to be more inclined (or non-coplanar)- the difference here, again, is that there was less gas and dust in outer solar system (back during the era of accretion, and still true today)

So, back to the original question. When there's lots of dust in a galaxy, the galaxy tends to collapse into the planar shape of a spiral galaxy (to maintain angular momentum and to minimize gravitational potential energy). Which is the same thing that happens in the inner solar system.

And why does that happen? Well, we first go into the answer here: http://www.quora.com/Astronomy/Why-are-some-galaxies-disk-shaped-and-not-spherical. As Leo C. Stein explains.

However, the story can be different for gas. Gas is interacting, unlike dark matter and stars. This means that it has a way to get rid of energy -- particles can collide, excite electrons, which later de-excite and turn that initial kinetic energy into light. This is how a gas cools. Gas can lose energy, but angular momentum is extremely difficult to get rid of. If a galaxy merger is gas-rich, and has a lot of angular momentum (which just depends on the initial conditions), there can be a lot of bulk rotation to the gas. As the gas cools (which the stars and dark matter can not, since they are non-interacting) and loses energy, it collapses into a disk. This is a lower energy configuration.

Earlier, I claimed that stars are basically non-interacting and won't collapse to a disk, in the same way that dark matter won't collapse. But even earlier, I said that stars are different from dark matter. This difference is that stars are born in gas clouds, so they trace the distribution of (molecular) gas in a galaxy.

So then, how is this a lower energy configuration? Well, we go into a Reddit AskScience thread, and use Astrokiwi's nice explanation at http://www.reddit.com/r/askscience/comments/ik4jj/during_spiral_galaxy_formation_how_does_gas/

To reduce the kinetic energy of the system, you want the particles to lose as much speed as possible. The gas and stars in a sphere have upwards and downwards motion, inwards and outwards motion, and circular motion. You can't get rid of the circular motion, because angular momentum is conserved. Once you get reduce these motions, everything will be going in nice circular orbits.

But why are all these circular orbits in the same plane? Well, you also want to reduce the potential energy. The closer particles are to each other, the lower their gravitational potential.

A disc is the closest you can get these particles to each other while still keeping them in circular orbits (as required by conservation of angular momentum).

## Why Do Galaxies Collide?

The sound waves in the presence of the moving object in its back will be compressed and will reach our ears at very high frequency. If these sound waves are present behind the moving object then they are expanded and will sound with deeper tone. This is explained clearly in Doppler Effect. The theory of big bang is supported by this concept of Doppler Effect. It also reveals that Universe is expanding. The changes in expansion of Universe are observed by red shift or calculus. The Hubble expansion of the Universe indicates that Universe is expounding and the Galaxies are also departing from each other.

If the Universe is unfolding and the galaxies are moving apart further, then is there any possibility for galaxies to collide with one another? There is an opinion that the galaxies will collide when they are attracted by gravitational pull. But this opinion cannot be considered valid in one perspective as the gravitational force that exists between all the matter might have made the Universe to get united after the Big Bang before the galaxies are formed. But, it can also be thought that the trajectory formation might have made the objects in the Universe to remain away from one another in a given time.

If the gravitational pull concept is correct, then the galaxies must be moving with certain acceleration in order to not collide with each other. When the Space is expanding, the local gravity can keep those galaxies to exist together and overcome the unfolding of the Universe. Some of the Galaxies like Andromeda and few others do not generally involve in expansion as they are bound together by their mutual gravity. Objects which are farther from one another in space move faster, while those which are closer to each other will move at slower speed. This can make them to reach other objects. Likewise galaxies may collide and form new galaxies. Galaxies meet and fuse in a long period of time to form a large galaxy. In terms of our time scale, when two galaxies collide, nothing will happen to the celestial bodies existing in them.

## How Do Galaxies Die?

Everything eventually dies, even galaxies. So how does that happen? Time to come to grips with our galactic mortality. Not as puny flesh beings, or as a speck of rock, or even the relatively unassuming ball of plasma we orbit.

Today we’re going to ponder the lifespan of the galaxy we inhabit, the Milky Way. If we look at a galaxy as a collection of stars, some are like our Sun, and others aren’t.

The Sun consumes fuel, converting hydrogen into helium through fusion. It’s been around for 5 billion years, and will probably last for another 5 before it bloats up as a red giant, sheds its outer layers and compresses down into a white dwarf, cooling down until it’s the background temperature of the Universe.

So if a galaxy like the Milky Way is just a collection of stars, isn’t that it? Doesn’t a galaxy die when its last star dies?

But you already know a galaxy is more than just stars. There’s also vast clouds of gas and dust. Some of it is primordial hydrogen left from the formation of the Universe 13.8 billion years ago.

All stars in the Milky Way formed from this primordial hydrogen. It and other similar sized galaxies produce 7 bouncing baby stars every year. Sadly, ours has used up 90% of its hydrogen, and star formation will slow down until it both figuratively, and literally, runs out of gas.

The Milky Way will die after it’s used all its star-forming gas, when all of the stars we have, and all those stars yet to be born have died. Stars like our Sun can only last for 10 billion years or so, but the smallest, coolest red dwarfs can last for a few trillion years.

The Andromeda Galaxy. Credit: Adam Evans

That should be the end, all the gas burned up and every star burned out. And that’s how it would be if our Milky Way existed all alone in the cosmos.

Fortunately, we’re surrounded by dozens of dwarf galaxies, which get merged into our Milky Way. Each merger brings in a fresh crop of stars and more hydrogen to stoke the furnaces of star formation.

There are bigger galaxies out there too. Andromeda is bearing down on the Milky Way right now, and will collide with us in the next few billion years.

When that happens, the two will merge. Then there’ll be a whole new era of star formation as the unspent gas in both galaxies mix together and are used up.

Eventually, all galaxies gravitationally bound to each other in this vicinity will merge together into a giant elliptical galaxy.

We see examples of these fossil galaxies when we look out into the Universe. Here’s M49, a supermassive elliptical galaxy. Who knows how many grand spiral galaxies stoked the fires of that gigantic cosmic engine?

Eta Carinae shines brightly in X-rays in this image from the Chandra X-Ray Observatory.

Elliptical galaxies are dead galaxies walking. They’ve used up all their reserves of star forming gas, and all that’s left are the longer lasting stars. Eventually, over vast lengths of time, those stars will wink out one after the other, until the whole thing is the background temperature of the Universe.

As long as galaxies have gas for star formation, they’ll keep thriving. Once it’s gonzo, or a dramatic merger uses all the gas in one big party, they’re on their way out.

What could we do to prolong the life of our galaxy? Let’s hear some wild speculation in the comments below.