How Fast Do Galaxies Circle Each Other?

How fast are both galaxies orbiting each other? Is there a NASA reference?
NGC 5256 is a pair of galaxies in its final stage of merging. It was previously observed by Hubble as part of a collection of 59 images of merging galaxies, released on Hubble’s 18th anniversary on 24 April 2008. The new data make the gas and dust being whirled around inside and outside the galaxy more visible than ever before. This image is composed of data gathered with the Advanced Camera for Surveys and the Wide-Field Camera 3. Image credit: ESA/Hubble, NASA

NGC 5256 is a pair of galaxies in its final stage of merging. It was previously observed by Hubble as part of a collection of 59 images of merging galaxies, released on Hubble’s 18th anniversary on 24 April 2008. The new data make the gas and dust being whirled around inside and outside the galaxy more visible than ever before. This image is composed of data gathered with the Advanced Camera for Surveys and the Wide-Field Camera 3. Image credit: ESA/Hubble, NASA

Originally posted on Forbes!

The Milky Way and Andromeda are plunging towards each other, aimed nearly directly at each other, and proceeding at a pace of about 110 kilometers every second. Given the enormous distance between our two galaxies, these two will still take some three billion years to bridge the space between them, even though 110 kilometers every second would get you from New York to Tokyo in about a minute and a half.  At that speed, you could travel to the Moon in about an hour, and jet between the Earth and Pluto in about two years. By comparison, New Horizons’  journey took nearly 10 years to cross that same distance.

This is not a particularly rapid pace for a collision between galaxies - most studies of interactions between two galaxies (at least in the relatively nearby universe) choose galaxies which are moving at less than 300 kilometers per second, relative to their companion. Even then, the typical encounters happen at a relatively slow pace, slightly less than 100 km/s. If you’re interested in collisions, the slower the speed the better.

Why is that? Similar to why ‘Oumuamua didn’t hit the Sun after travelling for so long, if galaxies pass by each other at very high speeds, they don’t spend very long influencing each other. If you go to clasp hands with a friend, when you’re both walking at reasonable, low speeds, you’ll find it easy to grab onto each other and keep your hands clasped for a little while. If you imagine trying to grab onto a hand extended from a car (don’t do this), the length of time that your hands could possibly be in contact with each other is so short that at best you’re looking for a high-energy high five instead of a handshake.

Similarly, the longer two galaxies spend near each other, which they do when they’re moving slowly, there’s much more time for the two galaxies to distort each other into fantastical shapes, and the slower they go, the less energy they have in order to escape the gravitational clutches of the other galaxy. If the two galaxies are moving slowly enough, then they will sink together and scramble themselves into a single, messier, larger galaxy, keeping all the stars that had made them up before their crash together. This is the future for the Milky Way and Andromeda - while the Sun will remain in orbit around the new center of our newly enlarged galaxy, the skies will be dramatically changed.

Studies have revealed that as galaxies approach one another massive amounts of gas are pulled from each galaxy towards the center of the other, until ultimately, the two merge into one massive galaxy. NGC 2623 is in the late stages of the merging process, with the centers of the original galaxy pair now merged into one nucleus, but stretching out from the center are two tidal tails of young stars, a strong indicator that a merger has taken place. Image credit: NASA, ESA and A. Evans (Stony Brook University, New York, University of Virginia & National Radio Astronomy Observatory, Charlottesville, USA)

Studies have revealed that as galaxies approach one another massive amounts of gas are pulled from each galaxy towards the center of the other, until ultimately, the two merge into one massive galaxy. NGC 2623 is in the late stages of the merging process, with the centers of the original galaxy pair now merged into one nucleus, but stretching out from the center are two tidal tails of young stars, a strong indicator that a merger has taken place. Image credit: NASA, ESA and A. Evans (Stony Brook University, New York, University of Virginia & National Radio Astronomy Observatory, Charlottesville, USA)

There are lots of places in the universe where galaxies can orbit at much much faster speeds. We don’t expect them to collide in the same spectacular fashion as the Milky Way and the Andromeda galaxy do, because they’re moving so much faster. In galaxy clusters, which are home to hundreds or thousands of galaxies, the relative speeds between any two galaxies can be much, much faster - up to thousands of kilometers per second. At a thousand kilometers per second you’d reach Tokyo from New York in ten seconds flat, take six and a half minutes to get to the Moon, and make it to Pluto in a little under three months. At that speed, even if the galaxies come near each other, they're the equivalent of trying to grab your friend's hand from the window of a high speed train - over very quickly. Only a direct hit between the disks of two galaxies would cause these same kinds of streamers to appear we see from the slower collisions. Given the amount of space between galaxies, even in the relatively dense regions of richly populated clusters, that almost never happens.

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Haven't The Stars In The Oldest Galaxies Died By Now?

If it has taken 13 or so billion light years for the light from the most distant observable stars to reach Earth, is it not probable that those stars no longer exist but have “gone nova”?

Originally posted on Forbes!

It’s not just probable; it’s a certainty! Most of the brightest stars have lifetimes that only keep them going for a few hundred million...

It’s not just probable; it’s a certainty! Most of the brightest stars have lifetimes that only keep them going for a few hundred million years. Even if the light caught in our detectors was generated in that star’s earliest days, by the time that light reached us, 13 billion years later, that star must be long gone. Our own Sun, which has a comparatively lengthy 8 billion year lifespan, wouldn’t have survived for the length of time it takes for the light from the most distant known galaxies to reach us.

What does that mean for the galaxy as we see it? For starters, it means that its population of stars has changed almost entirely since that light began its travels. The brightest stars have all died, exploding out to recycle their gas into their stellar neighborhood, and possibly triggering the formation of a new round of stars. In 13 billion years, this may happen many, many times. But the stellar recycling act isn’t perfect. For every large, extremely bright star that’s formed, we typically expect a number of smaller, fainter stars to also form. These smaller stars (like our Sun) live longer and don’t explode as violently (or at all) at the end of their lifetimes. Stars which are even smaller and redder than our own will live even longer, and may even persist for the entire length of the trip the light took to reach us. This means that over time, the galaxy will build up a reservoir of faint, reddish stars, which limits the amount of gas present in the galaxy available to make new stars. This sequence of events is one of the suggestions for how we end up with the giant elliptical galaxies in the nearby universe -- their population of stars is mostly red, and they seem to have very little gas.

A lone source shines out brightly from the dark expanse of deep space, glowing softly against a picturesque backdrop of distant stars and colorful galaxies. This scene shows PGC 83677, a lenticular galaxy — a galaxy type that sits between the more familiar elliptical and spiral varieties in the Hubble sequence. Image credit: ESA/Hubble & NASA | Acknowledgements: Judy Schmidt (Geckzilla)

A lone source shines out brightly from the dark expanse of deep space, glowing softly against a picturesque backdrop of distant stars and colorful galaxies. This scene shows PGC 83677, a lenticular galaxy — a galaxy type that sits between the more familiar elliptical and spiral varieties in the Hubble sequence. Image credit: ESA/Hubble & NASA | Acknowledgements: Judy Schmidt (Geckzilla)

But it’s not the only pathway open to that distant galaxy -- it’s also possible to refill the galaxy with gas, allowing the galaxy to continue forming the brightest, bluest, shortest-lived stars for a longer period of time. Depending on whether or not the galaxy finds itself surrounded by smaller, gas-filled galaxies, or on its own in a more lonely part of the Universe, that distant galaxy’s course will change again. If we could sit and watch that distant galaxy’s evolution for a few billion more years, we would be able to say for sure which pathway that galaxy was sent down. I don’t know about you, but I certainly don’t have a hundred million years to wait.

So without millions or billions of years to wait for updates from that galaxy, we’re a bit stuck. We effectively have a snapshot of this earliest galaxy as it was, and no ability to check what it did later, or how the galaxies around it changed with time. What we do have is another snapshot of the Universe later, where a different set of galaxies exist, with the same inability to watch where they go forward in their path through their own lives. Trying to piece together which galaxies in the distant universe might evolve into the galaxies we see at more recent times, at less extreme distances, is one of the fundamental puzzles of observational astronomy.

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How Big Is The Biggest Galaxy, And How Small Is The Smallest?

This image from the NASA/ESA Hubble Space Telescope shows the rich galaxy cluster Abell 3827. The strange blue structures surrounding the central galaxies are gravitationally lensed views of a much more distant galaxy behind the cluster. Observations of the central four merging galaxies have provided hints that the dark matter around one of the galaxies is not moving with the galaxy itself, possibly implying dark matter-dark matter interactions of an unknown nature are occuring. Image credit: ESO

This image from the NASA/ESA Hubble Space Telescope shows the rich galaxy cluster Abell 3827. The strange blue structures surrounding the central galaxies are gravitationally lensed views of a much more distant galaxy behind the cluster. Observations of the central four merging galaxies have provided hints that the dark matter around one of the galaxies is not moving with the galaxy itself, possibly implying dark matter-dark matter interactions of an unknown nature are occuring. Image credit: ESO

Originally posted on Forbes!

Of all the galaxies in our surveys of the sky, the biggest, most massive systems are always giant elliptical galaxies. These galaxies are thought to be the end product of repeated galactic collisions, and so the end result is an object, which, though full of stars, no longer resembles the galaxy it was, very early in the lifetime of the Universe.

After so many collisions, the stars within these galaxies no longer orbit in an orderly fashion, like those in our own Milky Way, but are more or less moving randomly with respect to each other. Each star follows its own particular elliptical orbit within the gravitational pull of the galaxy, but with no particular regard for what the other stars are doing around it. As a result, the galaxy has no particular directionality to it. If you could hold it in your hands, you could turn it over and around without finding any particular features to give it an orientation.

This NASA/ESA Hubble Space Telescope image shows an elliptical galaxy known as IC 2006. Massive elliptical galaxies like these are common in the modern Universe, but how they quenched their once furious rates of star formation is an astrophysical mystery.  The quenching of star formation seems to have started in the cores of the galaxies and then spread to the outer parts. Image credit: ESA/Hubble & NASA

This NASA/ESA Hubble Space Telescope image shows an elliptical galaxy known as IC 2006. Massive elliptical galaxies like these are common in the modern Universe, but how they quenched their once furious rates of star formation is an astrophysical mystery.  The quenching of star formation seems to have started in the cores of the galaxies and then spread to the outer parts. Image credit: ESA/Hubble & NASA

The very largest of these ellipticals are found in areas of the Universe where many galaxies gather together; with lots of galaxies in the area, there’s more chance for the galaxy to grow into the size it is by devouring its neighbors. The biggest of all are found lurking at the center of galaxy clusters, which are huge associations of thousands of galaxies. The galaxies within the cluster go whirling around the center of mass of those thousands of galaxies, and like the stars within an elliptical, they do this with no particular order.

There’s a little bit of order; the most massive galaxies sink to the center. And so this is where we find the most massive galaxies in our observable Universe. The current heavyweight sits at the center of the cluster Abell 3827 (shown at the top of the page), and has the entirely unpronounceable name of ESO 146-IG 005. It’s in the process of consuming a number of other galaxies, rapidly growing its own mass in the process. This galaxy is currently measured to be 27 trillion times the mass of our sun, which puts it more then ten times the mass of the Milky Way - it is definitely a giant.

Dwarf Galaxy Pisces A. Image credit: NASA, ESA, and E. Tollerud (STScI

Dwarf Galaxy Pisces A. Image credit: NASA, ESA, and E. Tollerud (STScI

The least massive galaxy, on the other hand, is much harder to find, because it's not very bright. By definition, small galaxies have very few stars, so finding that faint light is an observational challenge. These faintest galaxies are extra hard to find because we’re limited to those which are nearby. A faint galaxy too distant from us will be doubly faint, and impossible to spot.

The other problem with defining the least massive galaxy is that the definition of a galaxy gets a little messy at the low mass end. However, if we use the definition that a galaxy has to have some amount of dark matter surrounding it, the current least massive galaxy seems to be Segue 2Segue 2 is about 1000 stars, held together by dark matter, orbiting our Milky Way, and is only about 800 times brighter than our Sun! 

It's worth noting that both of these objects are likely to be holding temporary titles. As observational methods improve, we may find a galaxy was more massive than we had thought, or find dark matter surrounding stars that we had thought had none. In any case, these two objects are the far ends of the galaxy population - from a thousand stars, loosely held together, in the thrall of the Milky Way’s gravitational pull, to something ten times more massive than our entire Milky Way, itself doing the devouring of other galaxies.

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Why Don't Galaxies Just Pass Through Each Other Instead Of Colliding?

When two galaxies collide they eventually coalesce to form a single (larger) galaxy. By far most of the mass of a galaxy is Dark Matter which feels no friction and suffers not from collisions. Why then do not the dark matter components of the galaxies simply pass through each other and continue going? To suggest that the movement of these dark components is simply governed by the behavior of the visible portions seem to be like the tail wagging the dog.
NGC 6621/2 (VV 247, Arp 81) is a strongly interacting pair of galaxies, seen about 100 million years after their closest approach. It consists of NGC 6621 (to the left) and NGC 6622 (to the right). NGC 6621 is the larger of the two, and is a very disturbed spiral galaxy. The encounter has pulled a long tail out of NGC 6621 that has now wrapped behind its body. Image credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and W. Keel (University of Alabama, Tuscaloosa)

NGC 6621/2 (VV 247, Arp 81) is a strongly interacting pair of galaxies, seen about 100 million years after their closest approach. It consists of NGC 6621 (to the left) and NGC 6622 (to the right). NGC 6621 is the larger of the two, and is a very disturbed spiral galaxy. The encounter has pulled a long tail out of NGC 6621 that has now wrapped behind its body. Image credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and W. Keel (University of Alabama, Tuscaloosa)

Originally posted on Forbes!

Your typical galaxy does indeed have much more mass in its dark matter reservoir than it has in its stars. Unfortunately for us observers, stars are the only easily visible part of a galaxy. With the exception of a few nearby galaxies, where we can measure how much of this dark matter is hanging around a galaxy more directly, we have to assume that there’s some extra mass, but it's hard to know exactly how much. In general, there’s something like two to five times as much dark matter surrounding a galaxy as there is matter in stars and gas.

You’re also completely correct that the dark matter component to a galaxy doesn’t seem to collide with anything, and doesn’t feel friction or any kind of aerodynamic drag. As far as we can tell, dark matter only interacts with other kinds of matter (and with itself) through the force of gravity. So with this setup, you would expect two blobs of dark matter to simply pass through each other if they are on a collision course. And yet, we see that galaxies do collide; how is it possible to pull two objects together without any friction?

To get a galactic collision, you need two things. The first is to have two objects moving along a path where they will eventually pass near each other. However, if these two objects are moving too quickly relative to each other, they may just slingshot past one another, never to encounter each other again. This is called a flyby encounter, and happens a lot in clusters of galaxies. Galaxy clusters contain thousands of galaxies, all moving very quickly, and so each galaxy may swing past a number of others, but is moving too quickly to stop and merge with any of them.

This Hubble image displays a beautiful pair of interacting spiral galaxies with swirling arms. The smaller of the two, dubbed LEDA 62867 and positioned to the left of the frame, seems to be safe for now, but will probably be swallowed by the larger spiral galaxy, NGC 6786 (to the right) eventually. Image Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

This Hubble image displays a beautiful pair of interacting spiral galaxies with swirling arms. The smaller of the two, dubbed LEDA 62867 and positioned to the left of the frame, seems to be safe for now, but will probably be swallowed by the larger spiral galaxy, NGC 6786 (to the right) eventually. Image Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

So let’s say you have both pieces; your galaxies are on a collision course, and moving slow enough so they won’t go zipping past each other. The speed is critical. The two galaxies need to have some time to influence each other - the longer the galaxies spend near each other, the longer they have to be influenced by the other galaxy. But how do you slow down the galaxy enough to capture it, and eventually merge it into a single galaxy? The slowing is due to a process called dynamical friction. Even if the individual particles which make up a galaxy (which is mostly dark matter and stars) never physically touch each other, they still influence each other through gravity.

Galaxies on a collision course will swing past each other - let’s say they’re moving past each other like two people on escalators going opposite directions (not a bad approximation). The nearest stars in one galaxy pull gravitationally on the nearest stars in the other galaxy, and both sets of stars wind up slowing down as a result. It would be like reaching across the barrier and clasping hands with the person on the escalator going the opposite direction as you; that brief pull between you would pull both of you off balance. Both of you would get pulled towards each other, against the direction of your escalator. (I don’t recommend trying this experiment.) Over time, as every star does this with every other star, and the dark matter particles do this with every other dark matter particle, the stars in both galaxies will come to rest relative to each other. You’ve created a new, single galaxy.

Arp 256 is a stunning system of two spiral galaxies in an early stage of merging. The Hubble image displays two galaxies with strongly disrupted shapes and an astonishing number of blue knots of star formation that look like exploding fireworks. The galaxy to the left has two extended ribbon-like tails of gas, dust and stars. The system is a luminous infrared system radiating more than a hundred billion times the luminosity of our Sun. Image Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University), and G. Ostlin (Stockholm University)

Arp 256 is a stunning system of two spiral galaxies in an early stage of merging. The Hubble image displays two galaxies with strongly disrupted shapes and an astonishing number of blue knots of star formation that look like exploding fireworks. The galaxy to the left has two extended ribbon-like tails of gas, dust and stars. The system is a luminous infrared system radiating more than a hundred billion times the luminosity of our Sun. Image Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University), and G. Ostlin (Stockholm University)

This dynamical friction trick works great for individual galaxies, because both the dark matter and the stars are basically collisionless. Dark matter doesn’t interact with anything except by gravity, and the stars within a galaxy are so far apart that they’re incredibly unlikely to ever collide with another star. But galaxies aren’t uniquely stars and dark matter - there’s also gas: the stuff of nebulae. Gas is different from stars in that it can be compressed easily, and it collides with itself much more easily than the stars do. If you get enough galaxies together (as you do in a cluster), this gas can get pulled out of the galaxy, and heated to such a high temperature that it glows in X-rays.

So what happens if you take a cluster of galaxies, which is made of a huge mess of dark matter, several hundred galaxies made of stars, and a giant cloud of gas, permeating the whole cluster, and fling it at another cluster?

The Bullet Cluster. Dark purple indicates the location of the visible + dark matter mass. Red shows the X-ray emitting gas. Individual galaxies are seen in the optical image. Image credits: X-ray: NASA/CXC/CfA/ M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al., Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al

The Bullet Cluster. Dark purple indicates the location of the visible + dark matter mass. Red shows the X-ray emitting gas. Individual galaxies are seen in the optical image. Image credits: X-ray: NASA/CXC/CfA/ M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al., Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al

You get the Bullet Cluster, just above. This is one of the most famous clusters because it gave us really solid proof that dark matter was real, observable, and collisionless. The Bullet Cluster is actually two clusters in the early stages of merging together. We can see the two bundles of galaxies, on the left and right of the image. If you trace the amount of mass in each cluster, and then check where the X-ray gas is, you can see that the gas slammed into the gas from the other cluster like a brick wall - the stars and dark matter sailed right on through.

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How Can We Count Galaxies If They're Evolving Or Dying Out?

Does the estimate of the number of galaxies in the universe only contain those that exist now, or those that we can see now that their light has traveled to us? I mean, some of what we see now must have evolved, and died out. When you look into the sky are you seeing the same thing repeating, like a star for example, only at different times and distances?

Originally posted at Forbes!

Trying to count the total number of galaxies in the universe is a difficult task, made harder by the part where no one wants to spend an infinite amount of time counting galaxies. Instead, what we usually do is count the number of galaxies in a very small area of the sky. Usually what happens is that we point a telescope at a very empty, dark patch of sky, and wait for a while. We’ve done this a few times with Hubble, creating what we now call the Deep Fields. We now have the Hubble Deep Field, the Hubble Ultra Deep Field, and the Hubble eXtreme Deep Field. (Once more, astronomers prove themselves eminently practical namers.) Once we have a really deep image, we can then assume every other patch of the sky is roughly going to look the same (as far as we can tell, a valid assumption). We can then multiply the number of galaxies in that one piece of sky by the fraction of sky we looked at, and get a very rough estimate of the total number of galaxies. Hey presto: several hundred billion galaxies!

Like photographers assembling a portfolio of best shots, astronomers have assembled a new, improved portrait of mankind’s deepest-ever view of the universe. Called the eXtreme Deep Field, or XDF, the photo was assembled by combining 10 years of NASA Hubble Space Telescope photographs taken of a patch of sky at the center of the original Hubble Ultra Deep Field. The XDF is a small fraction of the angular diameter of the full Moon. The Hubble Ultra Deep Field is an image of a small area of space in the constellation Fornax, created using Hubble Space Telescope data from 2003 and 2004. By collecting faint light over many hours of observation, it revealed thousands of galaxies, both nearby and very distant, making it the deepest image of the universe ever taken at that time. The new full-color XDF image reaches much fainter galaxies, and includes very deep exposures in red light from Hubble’s new infrared camera, enabling new studies of the earliest galaxies in the universe. The XDF contains about 5,500 galaxies even within its smaller field of view. The faintest galaxies are one ten-billionth the brightness of what the human eye can see. Image Credit: NASA, ESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team

Like photographers assembling a portfolio of best shots, astronomers have assembled a new, improved portrait of mankind’s deepest-ever view of the universe. Called the eXtreme Deep Field, or XDF, the photo was assembled by combining 10 years of NASA Hubble Space Telescope photographs taken of a patch of sky at the center of the original Hubble Ultra Deep Field. The XDF is a small fraction of the angular diameter of the full Moon. The Hubble Ultra Deep Field is an image of a small area of space in the constellation Fornax, created using Hubble Space Telescope data from 2003 and 2004. By collecting faint light over many hours of observation, it revealed thousands of galaxies, both nearby and very distant, making it the deepest image of the universe ever taken at that time. The new full-color XDF image reaches much fainter galaxies, and includes very deep exposures in red light from Hubble’s new infrared camera, enabling new studies of the earliest galaxies in the universe. The XDF contains about 5,500 galaxies even within its smaller field of view. The faintest galaxies are one ten-billionth the brightness of what the human eye can see. Image Credit: NASA, ESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team

But you’re absolutely on the money with your suggestion that we can only see the galaxies whose light has reached us. This fundamentally limits our observing, so that the galaxies which are most distant from us are also very far removed from our current time.

And here we begin a giant puzzle, because the most distant galaxies, which we see the farthest in the past, are very different from the galaxies we see in our local, nearby universe, which we see much closer to our current time. This means, again as you suggest, that the ancient galaxies we can spot in images such as the Hubble Deep Fields, must have evolved and changed between the time when the light we observe left them, and now. Galaxies which were once independent will have merged in the billions of years which have passed while that light was making its long way to us. Some galaxies will have used up or lost the gas they need to create new stars – one of the few ways a galaxy can “die out”, though its existing stars will disagree with you on how dead they are. Some galaxies will have uneventful evolutions, though they will still evolve. At a base level, galaxies will be creating more stars over time and adding to their own mass, though the number of new stars they make each year will drop over time.

It is known today that merging galaxies play a large role in the evolution of galaxies and the formation of elliptical galaxies in particular. However there are only a few merging systems close enough to be observed in depth. The pair of interacting galaxies picture seen here — known as NGC 3921 — is one of these systems. NGC 3921 — found in the constellation of Ursa Major (The Great Bear) — is an interacting pair of disc galaxies in the late stages of its merger. Observations show that both of the galaxies involved were about the same mass and collided about 700 million years ago. You can see clearly in this image the disturbed morphology, tails and loops characteristic of a post-merger. The clash of galaxies caused a rush of star formation and previous Hubble observations showed over 1000 bright, young star clusters bursting to life at the heart of the galaxy pair. Image credit: ESA/Hubble & NASA

It is known today that merging galaxies play a large role in the evolution of galaxies and the formation of elliptical galaxies in particular. However there are only a few merging systems close enough to be observed in depth. The pair of interacting galaxies picture seen here — known as NGC 3921 — is one of these systems. NGC 3921 — found in the constellation of Ursa Major (The Great Bear) — is an interacting pair of disc galaxies in the late stages of its merger. Observations show that both of the galaxies involved were about the same mass and collided about 700 million years ago. You can see clearly in this image the disturbed morphology, tails and loops characteristic of a post-merger. The clash of galaxies caused a rush of star formation and previous Hubble observations showed over 1000 bright, young star clusters bursting to life at the heart of the galaxy pair. Image credit: ESA/Hubble & NASA

Untangling the complex line which can connect a nearby galaxy to the sort of galaxy it might have been, billions of years ago, is a whole subfield of astronomy, under the moniker of galaxy evolution.

It’s important to keep in mind that it’s not quite as simple as seeing the same things repeated over and over again. The galaxies we see much earlier in their lives than our own are truly, physically, very far away, which is why we see them so far removed in time. They will be evolving over time in their own physical space, but they should evolve into something that looks like the galaxies near us. Distant galaxies seem to be the same everywhere we look, so we shouldn’t be looking at a special group of distant galaxies that would evolve in a unique way. They’re not the same galaxies as the ones that built our own galaxy, but they should be pretty similar. It’s up to us to learn what the pathway between ancient and current day must have been.

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