A Blog by Nadia Drake

Milky Way Has 4 Billion Years to Live — But Our Sun Will Survive

Four billion years from now, our galaxy, the Milky Way, will collide with our large spiraled neighbor, Andromeda.

The galaxies as we know them will not survive.

In fact, our solar system is going to outlive our galaxy. At that point, the sun will not yet be a red giant star – but it will have grown bright enough to roast Earth’s surface. Any life forms still there, though, will be treated to some pretty spectacular cosmic choreography.

Currently, Andromeda and the Milky Way are about 2.5 million light-years apart. Fueled by gravity, the two galaxies are hurtling toward one another at 402,000 kilometers per hour. But even at that speed, they won’t meet for another four billion years. Then, the two galaxies will collide head-on and fly through one another, leaving gassy, starry tendrils in their wakes. For eons, the pair will continue to come together and fly apart, scrambling stars and redrawing constellations until eventually, after a billion or so years have passed, the two galaxies merge.

Then, the solar system will have a new cosmic address: A giant elliptical galaxy, formed by the collision and merger of the Milky Way and Andromeda.

This isn’t a chapter ripped from science fiction – it’s a real, scientific prediction. That science can forecast such events was the focus of the third episode of Cosmos: A Spacetime Odyssey. That Newton could describe the orbits of planets, and Halley the return of his eponymous comet, and contemporary astronomers, the end of the Milky Way – this gift of foresight is really a mathematical understanding of the physical laws that govern the movements of celestial bodies.

“Using nothing more than Newton’s laws of gravitation, we astronomers can confidently predict that several billion years from now, our home galaxy, the Milky Way, will merge with our neighboring galaxy, Andromeda,” host Neil DeGrasse Tyson says. “Because the distances between the stars are so great compared to their sizes, few if any stars in either galaxy will actually collide. Any life on the worlds of that far-off future should be safe, but they will be treated to an amazing, billion-year long light show.”

The galactic collision that closes out the third Cosmos episode follows the sequence in the animation below, which is based on a 2006 simulation by astrophysicist Brant Robertson*.

Video: NASA/Vimeo

Now, how on Earth do we know this is going to happen?

The story starts in the early 1900s, when astronomer Vesto Slipher measured the radial velocity of Andromeda — in other words, he calculated the speed at which the galaxy was moving toward or away from Earth. Slipher did this by looking for a telltale stretching or compression in the light from Andromeda arriving at Earth: Light from objects that are moving away from us is slightly stretched, or red-shifted. Light from objects moving toward us is blue-shifted, or compressed.

The result was a little bit surprising.

“We may conclude that the Andromeda Nebula is approaching the solar system with a velocity of about 300 kilometers per second,” Slipher wrote in the Lowell Observatory Bulletin in 1913 (Andromeda was called a nebula back then because astronomers didn’t realize it wasn’t part of the Milky Way; Slipher’s calculation strongly suggested that idea needed rejiggering).

So Andromeda was zooming toward us – that much at least seemed clear. Whether its arrival would mean the end of the Milky Way was still uncertain. For decades, scientists had no way of knowing whether Andromeda and the Milky Way would collide head-on, or if they would slip past one another like star-filled vessels in the cosmic night.

Turns out, it’s relatively easy to measure the velocity of faraway objects moving toward or away from us, but much more difficult to determine their sideways motion (something astronomers call “proper motion”). The farther away something is, the harder it is to measure its sideways motion, which doesn’t produce those telltale stretched or compressed wavelengths that astronomers can work with. Instead, astronomers rely on detailed observations of an object’s position relative to background stars – a small and subtle shift that without superior telescopes can take centuries to become apparent.

Artist's conception shows the future Earth, whose oceans have boiled away due to the Sun's increasing heat, exiled to the outskirts of the new merged galaxy that astronomers have dubbed "Milkomeda."  (David A. Aguilar, CfA)
Artist’s conception of the future Earth, whose oceans have boiled away due to the Sun’s increasing heat, exiled to the outskirts of the new merged galaxy that astronomers have dubbed “Milkomeda.” (David A. Aguilar, CfA)
Around 2007, Harvard University astrophysicist Avi Loeb decided to revisit the question of Andromeda’s impending arrival. “Most theorists are interested in reproducing systems from our past that are observed now, and are reluctant to make predictions that will be tested only billions of years from now,” Loeb says. “The rationale was unclear to me; I am curious about the future as much as I am about the past.”

Loeb and then post-doc T.J. Cox simulated the impending collision and merger of Andromeda and the Milky Way using estimates of Andromeda’s proper motion. Their results showed a better than decent chance of the two galaxies smashing into one another, and a pretty good possibility of the solar system being punted to the outskirts of the resulting elliptical galaxy, which Loeb named “Milkomeda.”

In 2012, a team of astronomers based at the Space Telescope Science Institute re-did the collision calculations, this time using direct measurements of Andromeda’s proper motion. After all those years, the team was able to get those measurements with the Hubble space telescope – and an observing campaign that used years of data, beginning with images snapped in 2002.

“We compared images taken at different times with the Hubble Space Telescope, and measured how much the Andromeda stars have moved relative to the fuzzy galaxies in the distant background,” says astronomer Sangmo Tony Sohn. “This gives us a sense of how fast the Andromeda stars moved across the sky.”

The team concluded that Andromeda’s proper motion was tiny – and that a head-on collision was pretty much inevitable. That might sound a little bit traumatic, but it’s not all that unusual for galaxies to merge. The Hubble space telescope has captured some glorious images of faraway mergers and collisions, and astronomer Halton Arp included a number of galactic interactions in his “Atlas of Peculiar Galaxies,” published in 1966. They’re all really pretty.

The good news is that, as Tyson says, stars are so far apart that even though galaxies are colliding, the probabilities of stellar collisions are small. So the sun and its planets will likely survive the birth of Milkomeda, though Earth will no longer be able to call the Milky Way home. And we’ll no longer live in a spiral galaxy: Milkomeda will be elliptical in shape, and it’ll probably look pretty red, which you can see toward the end of the 2012 team’s animation, and in the animation above.

So there’s no doubt this merger is going to be a spectacle – and there’s a good chance that the Triangulum, a smaller, nearby galaxy, will get sucked into the fray. I, for one, am disappointed that I won’t be able to watch this great cosmic light show. For now, the best I can do is enjoy the sequence of illustrations below.

Present day; 2 billion years from now; 3.75 billion years; 3.85 billion years; 3.9 billion years; 4 billion years; 5.1 billion years from now; and 7 billion years from now, when the galaxies have formed a huge elliptical galaxy. (NASA/ESA/Z. Levay and R. van der Marel, STScI/T. Hallas and A. Mellinger
Present day; 2 billion years from now; 3.75 billion years; 3.85 billion years; 3.9 billion years; 4 billion years; 5.1 billion years from now; and 7 billion years from now, when the galaxies have formed a huge elliptical galaxy.

*9:45pm PDT, 3/24: This post has been updated to attribute the embedded animation to astrophysicist Brant Robertson, now at the University of Arizona, and his colleagues. NASA recently redid the animation.

41 thoughts on “Milky Way Has 4 Billion Years to Live — But Our Sun Will Survive

  1. Congratulations on a nice article! It was neat to see our simulation (done with Lars Hernquist at Harvard) in the Cosmos episode. The animation in the Cosmos episode was originally produced by the NCSA Visualization Lab for the James Webb Space Telescope website. For more information, see http://www.jwst.nasa.gov.

  2. Hmm… So Earth will come out “unscathed”? But wouldn’t the huge gravitational effects cause all kinds of disturbances with the large and small bodies in the Solar System? I mean, if the forming of the solar system created enough gravitational disturbance to change the planets’ configuration, how would a galactic collision not?

  3. Also it was noted that the Sun would grow in size and it would be much to hot for human life to exist here on Earth some time into the future. So NASA was right in trying to send out exploratory ships to Mars, as it is a colder planet today but in the future it will get hotter and who knows what changes will take place then. So also I think Mars will be too small for future of mankind to exist with the greater population. But a great drop in planet on the way to our new home in four or so million years, as we move away from the sun.

  4. things are never the same.we cant know the future.in reality we dont even know how much time the world has exactly spent and how much will,the stars sun & moon etc all cant be expected to be the best for life existance for human beings but we can stop the over population and overgrazing & set the conditions of the earth .pix are amazing by the way.

  5. so 2 to 3 billion light years from two galaxies will will join with each other,how long have ie us earthling been studing the sky, ie the hubble telescope see,s pic,s of the universe a zillion lighty years away

    my confusion is , if we can see the light its already happened

    1. It would have already happened— if Andromeda had been traveling at the speed of light. Andromeda is traveling more slowly than the light that is emitted from it and reaches us. The article says that we are about 2.5 billion light years apart, which means the light took 2.5 billion years to get to us, so it’s a “little” delayed. However, again, Andromeda is still lagging far behind the light emitted from it, so the collision will happen in about 4-5 billion years. 2.5 billion light-years means 2.5 billion years for the light to reach us, not Andromeda.

  6. An interesting article. However – I am curious that, since “probabilities of stellar collisions are small”, most writers use the term “collision” and Ms. Drake talks of the two galaxies “smashing into one another”. Why is it that writers feel the need to use evocative language in an attemt to dress up reality, when the true nature of the universe is already quite astoundingly beautiful?

  7. There are some musteries that r nevr meant to be solved by us…it wil nly bring destruction….who knows how many planets are there…who knows are there any life forms…was there anythng even before the dinosaurs…no 1 knows how big bang happnd….these r nt meant to be solved by humans…we should nt try also…

  8. Henit (user above), this isn’t texting. *Spell* out your post. “There are some mysteries that are never meant to be solved by us…it will only bring destruction….who knows how many planets are there…who knows are there any life forms…was there anything even before the dinosaurs…no one knows how the Big Bang happened….these aren’t meant to be solved by humans…we shouldn’t try also…”

  9. Once The Milky Way and Andromeda collide to become a whole other galexy, what is to become of the name of this new elliptical, cosmic, star cluster? I am quite curious.

  10. I really enjoyed this. I think it would be interesting to see some examples of the stars and planets colliding, but then again I can always find pictures of that. Anyway, very helpful for my research paper.

  11. my question to brent
    it does not make sense if the earth is so hot after the collision due to the bright sun how will our future generations be able to see all those stuff

  12. my question to brent
    it does not make sense if the earth is so hot after the collision due to the bright sun how will our future generations be able to see all those stuff

  13. In his Cosmos series, Neil DeGrasse Tyson posits that the Earth’s orbit could be shifted away from the Sun by moving an Apophis sized asteroid close to it – In the future this may be accomplished with ion driven engines attached to an asteroid being controlled by an AI that we create & launch into space. The asteroid would just need to fly by the earth and nudge it about once every 10,000 years. I believe the Earth might make it longer than we can imagine because we’re not using enough imagination.. Set my cryo chamber to reanimate me in 5 billion years, it’s going to be a spectacular light show.
    In 20 billion years will our elliptical turn into a spiral once more?

  14. If the Andromeda Galaxy also has a supermasive black hole in it’s center when the galaxies collied won’t the black hole’s collied to? What do 2 supermassive black holes make?

  15. To Random Person……Two colliding black holes send ripples through the space-time fabric of the Universe that are called gravitational waves. It is possible for two black holes to collide. Once they come so close that they cannot escape each other’s gravity, they will merge to become one bigger black hole.

  16. As I understand it, (I don’t claim to understand much) the entire universe is thought to be currently expanding. The statement that “the two galaxies are hurtling toward one another at 402,000 kilometers per hour” says to me that the universe or our position in it is contracting. Am I missing something, like scientists have concluded that the universe is contracting rather than expanding?

    1. All the galaxies in the Virgo Supercluster are gravitationally bound to each other. We are not expanding from these galaxies. We will most likely merge with a few more galaxies in the next 100 billion years, then watch the rest of the universe fly away because dark energy, (the force driving the universe apart), outweights our gravitontional attraction.

  17. u-hsen is wrong.according to scientists when the sun expands and gets older it will envelope venus and mercury but earth will just be out of reach.and survive.earth will no doubt be too hot to live on but mars wont be cold anymore and the frozen water on mars should run again.and im sure man will make mars like earth and live the remaining life of humankind there.till the sun burns out.

    1. You do realize that National Geographic is a magazine made by scientists, right? And multiple well-known sources say that the Earth will also be consumed, so it seems to be the most commonly accepted prediction.

      1. what i said is based on scientists.it doesn’t matter anyways.the earth will be too hot for any human to live on.the oceans will be gone and the earth will be like venus.people will be living on mars to extend the human race.

  18. Let’s see. This is all going to happen in approximately 4 billion years, right? Well now, it was just 65 million years ago when the asteroid killed all the dinosaurs. That’s 65 m / 4 b = 1.625% of the time until then. Sixty Five million years ago, “humans” didn’t exist, unless you want to call a creature just slightly larger than a mouse, that had snout and whiskers and a long tail, “human.”

    In 4 billion years, the human race will have long since become extinct. In all probability, the human race will have become extinct in less than 1 million years. Remember that 1 million years ago, we looked more like chimpanzees than what we look like now. There were no humans 1 million years ago, and there very, very likely be no humans 1 million years from now.

    We might have descendants 1 million years from now, but they won’t be human. And in 4 billion years? We probably won’t even have that.

    1. you don’t know how long man has been on earth.you’re going by what we have found so far.in fact i read somewhere they found the remains of a human that dated back further than any remains of a human on record.scientists were baffled.

  19. Those who meditate on the word of the Lord will definitely welcome the descent of New Jerusalem as revealed to St. John in the last book of Holy Bible! Be truthful in Spirit to understand the secrets behind the symbolization in that book; for God has created the stars for signs & seasons; Praise be to Him alone. Amen!

  20. In 150 years from now neither one of us who even posted a comment will be living! Even if it happened tomorrow what we gone do about it? Nothing but pray and cry! So when you wake up in the morning eat you some eggs,bacon,grits,sausage, and a pile of pancakes or waffles and a tall glass of juice and just chill out! Dude! Then perform at your favorite hobby and your day will be much better! Thank you very much (Elvis voice)!

  21. Our Sun does not appear to be gravitationally bound to another star which would make our Sun part of a binary system. The motion of our Sun and the other stars in the galaxy do not follow repeatable Keplerian orbits around the Milky Way. Our Sun and the other stars in the Milky Way follow complicated looping paths which are determined by the gravitational influence of the collective swarm of stars, massive interstellar clouds and the Dark Matter imbedded throughout the Milky Way, plus they ‘bob’ up and down through the mid-plane of the Milky Way like horses on a merry-go-round. The central bulge region of the Milky Way rotates as a swarm of stars surrounding the central black hole in the same general direction as the rest of the galactic disk. The central black hole’s axis of rotation is inclined about 15 degrees to that of the Milky Way’s disk. The plane of our Solar System is inclined at a steep angle with respect to the plane of the Milky Way, almost perpendicular to it. The mass of the Milky Way’s central black hole is estimated at about 4 million solar masses. The total apparent mass of the Milky Way is approximately 1.25 trillion solar masses. That makes the central black hole about 1/300,000 the mass of the entire Milky Way. The pressure waves revolve around the Milky Way more slowly: thus, the stars and interstellar gas ‘catch up’ to them from behind and the gas can ‘pile up’ to induce star formation within the arms. The Solar System is the tiny system of planets orbiting one particular tiny star, our Sun. The Milky Way is a galaxy composed of approximately 100 billion to 400 billion of such stars. We now realize most of those stars probably host their own tiny planetary systems. Our Sun and all the other 100 billion to 400 billion of our neighbor stars don’t progressively spiral inward. Our Milky Way is currently drifting toward our neighbor galaxy, the great spiral in Andromeda, which will result in a merger to produce a single giant galaxy about 4 billion years from now.

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