Why Is Andromeda Coming Towards Us?

Article written: 22 Jan , 2015
Updated: 27 Feb , 2017
by

I don’t want to freak you out, but you should be aware that there’s a gigantic galaxy with twice our mass headed right for us. Naw, I’m just kidding. I totally want to freak you out. The Andromeda galaxy is going to slam head first into the Milky Way like it doesn’t even have its eyes on the road.

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Transcript

I don’t want to freak you out, but you should be aware that there’s a gigantic galaxy with twice our mass headed right for us. Naw, I’m just kidding. I totally want to freak you out. The Andromeda galaxy is going to slam head first into the Milky Way like it doesn’t even have its eyes on the road.

This collision will tear the structure of our galaxy apart. The two galaxies will coalesce into a new, larger elliptical galaxy, and nothing will ever be the same again, including your insurance premiums. There’s absolutely nothing we can do about it. It’s like those “don’t text and drive commercials” where they stop time and people get out and have a conversation about their babies and make it clear that selfish murderous teenagers are really ruining everything for all of us all the time.

The Andromeda Galaxy will collide with the Milky Way in the future. Credit: Adam Evans

The Andromeda Galaxy will collide with the Milky Way in the future. Credit: Adam Evans

And now that we know disaster is inbound, all we can do is ask WHY? Why this is even happening? Isn’t the Universe expanding, with galaxies speeding away from us in all directions? Shouldn’t Andromeda be getting further away, and not closer? What the hay, man!

Here’s the thing, the vast majority of galaxies are travelling away from us at tremendous speed. This was the big discovery by Edwin Hubble in 1929. The further away a galaxy is, the faster it’s moving away from us. The most recent calculation by NASA in 2013 put this amount at 70.4 kilometers per second per megaparsec. At a billion light-years away, the expansion of the Universe is carrying galaxies away from us at 22,000 km/s, or about 7% of the speed of light. At 100 million light-years away, that speed is only 2,200 km/s.

Which actually doesn’t seem like all that much. Is that like Millenium Falcon fast or starship Enterprise Warp 10 fast? Andromeda is only 2.5 million light-years away. Which means that the expansion of the Universe is carrying it away at only 60 kilometers per second. This is clearly not fast enough for our purposes of not getting our living room stirred into the backyard pool. As the strength of gravity between the Milky Way and Andromeda is strong enough to overcome this expansive force. It’s like there’s an invisible gravity rope connecting the two galaxies together. Dragging us to our doom. Curse you, gravity doom rope!

The Hubble Space Telescope's extreme close-up of M31, the Andromeda Galaxy. Picture released in January 2015. Credit: NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler

The Hubble Space Telescope’s extreme close-up of M31, the Andromeda Galaxy. Picture released in January 2015. Credit: NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler

Andromeda is speeding towards us at 110 kilometers per second. Without the expansion of the Universe, I’m sure it would be faster and even more horrifying! It’s the same reason why the Solar System doesn’t get torn apart. The expansion rate of the Universe is infinitesimally small at a local level. It’s only when you reach hundreds of millions of light-years does the expansion take over from gravity.

You can imagine some sweet spot, where a galaxy is falling towards us exactly as fast as it’s being carried away by the expansion of the Universe. It would remain at roughly the same distance and then we can just be friends, and they don’t have to get all up in our biz. If Andromeda starts complaining about being friend-zoned, we’ll give them what-for and begin to re-evaluate our friendship with them, because seriously, no one has time for that.

The discovery of dark energy in 1998 has made this even more complicated. Not only is the Universe expanding, but the speed of expansion is accelerating. Eventually distant galaxies will be moving faster away from us than the speed of light. Only the local galaxies, tied together by gravity will remain visible in the sky, eventually all merging together. Everything else will fall over the cosmic horizon and be lost to us forever.

This annotated artist's conception illustrates our current understanding of the structure of the Milky Way galaxy. Image Credit: NASA

This annotated artist’s conception illustrates our current understanding of the structure of the Milky Way galaxy. Image Credit: NASA

All things in the Universe are speeding away from us, it’s just that gravity is a much stronger force at local levels. This is why the Solar System holds together, and why Andromeda is moving towards us and in about 4 billion years or so, the Andromeda galaxy is going to slam into the Milky Way.

So, if by chance you only watched the first part of this video, freaked out, sold your possessions and joined some crazy silver jumpsuit doomsday cult, and are now, years later watching the conclusion… you may feel a bit foolish. However, I hope that you at least made some lifelong friendships and got to keep the jumpsuit.

Really, there’s nothing to worry about. Stars are spread so far apart that individual stars won’t actually collide with each other. Even if humanity is still around in another 4 billion years or so, which is when this will all go down. This definitely isn’t something we’ll be concerned with. It’s just like climate change. Best of luck future generations!

What do you think, will humans still be around in 4 billion years to enjoy watching the spectacle of the Milky Way and Andromeda collide?

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12 Responses

  1. Abes48 says

    Best video yet, Fraser. Nice job!

  2. tonybudz says

    Another very good reason to admit that our time on this planet is limited. We have less than 3.5 billion years to build the technology and create a plan to escape annihilation. Afterwards we have less than 50 million years to actually escape the boundaries of both galaxies, if we survive that long.

    • ed b says

      For whatever it is worth, we will have 500 million years to escape.
      4 billion – 3.5 billion = .5 billion or 500 million. Otherwise your logic is “right on”.

    • mewo says

      It is unlikely that the collision between the Milky Way and Andromeda will negatively affect us. Galaxies are mostly empty space, and the chance that another star, gas cloud, or anything else will affect the Solar System is minimal. The same goes for any other solar system we can escape to. The only real risk is that our home solar system could be ejected (intact) into intergalactic space.

  3. Icee Red says

    First off the earth will be uninhabitable for human life within the next billion years or so, simply because the sun is increasing in luminosity over time. Citing Wikipedia on The Future of the Earth: “During the next four billion years, the luminosity of the Sun will steadily increase, resulting in a rise in the solar radiation reaching the Earth. This will cause a higher rate of weathering of silicate minerals, which will cause a decrease in the level of carbon dioxide in the atmosphere. In about 600 million years, the level of CO2 will fall below the level needed to sustain C3 carbon fixation photosynthesis used by trees. Some plants use the C4 carbon fixation method, allowing them to persist at CO2 concentrations as low as 10 parts per million. However, the long-term trend is for plant life to die off altogether. The extinction of plants will be the demise of almost all animal life, since plants are the base of the food chain on Earth.”

    Also “In about 1.1 billion years, the solar luminosity will be 10% higher than at present. This will cause the atmosphere to become a “moist greenhouse”, resulting in a runaway evaporation of the oceans. As a likely consequence, plate tectonics will come to an end, and with it the entire carbon cycle.”

    As for galactic collisions, merger would be a better word. As mentioned, the space between stars is so large that galaxies can pass thru each other with the stars themselves intact, just gravitationally pulled in new directions. Our Milky Way galaxy and Andromeda have swallowed smaller galaxies in the past and will one day merge. Stars and planetary systems actually physically colliding would be a rarity.

    Anthropogenic (human caused) climate change on the other hand is a present day occurrence with consequences in the near term and one that is capable of being addressed by humanity as a whole, as opposed to oncoming galaxies and the changing luminosity of our sun which we cannot alter.

  4. RAYMOND_T says

    Crash! Burn! Yeah! I can’t wait to see it happen!
    I wonder where this so-called “Sweet Spot” is in the Universe, it could make for some interesting physics.

  5. TedH says

    No Fraser, humans won’t be around in app. 4billion years. If we don’t wipe out our race ourself… evolution goes on and we will be so different to what we are now like we are now compared to the stromatolites, or like the Spirit of Saint Louis compared to an A380 😉

  6. mzxo says

    ah how I love Science Fiction

  7. Member
    Gozlemci says

    Don’t miss: Rogue Star HIP 85605 is coming…!
    You, probably, may have time only 239 000 years+ a couple dozen weeks…!

  8. BrianFraser says

    You have pointed out something that astronomy seems not to recognize: that there is an equilibrium “sweet spot” (as you call it) where the inward force of gravitation just balances with the outward “force” of the expansion of the Universe. That would nicely explain the stability of globular clusters. It also would explain why galaxies rotate as a disk (more or less). No Dark Matter is needed!

    The location of the “sweet radius” or “interstellar gravipause” (as I will call it) depends on the mass of the object under consideration. It is evidently a few light years for stars and a few million light years for galaxies. Inside the radius, gravity holds sway and matter will accrete towards a center; outside this distance, matter will disperse with the expansion. That is why stars are separated by light hours or light years but not light months.

    Someone should check the math. The gravitational retardation will be proportional to 1/d^2 . The Hubble retardation will be proportional to 1/d^1 . Eventually it will become 1/d^0 . THAT, incidentally, implies that the Hubble “constant” is not truly constant; it is, to some extent, distance dependent, which may explain some of the discrepant measurements.

    We would still see the LIGHT from an object moving at speeds above that of light, but such light would be “non-local” from the standpoint of a spatial reference system. We would not see the object itself, because its light would be scattered uniformly all over the sky–as background radiation.

  9. koolduck says

    We would need to leave here to get a good view. We would need to leave now.

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