<p>4 billion years is the same timeframe of the life remaining to our Sun.</p>

<p>So if we have not yet invented interstellar voyages, we're screwed, with or without Andromeda.</p>

<p>Besides, stars do not interact directly with each other in a galactic collision. What we will notice from the several stars we are on is that star orbits around the galactic centre will be altered by the massive gravitational perturbation made by the other galaxy. Almost all stars will change orbits from that of orbiting around the galactic centre to orbiting around the center of mass of both galaxies. Some stars will be ejected out of the new bigger galaxy. It is quite safe to think that planets will continue orbiting their stars, but not that they will not have some alterations in their orbits.</p>

<p>On the other hand, from the gas to gas interaction, there will be a lot of new pressure waves in the interstellar medium, which will account for the formation of billions of new stars in new nebulae.</p>

CC BY-SA 3.0
2015-03-25T10:28:34.783Z
@envite: +1 for  **if we have not yet invented interstellar voyages, we're screwed, with or without Andromeda.**
Pritam
CC BY-SA 3.0
2017-09-06T23:36:25.790Z
4 billion years is not the remaining lifetime of the Sun. It will still be on the main sequence in 4 billion years. Perhaps you meant: similar timescale ?
ProfRob
CC BY-SA 3.0
2014-03-04T12:03:46.347Z
so are there any chances of evaporation of earths due to large heat due to gases ? So there is no chance to survival of human being. How long before we should think about inter-stellar voyage?
AmitG
CC BY-SA 3.0
2014-03-04T12:39:53.363Z
The point is that over the timeframe of 4 billion years many stars cease to exist and planets will become uninhabitable (including Sun/Earth) no matter what happens with the galaxy collision.  So by the time of Andromeda/Milkyway collision either humans will be an interstellar migratory species and able to work around it, or will be extinct before it happens.
Peteris
CC BY-SA 3.0
2014-03-05T23:45:38.077Z
@AmitG: I don't have the numbers, but I'd think that any heat from colliding interstellar gas clouds would be a tiny fraction of the energy we already receive from that nuclear furnace just 1 AU away.
Keith Thompson
CC BY-SA 3.0
2014-05-25T23:23:19.800Z
Heat from pressure waves in interstellar gas is not likely to be a problem. Increased star formation, including very large, short-lived stars that will go supernova in the neighborhood might be a problem.
Marc
CC BY-SA 3.0
2014-03-04T11:30:31.353Z
2014-03-04T11:30:31.353Z
Envite
<p>Direct collisions between stars and planets is highly unlikely, due to the relatively low density of objects in the Milky Way and Andromeda. For instance, the stellar density in the solar neighborhood is only 0.004 stars per cubic light year.</p>

<p>The problem is that gravitational interactions between objects is not low. Stars that eventually pass too close to other systems might disrupt the orbits of planets, asteroids and comets, and this can be problematic if we are still around. I would speculate that planets and other objects would be flung out everywhere, and the system may become a shooting gallery.</p>

CC BY-SA 3.0
2016-11-13T01:34:37.200Z
I just want to say, I think you touched on a very important point that's often overlooked.   Planetary disruption probably stays rare, you need a very close passing star to accomplish that, The biggest effect would be oort cloud disruption, not just of our solar-system but numerous solar-systems and oort cloud objects would get scattered every which way.   The danger to "Earth" assuming there's anyone still here in 4 billion years, is hard to say, but likely a pretty big increase in impacts, say, once every 10,000 year events might happen every 100 years or so, as a very rough guess.
userLTK
CC BY-SA 3.0
2014-03-04T12:00:04.247Z
2014-03-04T12:00:04.247Z
SkydivingPanda
<p>First, note that by the time Andromeda is close enough for collisions with wandering stars to become a concern, Earth's average temperature will have changed significantly, and the planet will be unrecognizable.</p>

<p>When Sol is 8.5 billion years old, it will still have hydrogen available for fusion, but as it fuses it contracts and expands differentially.  The contraction causes hydrogen fusion to become more favorable, so that Sol will have 50% greater power output ($6 \times {10}^{26}\ \mathrm{W}$) and 3% greater effective temperature ($6000\ \mathrm{K}$).  Fusion also causes Sol to lose mass at a prodigious rate (currently $4 \times {10}^9 \mathrm{kg/s}$); it will release $6 \times {10}^{43} \mathrm{J}$ from fusion, which corresponds to $7 \times {10}^{26}\ \mathrm{kg}$.  That is about one hundred Earth masses of sunlight but only $1 \over 3000$ the mass of Sol.  Gravitation with Earth decreases proportionally, so Earth's orbit might on average expand $3000\ \mathrm{km}$ per billion years.  Other gravitational effects might change Earth's average distance by as much as $6 \times {10}^5\ \mathrm{km}$, 4‰ of an astronomical unit.  Expansion of Sol's outer layers due to reduced gravitation will increase its radius by 20%, $3 \times {10}^5\ \mathrm{km}$.  Thus Earth will receive nearly 50% more power as well.</p>

<p>The energy balance of Earth wrt Sol gives the expected surface temperature:</p>

<p>$$
\begin{align}
\bar{a} = &amp; 0.7 &amp; \small\text{(Average absorption)} \\
P_p = &amp; 1366\ \mathrm{W/m^2} &amp; \small\text{(Average solar flux incident on Earth at present)} \\
P_f = &amp; P_p \cdot 1.5 \approx 2000\ \mathrm{W/m^2} &amp; \small\text{(In future)} \\
\sigma = &amp; 5.670373 \times {10}^{-8}\ \mathrm{W/m^2/K^4} &amp; \small\text{(Stefan-Boltzmann constant)} \\
\\
T_p^4 = &amp; \frac{\bar{a} P_p}{4 \sigma} \\
\approx &amp; \frac{0.7 \cdot 1366\ \mathrm{W/m^2}}{2.268149 \times {10}^{-7}\ \mathrm{W/m^2/K^4}} \\
\approx &amp; 4.2 \times {10}^9\ \mathrm{K^4} \\
T_p \approx &amp; 250\ \mathrm{K} \\
\\
T_f \approx &amp; T_p \cdot {1.5}^{1/4} \approx T_p \cdot 1.11 \\
\approx &amp; 280\ \mathrm{K}
\end{align}
$$</p>

<p>Since the average surface temperature on Earth is not $-20\ \mathrm{°C}$ — it is $+15\ \mathrm{°C}$ and already around $8\ \mathrm{K}$ warmer than in an airless future — we can see the atmosphere has a significant role in retaining heat.  Assuming increasing cooling needs <em>do not</em> lead to the atmosphere retaining more heat, the average surface temperature can be expected to rise to $+50\ \mathrm{°C}$.</p>

<p>The average temperature of Antarctica is now $240\ \mathrm{K}$ in winter and $270\ \mathrm{K}$ in summer.  These can be expected to rise to $270\ \mathrm{K}$ (just below freezing) and $300\ \mathrm{K}$ (well above freezing) respectively, and this is a best-case scenario.  Antarctica will melt.  That will produce the largest component (60%) of sea level increase, in total around $100\ \mathrm{m}$.</p>

<hr>

<p>If Earth were still inhabited four billion years from now, it is extremely unlikely that Earth would fall into a star from Andromeda.</p>

<blockquote>
  <p>Space is big.  Really big.  You just won't believe how vastly, hugely, mindbogglingly big it is.</p>
  
  <p>— Douglas Adams, <em>The Hitchhiker's Guide to the Galaxy</em></p>
</blockquote>

<p>The Milky Way is about 100,000 light years in diameter and contains about 400 billion stars.  Andromeda is bigger and denser; it may have one trillion stars and a diameter of 140,000 light years.  It is 2.5 million light years away but appears six times larger than Sol.</p>

<p>$$
\begin{align}
d_M \approx &amp; \frac{4 \times {10}^{11}\ \mathrm{stars}}{{10}^{10} \pi/4\ \mathrm{{ly}^2}} \\
\approx &amp; 50\ \mathrm{stars/{ly}^2} \\
\\
d_A \approx &amp; \frac{{10}^{12}\ \mathrm{stars}}{2 \times {10}^{10} \pi/4\ \mathrm{{ly}^2}} \\
\approx &amp; 60\ \mathrm{stars/{ly}^2} \\
\end{align}
$$</p>

<p>If the two galaxies were simply superposed, there would be about one hundred stars per square light year, viewed from infinitely far along the rotation axis.  However, the Milky Way is a 2:1 ellipse as seen from Andromeda, while we see Andromeda as a 3:1 ellipse.  Projecting both onto a plane between them, perpendicular to a line between their central black holes, would give a region of overlap with dimensions between $50 \times 50\ \mathrm{{kly}^2}$ and $50 \times 100\ \mathrm{{kly}^2}$, with at most half the Milky Way outside it.  Sol is likely to be involved in the collision, since it is about 27,200 light years from the galactic center.</p>

<p>That doesn't mean, though, that Earth will come close to another star, that Sol might collide, or that the solar system will be disrupted.  </p>

<p>Considering probabilistically the worst-case scenario (the entire Milky Way falls through Andromeda on their first pass), there is a mean free path for stars.  The actual stellar density of the colliding galaxies is:</p>

<p>$$
\rho \approx 1.4 \times {10}^{12}\ \mathrm{stars}\ /\ V_{A \cup M}
$$</p>

<p>where the union of the two galaxies' volumes would be a very complicated expression.  <em>Very roughly</em>, their volumes can be described as joined cones, ignoring their spheroidal dark matter halos (which are mostly harmless).</p>

<p>$$
\begin{align}
\rho \approx &amp; \frac{1.4 \times {10}^{12}\ \mathrm{stars}}
{\left(
\frac{1}{2} \cdot
\left(
{10}^3\ \mathrm{ly} \cdot {10}^{10} \pi/4\ \mathrm{{ly}^2} +
1.4 \times {10}^3\ \mathrm{ly} \cdot 2 \times {10}^{10} \pi/4\ \mathrm{{ly}^2}
\right)
\cdot \frac{1}{3}
\right)} \\
\approx &amp; 0.28\ \mathrm{stars/{ly}^3} \\
\\
V_\star \approx &amp; 3.6\ \mathrm{{ly}^3} \\
\\
r_\star \approx &amp; {\left( V_\star \cdot \frac{3}{4 \pi} \right)}^{1/3} \\
\approx &amp; 0.95\ \mathrm{ly}
\end{align}
$$</p>

<p>At a distance of 1.9 light years, Betelgeuse would look a lot like Mars.  If we assume disaster results from a star closer than the diameter of the heliosphere (about 200 AU), then:</p>

<p>$$
\begin{align}
m = &amp; \frac{1\ \mathrm{star}}{\rho \cdot \pi \cdot 4 \times {10}^4\ \mathrm{{AU}^2}} \\
\approx &amp; 1.1 \times {10}^{21}\ \mathrm{m} \\
\approx &amp; 7.2 \times {10}^9\ \mathrm{AU} \\
\approx &amp; 1.1 \times {10}^5\ \mathrm{ly}
\end{align}
$$</p>

<p>On average, a star can travel 110 thousand light years before it grazes past another, slightly less than the diameter of Andromeda.  The proportion of stars from the Milky Way that do not approach within 200 AU of stars in Andromeda is at least $1/e^{1.4 / 1.1} \approx 100/400\ \mathrm{billion\ stars}$.  For Earth to approach within 4 AU of another star (one Betelgeuse radius), it can be expected to travel at least 2500 times farther, which at a relative velocity of 300 km/s would take $9 \times {10}^{18}\ \mathrm{s} \approx 300\ \mathrm{billion\ years}$.</p>

CC BY-SA 3.0
2014-03-06T08:37:30.913Z
In only worst case scenarion, 1> will life exist? 2> If we are about to collide in next 4 billion year then how long before we should take action for inter-stellar voyage? 3> Are scientists working on such projects for inter-stellar voyage? 4>Will we be able to get the portal like earth ?
AmitG
CC BY-SA 3.0
2014-03-06T09:12:30.097Z
Earth seems to be still habitable (but unpleasant) four billion years from now.  When Sol runs out of hydrogen to fuse, several hundred million years later, it is likely to expand much more rapidly and either melt or vaporize the entire planet.  A greater immediate threat is an asteroid or a light-year-wide beam of gamma rays from something like WR-104.  There are no plans currently for escaping those; if it becomes possible, making a destination habitable should be easier than leaving Sol.
user130144
CC BY-SA 3.0
2014-03-06T10:25:19.510Z
Thanks for the links.  I have thought about changing my ID... but this is a _very unusual_ randomly chosen number.
user130144
CC BY-SA 3.0
2014-03-07T07:15:41.360Z
one question: what reference do you have for all the numbers you use in this answer?
usethedeathstar
CC BY-SA 3.0
2014-03-07T23:58:46.660Z
Stackexchange makes it difficult to link to anything.  For things like the masses and dimensions of stars and galaxies, Wikipedia is the most quickly available reference.  Calculators and NIST have fundamental constants such as $\sigma$, the [Stefan-Boltzmann constant](http://physics.nist.gov/cgi-bin/cuu/Value?sigma) (although extra digits do no good here).  UNL has a helpful simulation for the evolution of Sol which predicts its future luminosity.  It isn't clear to me how they determine when Earth will leave the habitable zone.  With less carbon, Venus might still be habitable.
user130144
CC BY-SA 3.0
2014-03-11T08:47:38.763Z
@user130144 the problem with those numbers is that masses of galaxies and distances and so on, the errorbars are astronomical (pun intended). It is quite difficult for some things to accurately say how fast it will happen since the distancescales and timescales on these things are so massive that they are hard to determine exact numbers for certain quantities. That being said though, the numbers you give are "the best estimate" that we can do with current technology, so i am not saying it is wrong, just that some of these numbers can be quite different ten years from now (better instruments).
usethedeathstar
CC BY-SA 3.0
2014-03-05T08:59:25.837Z
2014-03-05T22:47:29.647Z
2014-03-05T22:47:29.647Z
user130144
<p>Normally when two galaxies collide, it is the gas that interacts with each other. The odds of stars impacting each other are nearly zero due to the huge distances between the stars. The same goes for planets hitting each other. </p>

<p>The timescales on this happening are so large that it is difficult for our mind to understand these distances (and the timescales at which these processes take place), and by that time, many other things might have happened that change the situation of our solar system making it difficult to predict what will happen to humans. For example a meteorite impact like the one that killed the dinosaurs may kill all life on earth one billion years from now, so that we are not even there to see what happens to human life when andromeda interacts with the milky way. </p>

CC BY-SA 3.0
2014-03-04T09:02:08.117Z
so will it impact on human being if we suppose human being will survive till next 4billion year from now?
AmitG
CC BY-SA 3.0
2014-03-04T09:04:29.023Z
not likely, if you calculate the density of the stars in either galaxy, you can calculate the probability of star-star collisions, and that probability is extremely low
usethedeathstar
CC BY-SA 3.0
2014-03-04T08:59:04.500Z
2014-03-04T08:59:04.500Z
usethedeathstar
<p>If the sun was 10 cm wide, Alpha Centauri would be 3200 kilometers away. <a href="http://www.wolframalpha.com/input/i=%281mm%2Fearth+diameter+%29+*alpha+centauri+distance" rel="nofollow">1</a></p>

<p>If the sun was an orange, the termination shock, the extreme boundary of the solar system, where the interstellar winds are more consequent than solar wind, is about 1 km diameter. The different stars wouldnt tend to interfere with each other's termination shock boundary. you wouldn't even see a degree of change in earth temperature from another star passing near to the solar system.</p>

<p>If the most dense parts of our galaxy are 3000 times more star dense than our neighborhood, we are talking about a cloud of oranges, each of which is separated by a kilometer, in the most dense scenario.</p>

<p>Most probably it's the dramatic collision of a cloud of oranges each separated by 3000 km, the Milky Way in total would be 10 million kilometers across, if the sun was an orange.</p>

<p>Wiki sais that it would be negligable probability of even two stars colliding.</p>

<p>The collision of the galaxies doesn't change the challenge of transporting life away from our planet, compared to what it already is. we already have to cross the 32000km to a dozen nearby stars, if we humans measured 1.4 Angstroms tall... alpha centaur would be 3200 km walk away.</p>

<p><a href="https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision#Stellar_collisions" rel="nofollow">https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision#Stellar_collisions</a></p>

<p>1- <a href="http://www.wolframalpha.com/input/i=%281mm%2Fearth+diameter+%29+*alpha+centauri+distance" rel="nofollow">http://www.wolframalpha.com/input/i=%281mm%2Fearth+diameter+%29+*alpha+centauri+distance</a></p>

CC BY-SA 3.0
2015-12-02T10:39:22.787Z
incidentally if earth was 1mm, sun is 10cm, aldebaran and beteljeuse are about 87 meters and 65 meters, sun is kilometers away, alpha centauri is 1000ds of kilometers, galaxy width is same as eart sun distance or something.  I found it all doing earth diameter / 1mm * sun diameter
bandybabboon
CC BY-SA 3.0
2015-10-03T07:17:31.333Z
2015-10-03T07:56:10.130Z
2015-10-03T07:56:10.130Z
bandybabboon
<blockquote>
  <p>What will happen to life on earth or human beings on earth?</p>
</blockquote>

<p>Assuming that human beings, or life, still exists on Earth at that time, they will have survived so much due to the ongoing death of the sun, that the gravitational pertubations due to the galactic collision will be nothing.</p>

<p>Keep in mind that in about 1-2 billion years, the sun will be so hot and large that all the water will have boiled off the earth into space.  About 3 billion years from now, the surface of the Earth will be so hot that metals will be melting.</p>

<p>Any life that has survived those events and still lives on Earth will surely take a galactic collision in stride.</p>

<p>I imagine, though, that most humans will have fled Earth - if not for distant star systems, then at least for planets in our own system that are going to be warming up enough for human habitation.</p>

<blockquote>
  <p>If we are about to collide in next 4 billion year then how long before we should take action for inter-stellar voyage? </p>
</blockquote>

<p>As soon as possible. When interstellar voyage becomes possible, we should start sending out ships to colonize other planets and star systems.  This will likely take a long time, but if we are to survive more than a billion years, it is necessary. Keep in mind that the Sun and Andromeda are events we can predict.  We don't know, and can't predict, the next cataclysmic asteroid strike, which is likely to happen in a shorter time than a billion years.  There are lots of reasons to exit the planet, we should be worried about the ones we can't predict or see, not the ones we can predict.</p>

<blockquote>
  <p>Are scientists working on such projects for inter-stellar voyage?</p>
</blockquote>

<p>Yes, but in small steps.  Manned missions to space, to the moon, and living aboard the ISS have provided significantly valuable information that will be used in such interstellar missions.  As we continue to push the boundaries of our ability to survive in space we eventually will be able to live in space, perhaps whole lifetimes will be spent in space.  As engine technology progresses beyond simply lifting people out of Earth's gravitational well, we will eventually be sending people on long voyages outside our solar system.</p>

<p>It's a very, very long way off, but each advance takes us closer to that eventual goal.</p>

CC BY-SA 4.0
2020-10-13T18:57:57.260Z
I like your optimism about interstellar voyage. I hope you are right, but there is no guarantee that we will ever be able to traverse the distance in a human lifetime. Maybe we could send out an "ark" someday, but if you look at arguments like those presented in Kim Stanley Robinson's book, "Aurora", sustaining such an ark may be a lot harder than people realize. And we have no idea where to go yet. I think our best option will be space colonies that can be relocated to different distances from our star where we can continue to get solar energy and mine resources from moons and asteroids.
Jack R. Woods
CC BY-SA 4.0
2022-10-10T18:01:54.150Z
@JackR.Woods I like your optimism about any significant amount of mass being able to overcome the gravity well of our planet, in the first place. All orbital launches combined amount to 15 kilotons of mass. A very, *very* small number of those left earth orbit. Perish the thought of "generation ships" or anything like that as the Science Fiction it is.
DevSolar
CC BY-SA 3.0
2014-10-22T17:28:38.847Z
I'm hoping we'll move Earth to a safe distance by that time, for sentimental reasons if nothing else.
Keith Thompson
CC BY-SA 3.0
2014-10-22T17:31:19.557Z
@KeithThompson It'll be a brownfield site by then, a dead bird hanging around the human race's neck.  Might as well let the star ingest it and hope other civilizations that we come across don't find out until after we've stripped their planet(s). ;)
Adam Davis
CC BY-SA 3.0
2015-10-05T08:48:01.707Z
You should provide evidence (via links to respectable sources) for your claims that in 1-2 billion years water on Earth will be evaporated.
Walter
CC BY-SA 3.0
2015-10-05T15:14:56.247Z
@Walter It is fairly well accepted by the scientific community that the Sun's next phase is red giant, with a diameter well beyond the current Earth's orbit.  You can do an internet search for "When will the oceans boil away" and choose which source you find most respectable, ignoring, if you like, the global climate change estimates which move the models up a few million years. An interesting recent paper on this subject is "Distant future of Sun and Earth revisited" at http://arxiv.org/abs/0801.4031 *"...it is clear that Earth
will come to leave the HZ already in about a billion years
time"*
Adam Davis
CC BY-SA 3.0
2015-10-06T07:43:18.443Z
I was not asking for a comment, but for improvement to your answer, and I was not asking because I doubted it, but to give you a chance to improve your answer (though I was a bit surprised at the time scale of only 1Gyr).
Walter
CC BY-SA 3.0
2014-03-04T16:05:26.310Z
2014-03-04T16:16:56.603Z
2014-03-04T16:16:56.603Z
Adam Davis
<p>It is said that the Andromeda and Milky Way galaxies are coming close to each other with a speed of approximately 400000 km/hour. They will be together in the next 4 billion years.</p>

<ol>
<li>What will happen to life on Earth or human beings on Earth?</li>
<li>If we are about to collide in the next 4 billion years then how long before we should take action for interstellar voyage?</li>
<li>Are scientists working on such projects for an interstellar voyage?</li>
<li>Will we be able to get the something like Earth to go away from this galaxy/Earth/ solar system and, considering speed of human instruments/spaceships, how long will it take to go to a safe place?</li>
</ol>

CC BY-SA 3.0
2017-09-11T21:37:01.750Z
If "We" humans are still around on Earth in 4 billion years - no small trick, we'd need to move or shield the planet or terraform Mars, but lets pretend we are.   The collision with Andromeda might be wonderful thing, many more stars and much greater stellar movement will make inter-stellar travel easier.    The galactic collision might be very good, replenishing the galaxy with new star formation and increasing the number of stars passing not too many light years away.   Not harmful but wonderful (assuming we're still here in 4 billion years, and assuming no hostile aliens.  :-)
userLTK
CC BY-SA 3.0
2014-05-25T01:12:55.093Z
Apart from all the scientific reasons mentioned, we will get an amazing view. It will be fun to witness them collide, at least before the shock reaches us, and if you're planning to live that long.
Cheeku
CC BY-SA 3.0
2015-10-05T08:46:56.060Z
Why do you assume that any humans or even any observing life forms will be around on Earth in 4billion years?
Walter
<galaxy><milky-way><life>
What will happen to life on Earth when the Andromeda and Milky Way galaxies collide?
AmitG
# What will happen to life on Earth when the Andromeda and Milky Way galaxies collide?

(Asked by: username_0 on 2014-03-04)

It is said that the Andromeda and Milky Way galaxies are coming close to each other with a speed of approximately 400000 km/hour. They will be together in the next 4 billion years.

1. What will happen to life on Earth or human beings on Earth?
2. If we are about to collide in the next 4 billion years then how long before we should take action for interstellar voyage?
3. Are scientists working on such projects for an interstellar voyage?
4. Will we be able to get the something like Earth to go away from this galaxy/Earth/ solar system and, considering speed of human instruments/spaceships, how long will it take to go to a safe place?

**Tags:** galaxy, milky-way, life

> **Comments:**
> **username_2**: Apart from all the scientific reasons mentioned, we will get an amazing view. It will be fun to witness them collide, at least before the shock reaches us, and if you're planning to live that long.  
> **username_3**: Why do you assume that any humans or even any observing life forms will be around on Earth in 4billion years?  
> **username_1**: If "We" humans are still around on Earth in 4 billion years - no small trick, we'd need to move or shield the planet or terraform Mars, but lets pretend we are.   The collision with Andromeda might be wonderful thing, many more stars and much greater stellar movement will make inter-stellar travel easier.    The galactic collision might be very good, replenishing the galaxy with new star formation and increasing the number of stars passing not too many light years away.   Not harmful but wonderful (assuming we're still here in 4 billion years, and assuming no hostile aliens.  :-)  

---

## 6 answers

### Answer by username_14 on 2014-03-04. Score: 24 | Accepted answer

> What will happen to life on earth or human beings on earth?

Assuming that human beings, or life, still exists on Earth at that time, they will have survived so much due to the ongoing death of the sun, that the gravitational pertubations due to the galactic collision will be nothing.

Keep in mind that in about 1-2 billion years, the sun will be so hot and large that all the water will have boiled off the earth into space. About 3 billion years from now, the surface of the Earth will be so hot that metals will be melting.

Any life that has survived those events and still lives on Earth will surely take a galactic collision in stride.

I imagine, though, that most humans will have fled Earth - if not for distant star systems, then at least for planets in our own system that are going to be warming up enough for human habitation.

> If we are about to collide in next 4 billion year then how long before we should take action for inter-stellar voyage?

As soon as possible. When interstellar voyage becomes possible, we should start sending out ships to colonize other planets and star systems. This will likely take a long time, but if we are to survive more than a billion years, it is necessary. Keep in mind that the Sun and Andromeda are events we can predict. We don't know, and can't predict, the next cataclysmic asteroid strike, which is likely to happen in a shorter time than a billion years. There are lots of reasons to exit the planet, we should be worried about the ones we can't predict or see, not the ones we can predict.

> Are scientists working on such projects for inter-stellar voyage?

Yes, but in small steps. Manned missions to space, to the moon, and living aboard the ISS have provided significantly valuable information that will be used in such interstellar missions. As we continue to push the boundaries of our ability to survive in space we eventually will be able to live in space, perhaps whole lifetimes will be spent in space. As engine technology progresses beyond simply lifting people out of Earth's gravitational well, we will eventually be sending people on long voyages outside our solar system.

It's a very, very long way off, but each advance takes us closer to that eventual goal.

> **Comments:**
> **username_8**: I'm hoping we'll move Earth to a safe distance by that time, for sentimental reasons if nothing else.  
> **username_14**: @KeithThompson It'll be a brownfield site by then, a dead bird hanging around the human race's neck.  Might as well let the star ingest it and hope other civilizations that we come across don't find out until after we've stripped their planet(s). ;)  
> **username_3**: You should provide evidence (via links to respectable sources) for your claims that in 1-2 billion years water on Earth will be evaporated.  
> **username_14**: @username_3 It is fairly well accepted by the scientific community that the Sun's next phase is red giant, with a diameter well beyond the current Earth's orbit.  You can do an internet search for "When will the oceans boil away" and choose which source you find most respectable, ignoring, if you like, the global climate change estimates which move the models up a few million years. An interesting recent paper on this subject is "Distant future of Sun and Earth revisited" at http://arxiv.org/abs/0801.4031 *"...it is clear that Earth will come to leave the HZ already in about a billion years time"*  
> **username_3**: I was not asking for a comment, but for improvement to your answer, and I was not asking because I doubted it, but to give you a chance to improve your answer (though I was a bit surprised at the time scale of only 1Gyr).  
> **username_15**: I like your optimism about interstellar voyage. I hope you are right, but there is no guarantee that we will ever be able to traverse the distance in a human lifetime. Maybe we could send out an "ark" someday, but if you look at arguments like those presented in Kim Stanley Robinson's book, "Aurora", sustaining such an ark may be a lot harder than people realize. And we have no idea where to go yet. I think our best option will be space colonies that can be relocated to different distances from our star where we can continue to get solar energy and mine resources from moons and asteroids.  
> **username_16**: @JackR.Woods I like your optimism about any significant amount of mass being able to overcome the gravity well of our planet, in the first place. All orbital launches combined amount to 15 kilotons of mass. A very, *very* small number of those left earth orbit. Perish the thought of "generation ships" or anything like that as the Science Fiction it is.  

---

### Answer by username_4 on 2014-03-04. Score: 20

4 billion years is the same timeframe of the life remaining to our Sun.

So if we have not yet invented interstellar voyages, we're screwed, with or without Andromeda.

Besides, stars do not interact directly with each other in a galactic collision. What we will notice from the several stars we are on is that star orbits around the galactic centre will be altered by the massive gravitational perturbation made by the other galaxy. Almost all stars will change orbits from that of orbiting around the galactic centre to orbiting around the center of mass of both galaxies. Some stars will be ejected out of the new bigger galaxy. It is quite safe to think that planets will continue orbiting their stars, but not that they will not have some alterations in their orbits.

On the other hand, from the gas to gas interaction, there will be a lot of new pressure waves in the interstellar medium, which will account for the formation of billions of new stars in new nebulae.

> **Comments:**
> **username_0**: so are there any chances of evaporation of earths due to large heat due to gases ? So there is no chance to survival of human being. How long before we should think about inter-stellar voyage?  
> **username_7**: The point is that over the timeframe of 4 billion years many stars cease to exist and planets will become uninhabitable (including Sun/Earth) no matter what happens with the galaxy collision.  So by the time of Andromeda/Milkyway collision either humans will be an interstellar migratory species and able to work around it, or will be extinct before it happens.  
> **username_8**: @username_0: I don't have the numbers, but I'd think that any heat from colliding interstellar gas clouds would be a tiny fraction of the energy we already receive from that nuclear furnace just 1 AU away.  
> **username_9**: Heat from pressure waves in interstellar gas is not likely to be a problem. Increased star formation, including very large, short-lived stars that will go supernova in the neighborhood might be a problem.  
> **username_5**: @envite: +1 for  **if we have not yet invented interstellar voyages, we're screwed, with or without Andromeda.**  
> **username_6**: 4 billion years is not the remaining lifetime of the Sun. It will still be on the main sequence in 4 billion years. Perhaps you meant: similar timescale ?  

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### Answer by username_11 on 2014-03-05. Score: 18

First, note that by the time Andromeda is close enough for collisions with wandering stars to become a concern, Earth's average temperature will have changed significantly, and the planet will be unrecognizable.

When Sol is 8.5 billion years old, it will still have hydrogen available for fusion, but as it fuses it contracts and expands differentially. The contraction causes hydrogen fusion to become more favorable, so that Sol will have 50% greater power output ($6 \times {10}^{26}\ \mathrm{W}$) and 3% greater effective temperature ($6000\ \mathrm{K}$). Fusion also causes Sol to lose mass at a prodigious rate (currently $4 \times {10}^9 \mathrm{kg/s}$); it will release $6 \times {10}^{43} \mathrm{J}$ from fusion, which corresponds to $7 \times {10}^{26}\ \mathrm{kg}$. That is about one hundred Earth masses of sunlight but only $1 \over 3000$ the mass of Sol. Gravitation with Earth decreases proportionally, so Earth's orbit might on average expand $3000\ \mathrm{km}$ per billion years. Other gravitational effects might change Earth's average distance by as much as $6 \times {10}^5\ \mathrm{km}$, 4‰ of an astronomical unit. Expansion of Sol's outer layers due to reduced gravitation will increase its radius by 20%, $3 \times {10}^5\ \mathrm{km}$. Thus Earth will receive nearly 50% more power as well.

The energy balance of Earth wrt Sol gives the expected surface temperature:

$$
\begin{align}
\bar{a} = & 0.7 & \small\text{(Average absorption)} \\
P\_p = & 1366\ \mathrm{W/m^2} & \small\text{(Average solar flux incident on Earth at present)} \\
P\_f = & P\_p \cdot 1.5 \approx 2000\ \mathrm{W/m^2} & \small\text{(In future)} \\
\sigma = & 5.670373 \times {10}^{-8}\ \mathrm{W/m^2/K^4} & \small\text{(Stefan-Boltzmann constant)} \\
\\
T\_p^4 = & \frac{\bar{a} P\_p}{4 \sigma} \\
\approx & \frac{0.7 \cdot 1366\ \mathrm{W/m^2}}{2.268149 \times {10}^{-7}\ \mathrm{W/m^2/K^4}} \\
\approx & 4.2 \times {10}^9\ \mathrm{K^4} \\
T\_p \approx & 250\ \mathrm{K} \\
\\
T\_f \approx & T\_p \cdot {1.5}^{1/4} \approx T\_p \cdot 1.11 \\
\approx & 280\ \mathrm{K}
\end{align}
$$

Since the average surface temperature on Earth is not $-20\ \mathrm{°C}$ — it is $+15\ \mathrm{°C}$ and already around $8\ \mathrm{K}$ warmer than in an airless future — we can see the atmosphere has a significant role in retaining heat. Assuming increasing cooling needs *do not* lead to the atmosphere retaining more heat, the average surface temperature can be expected to rise to $+50\ \mathrm{°C}$.

The average temperature of Antarctica is now $240\ \mathrm{K}$ in winter and $270\ \mathrm{K}$ in summer. These can be expected to rise to $270\ \mathrm{K}$ (just below freezing) and $300\ \mathrm{K}$ (well above freezing) respectively, and this is a best-case scenario. Antarctica will melt. That will produce the largest component (60%) of sea level increase, in total around $100\ \mathrm{m}$.

---

If Earth were still inhabited four billion years from now, it is extremely unlikely that Earth would fall into a star from Andromeda.

> Space is big. Really big. You just won't believe how vastly, hugely, mindbogglingly big it is.
>
> — Douglas Adams, *The Hitchhiker's Guide to the Galaxy*

The Milky Way is about 100,000 light years in diameter and contains about 400 billion stars. Andromeda is bigger and denser; it may have one trillion stars and a diameter of 140,000 light years. It is 2.5 million light years away but appears six times larger than Sol.

$$
\begin{align}
d\_M \approx & \frac{4 \times {10}^{11}\ \mathrm{stars}}{{10}^{10} \pi/4\ \mathrm{{ly}^2}} \\
\approx & 50\ \mathrm{stars/{ly}^2} \\
\\
d\_A \approx & \frac{{10}^{12}\ \mathrm{stars}}{2 \times {10}^{10} \pi/4\ \mathrm{{ly}^2}} \\
\approx & 60\ \mathrm{stars/{ly}^2} \\
\end{align}
$$

If the two galaxies were simply superposed, there would be about one hundred stars per square light year, viewed from infinitely far along the rotation axis. However, the Milky Way is a 2:1 ellipse as seen from Andromeda, while we see Andromeda as a 3:1 ellipse. Projecting both onto a plane between them, perpendicular to a line between their central black holes, would give a region of overlap with dimensions between $50 \times 50\ \mathrm{{kly}^2}$ and $50 \times 100\ \mathrm{{kly}^2}$, with at most half the Milky Way outside it. Sol is likely to be involved in the collision, since it is about 27,200 light years from the galactic center.

That doesn't mean, though, that Earth will come close to another star, that Sol might collide, or that the solar system will be disrupted.

Considering probabilistically the worst-case scenario (the entire Milky Way falls through Andromeda on their first pass), there is a mean free path for stars. The actual stellar density of the colliding galaxies is:

$$
\rho \approx 1.4 \times {10}^{12}\ \mathrm{stars}\ /\ V\_{A \cup M}
$$

where the union of the two galaxies' volumes would be a very complicated expression. *Very roughly*, their volumes can be described as joined cones, ignoring their spheroidal dark matter halos (which are mostly harmless).

$$
\begin{align}
\rho \approx & \frac{1.4 \times {10}^{12}\ \mathrm{stars}}
{\left(
\frac{1}{2} \cdot
\left(
{10}^3\ \mathrm{ly} \cdot {10}^{10} \pi/4\ \mathrm{{ly}^2} +
1.4 \times {10}^3\ \mathrm{ly} \cdot 2 \times {10}^{10} \pi/4\ \mathrm{{ly}^2}
\right)
\cdot \frac{1}{3}
\right)} \\
\approx & 0.28\ \mathrm{stars/{ly}^3} \\
\\
V\_\star \approx & 3.6\ \mathrm{{ly}^3} \\
\\
r\_\star \approx & {\left( V\_\star \cdot \frac{3}{4 \pi} \right)}^{1/3} \\
\approx & 0.95\ \mathrm{ly}
\end{align}
$$

At a distance of 1.9 light years, Betelgeuse would look a lot like Mars. If we assume disaster results from a star closer than the diameter of the heliosphere (about 200 AU), then:

$$
\begin{align}
m = & \frac{1\ \mathrm{star}}{\rho \cdot \pi \cdot 4 \times {10}^4\ \mathrm{{AU}^2}} \\
\approx & 1.1 \times {10}^{21}\ \mathrm{m} \\
\approx & 7.2 \times {10}^9\ \mathrm{AU} \\
\approx & 1.1 \times {10}^5\ \mathrm{ly}
\end{align}
$$

On average, a star can travel 110 thousand light years before it grazes past another, slightly less than the diameter of Andromeda. The proportion of stars from the Milky Way that do not approach within 200 AU of stars in Andromeda is at least $1/e^{1.4 / 1.1} \approx 100/400\ \mathrm{billion\ stars}$. For Earth to approach within 4 AU of another star (one Betelgeuse radius), it can be expected to travel at least 2500 times farther, which at a relative velocity of 300 km/s would take $9 \times {10}^{18}\ \mathrm{s} \approx 300\ \mathrm{billion\ years}$.

> **Comments:**
> **username_0**: In only worst case scenarion, 1> will life exist? 2> If we are about to collide in next 4 billion year then how long before we should take action for inter-stellar voyage? 3> Are scientists working on such projects for inter-stellar voyage? 4>Will we be able to get the portal like earth ?  
> **username_11**: Earth seems to be still habitable (but unpleasant) four billion years from now.  When Sol runs out of hydrogen to fuse, several hundred million years later, it is likely to expand much more rapidly and either melt or vaporize the entire planet.  A greater immediate threat is an asteroid or a light-year-wide beam of gamma rays from something like WR-104.  There are no plans currently for escaping those; if it becomes possible, making a destination habitable should be easier than leaving Sol.  
> **username_11**: Thanks for the links.  I have thought about changing my ID... but this is a _very unusual_ randomly chosen number.  
> **username_12**: one question: what reference do you have for all the numbers you use in this answer?  
> **username_11**: Stackexchange makes it difficult to link to anything.  For things like the masses and dimensions of stars and galaxies, Wikipedia is the most quickly available reference.  Calculators and NIST have fundamental constants such as $\sigma$, the [Stefan-Boltzmann constant](http://physics.nist.gov/cgi-bin/cuu/Value?sigma) (although extra digits do no good here).  UNL has a helpful simulation for the evolution of Sol which predicts its future luminosity.  It isn't clear to me how they determine when Earth will leave the habitable zone.  With less carbon, Venus might still be habitable.  
> **username_12**: @username_11 the problem with those numbers is that masses of galaxies and distances and so on, the errorbars are astronomical (pun intended). It is quite difficult for some things to accurately say how fast it will happen since the distancescales and timescales on these things are so massive that they are hard to determine exact numbers for certain quantities. That being said though, the numbers you give are "the best estimate" that we can do with current technology, so i am not saying it is wrong, just that some of these numbers can be quite different ten years from now (better instruments).  

---

### Answer by username_10 on 2014-03-04. Score: 12

Direct collisions between stars and planets is highly unlikely, due to the relatively low density of objects in the Milky Way and Andromeda. For instance, the stellar density in the solar neighborhood is only 0.004 stars per cubic light year.

The problem is that gravitational interactions between objects is not low. Stars that eventually pass too close to other systems might disrupt the orbits of planets, asteroids and comets, and this can be problematic if we are still around. I would speculate that planets and other objects would be flung out everywhere, and the system may become a shooting gallery.

> **Comments:**
> **username_1**: I just want to say, I think you touched on a very important point that's often overlooked.   Planetary disruption probably stays rare, you need a very close passing star to accomplish that, The biggest effect would be oort cloud disruption, not just of our solar-system but numerous solar-systems and oort cloud objects would get scattered every which way.   The danger to "Earth" assuming there's anyone still here in 4 billion years, is hard to say, but likely a pretty big increase in impacts, say, once every 10,000 year events might happen every 100 years or so, as a very rough guess.  

---

### Answer by username_12 on 2014-03-04. Score: 10

Normally when two galaxies collide, it is the gas that interacts with each other. The odds of stars impacting each other are nearly zero due to the huge distances between the stars. The same goes for planets hitting each other.

The timescales on this happening are so large that it is difficult for our mind to understand these distances (and the timescales at which these processes take place), and by that time, many other things might have happened that change the situation of our solar system making it difficult to predict what will happen to humans. For example a meteorite impact like the one that killed the dinosaurs may kill all life on earth one billion years from now, so that we are not even there to see what happens to human life when andromeda interacts with the milky way.

> **Comments:**
> **username_0**: so will it impact on human being if we suppose human being will survive till next 4billion year from now?  
> **username_12**: not likely, if you calculate the density of the stars in either galaxy, you can calculate the probability of star-star collisions, and that probability is extremely low  

---

### Answer by username_13 on 2015-10-03. Score: 4

If the sun was 10 cm wide, Alpha Centauri would be 3200 kilometers away. [1](http://www.wolframalpha.com/input/i=%281mm%2Fearth+diameter+%29+*alpha+centauri+distance)

If the sun was an orange, the termination shock, the extreme boundary of the solar system, where the interstellar winds are more consequent than solar wind, is about 1 km diameter. The different stars wouldnt tend to interfere with each other's termination shock boundary. you wouldn't even see a degree of change in earth temperature from another star passing near to the solar system.

If the most dense parts of our galaxy are 3000 times more star dense than our neighborhood, we are talking about a cloud of oranges, each of which is separated by a kilometer, in the most dense scenario.

Most probably it's the dramatic collision of a cloud of oranges each separated by 3000 km, the Milky Way in total would be 10 million kilometers across, if the sun was an orange.

Wiki sais that it would be negligable probability of even two stars colliding.

The collision of the galaxies doesn't change the challenge of transporting life away from our planet, compared to what it already is. we already have to cross the 32000km to a dozen nearby stars, if we humans measured 1.4 Angstroms tall... alpha centaur would be 3200 km walk away.

<https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision#Stellar_collisions>

1- [http://www.wolframalpha.com/input/i=%281mm%2Fearth+diameter+%29+\*alpha+centauri+distance](http://www.wolframalpha.com/input/i=%281mm%2Fearth+diameter+%29+*alpha+centauri+distance)

> **Comments:**
> **username_13**: incidentally if earth was 1mm, sun is 10cm, aldebaran and beteljeuse are about 87 meters and 65 meters, sun is kilometers away, alpha centauri is 1000ds of kilometers, galaxy width is same as eart sun distance or something.  I found it all doing earth diameter / 1mm * sun diameter  

---

