Starship Asterisk*

Milky Way Galaxy Doomed: Collision with Andromeda Pending

Explanation: Will our Milky Way Galaxy collide one day with its larger neighbor, the Andromeda Galaxy? Most likely, yes. Careful plotting of slight displacements of M31's stars relative to background galaxies on recent Hubble Space Telescope images indicate that the center of M31 could be on a direct collision course with the center of our home galaxy. Still, the errors in sideways velocity appear sufficiently large to admit a good chance that the central parts of the two galaxies will miss, slightly, but will become close enough for their outer halos to become gravitationally entangled. Once that happens, the two galaxies will become bound, dance around, and eventually merge to become one large elliptical galaxy -- over the next few billion years. Pictured above is an artist's illustration of the sky of a world in the distant future when the central parts of each galaxy begin to destroy each other. The exact future of our Milky Way and the entire surrounding Local Group of Galaxies is likely to remain an active topic of research for years to come.

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Partial Lunar Eclipse Tonight: Gallery of photogenic past lunar eclipses

The lunar eclipse is before sunrise on the 4th.

• Starting 6:00 am EDT, 3:00 am PDT.
  Ending 8:06 am EDT, 5:06 am PDT.

http://asterisk.apod.com/viewtopic.php?t=28752

APOD Robot wrote:Once that happens, the two galaxies will become bound, dance around, and eventually merge to become one large elliptical galaxy -- over the next few billion years.

The two galaxies are already gravitationally bound, and have been dancing around each other in a complex way for billions of years.

Pardon the ignorance, but will the southern hemisphere or the northern hemisphere see more of the approaching andromeda galaxy ?
Depending on its, and our, trajectory presumeably ??
How close will it actually appear, or will its flyby still be at astronomical distances ?
Distances which someone can only guess at, at this stage ??

Chris Peterson wrote:

APOD Robot wrote:
Once that happens, the two galaxies will become bound, dance around, and eventually merge to become one large elliptical galaxy -- over the next few billion years.

The two galaxies are already gravitationally bound, and have been dancing around each other in a complex way for billions of years.

The two galaxies are already gravitationally bound but that doesn't necessarily imply that they have been "dancing around each other" in some complex fashion for billions of years.

At 2.6 million light-years distance the Andromeda Galaxy is currently approaching the Milky Way at ~120 kilometres per second (400 light-years every million years); it should accelerate to an average of ~195 kilometres per second (650 light-years every million years) so as to collide with us in ~4.5 billion years.

Run time backwards and the Andromeda Galaxy is receding from the Milky Way at about 120 kilometres per second. It is hard for me to imagine that this separation could possibly decelerate fast enough so that the two galaxies could have collided (in past times) in much less than 3 x 4.5 billion years. Which doesn't allow these two galaxies a whole lot of time to have "danced around each other" prior to somehow being violently flung apart. They most likely began their galactic lives gravitationally bound but moving apart and have never actually "danced with each other" before.

Since long time I have tried to imagine this situation. In my scenario there are much more disruptive, catastrophic details, even in the early phase of approach. The gravitation has a long arm... Hubbles pictures of interacting galaxies shows infernal materia flows and radiation storms in a hellish spiral of violence over enormous distances.

The illustration here is quite nice, friendly and peacefully. What happens in the next chapter? For the final one let's have a look to the big feast of CenA, which now digest a smaller spiral. The jet is a fart...

Talk about a train wreck… it’s interesting to think about but such a long way off… it’s hard to wrap my head around 4 billion years.

Maybe we will gain a closer inhabited solar system… close enough to visit!

I understand from previous postings that the likelihood of a direct head on collision with another cosmic body is not that great considering the typical expanse between objects. Still, my imagination escalates when I think of galaxies colliding.

I used to enjoy watching the figure eight jalopy car races near my hometown. Some drivers were fearless. Most weren’t. Maybe in 4 billion years we’ll know how to move to a safe location should the need arise.

Awh! It's like a marriage; And the two shall become one!

orin stepanek wrote:
Awh! It's like a marriage; And the two shall become one!

------------------------------------------------------
Blanche Bickerson awakens John Bickerson
http://www.otr.net/?p=bick
.....................................
Blanche: You used to be so considerate. Since you got married to me you haven't got any sympathy at all.

John: I have, too. I've got everybody's sympathy.

Blanche: Believe me, there's better fish in the ocean than the one I caught.

John: There's better bait, too.

Blanche: I don't see how you can go to bed without kissing me good night.

John: I can do it.

Blanche: You'd better say you're sorry for that, John.

John: Okay, I'm sorry, I'm sorry, I'm sorry.

Blanche: You are not.

John: I am too. I'm the sorriest man that was ever born.

Blanche: Is there any milk for breakfast?

John: No.

Blanche: Then you'll have to eat out.

John: I don't care, I've been doing it all week.

Blanche: What for? I left you enough food for six days. I cooked a whole bathtub full of rice. What happened to it?

John: I took a bath in it.

Blanche: Why didn't you eat it?

John: I've told you a million times I can't stand the sight of rice.

Blanche: Why not?

John: Because it's connected to the saddest mistake of my life.

Blanche: You stopped loving me the day we were married.

John: That wasn't the day at all.
-------------------------------------

neufer wrote:The two galaxies are already gravitationally bound but that doesn't necessarily imply that they have been "dancing around each other" in some complex fashion for billions of years.

But they have been. People have run time backwards in simulations, and the motion of the galaxies in the Local Group is complex- as you'd expect for a multiple body system.

Run time backwards and the Andromeda Galaxy is receding from the Milky Way at about 120 kilometres per second. It is hard for me to imagine that this separation could possibly decelerate fast enough so that the two galaxies could have collided (in past times) in much less than 3 x 4.5 billion years. Which doesn't allow these two galaxies a whole lot of time to have "danced around each other" prior to somehow being violently flung apart. They most likely began their galactic lives gravitationally bound but moving apart and have never actually "danced with each other" before.

They are in orbit around each other. I would call that a dance, regardless of which way they were moving at different times- a dance that is 13 billion years old. I did not suggest that they have collided in the past... the clean spiral structure of each strongly argues against that.

Clarification: The explanation say "central parts of each galaxy begin to destroy each other". They won't really destroy each other, but they will reform and merge. Perhaps the central black holes will combine and star formation will increase dramatically. But individual stars are unlikely to collide and destruction of the type E=mc^2 will not happen. So it hardly seems to be a doomsday scenario and the term "destroy" is over-dramatic.

Newtonian2 wrote:Clarification: The explanation say "central parts of each galaxy begin to destroy each other". They won't really destroy each other, but they will reform and merge. Perhaps the central black holes will combine and star formation will increase dramatically. But individual stars are unlikely to collide and destruction of the type E=mc^2 will not happen. So it hardly seems to be a doomsday scenario and the term "destroy" is over-dramatic.

It would be correct to say that the structures will be destroyed. What we see in other galaxy collisions is that where dust and gas regions collide, star forming regions are created. But these would only be locally destructive to any planets with life on them. As you say, actual collisions would be rare (or non-existent). The biggest threat to planetary systems would likely come from a high density region passing nearby. That would likely destabilize planetary orbits. Certainly, if Andromeda's central bulge intersected the region of the Milky Way where our Solar System is, I would consider it likely that our planetary system would not survive as it is. Of course, since the Sun will be well on its way to the red giant stage by then, life on Earth will no longer exist in any case.

I watched the NASAtv presentation of the Q&A session and it was refreshing to see a very young person so knowledgeable of the subject matter, and she was not on the panel seated next to the podium. Long live NASAtv.

Chris Peterson wrote: As you say, actual collisions would be rare (or non-existent).

Galaxies begins a merger with a gravitation dance. When Stars meets, they do the same; they leave their clockwork of mechanistic trajectories, attracting each other.
Why is it unlikely that stars would collide in such a giant disorder?

ThePiper wrote:Galaxies begins a merger with a gravitation dance. When Stars meets, they do the same; they leave their clockwork of mechanistic trajectories, attracting each other.
Why is it unlikely that stars would collide in such a giant disorder?

Unless the stars have velocity vectors that intersect, all they can do is pass near each other. Of course, they'll gravitationally perturb each other, but that just means they'll follow some sort of slingshot path. Gravity is not effective at making things collide (which is why bodies don't generally fall into black holes, either, but merely orbit them or have their existing orbits perturbed).

Stars are very small compared with the distances between them, even in dense regions. So collisions are vanishingly rare.

The explanation states that they will merge to form an elliptical galaxy. Considering that the two galaxies have supermassive black holes at the center of each (correct me if I'm wrong) wouldn't we end up with a state where the gravity of the black holes dominate the geometry of the system possibly resulting in a larger spiral galaxy maybe with some sort of halo?

Devil Particle wrote:The explanation states that they will merge to form an elliptical galaxy. Considering that the two galaxies have supermassive black holes at the center of each (correct me if I'm wrong) wouldn't we end up with a state where the gravity of the black holes dominate the geometry of the system possibly resulting in a larger spiral galaxy maybe with some sort of halo?

No. The black holes combined represent less than 0.003% of the total mass of the galaxies. Their effects are only very local.

Chris Peterson wrote: ... Of course, since the Sun will be well on its way to the red giant stage by then, life on Earth will no longer exist in any case.

Good point!

Devil Particle wrote:The explanation states that they will merge to form an elliptical galaxy. Considering that the two galaxies have supermassive black holes at the center of each (correct me if I'm wrong) wouldn't we end up with a state where the gravity of the black holes dominate the geometry of the system possibly resulting in a larger spiral galaxy maybe with some sort of halo?

Given that the two about-to-collide galaxies are spirals, and since spiral galaxies have large net angular momentum (as opposed to elliptical and spherical), then unless the two galaxies have anti-parallel or nearly anti-parallel angular momentum vectors (and nearly equal in magnitude) I would exect the resultant galaxy (after the merge) to also have large angular momentum, and eventually form another spiral. Isn't angular momentum still conserved when galaxies collide? And doesn't that mean the resultant galaxy will be a spiral?

flash wrote:Given that the two about-to-collide galaxies are spirals, and since spiral galaxies have large net angular momentum (as opposed to elliptical and spherical), then unless the two galaxies have anti-parallel or nearly anti-parallel angular momentum vectors (and nearly equal in magnitude) I would exect the resultant galaxy (after the merge) to also have large angular momentum, and eventually form another spiral. Isn't angular momentum still conserved when galaxies collide? And doesn't that mean the resultant galaxy will be a spiral?

I can see the possibility that two galaxies spinning in the same direction and meeting gently top to bottom might remain a spiral after merging, but that would be an extremely rare type of merger. It's more likely they will be at some angle to each other and, about 50% of the time, will not be spinning in the same direction. So the resulting merger will be messy (that's a technical term ), and though individual stars and systems will indeed maintain much of their individual momentum, there isn't much reason to assume that the overall momentum will be all pointing in a tidy spiral-type pattern.

Try tossing two pizza doughs in the air and getting them to combine into one larger dough. Messy!

Rob

rstevenson wrote:

flash wrote:Given that the two about-to-collide galaxies are spirals, and since spiral galaxies have large net angular momentum (as opposed to elliptical and spherical), then unless the two galaxies have anti-parallel or nearly anti-parallel angular momentum vectors (and nearly equal in magnitude) I would exect the resultant galaxy (after the merge) to also have large angular momentum, and eventually form another spiral. Isn't angular momentum still conserved when galaxies collide? And doesn't that mean the resultant galaxy will be a spiral?

I can see the possibility that two galaxies spinning in the same direction and meeting gently top to bottom might remain a spiral after merging, but that would be an extremely rare type of merger. It's more likely they will be at some angle to each other and, about 50% of the time, will not be spinning in the same direction. So the resulting merger will be messy (that's a technical term ), and though individual stars and systems will indeed maintain much of their individual momentum, there isn't much reason to assume that the overall momentum will be all pointing in a tidy spiral-type pattern.

Try tossing two pizza doughs in the air and getting them to combine into one larger dough. Messy!

Rob

It doesn't (shouldn't) matter how messy it is: angular momentum is angular momentum: It must be conserved. Just as with linear momentum: If you could somehow get 1million BBs to simultaneously collide with a bowling ball, their combined center of mass will move in exactly the same manner before, during, and after the collision. The same must be true of the angular momentum of the galaxies. Before, during, and after the collision. the combined angular momentum vector must remain constant. I think that means that the result will necessarily be a spiral galaxy (if you wait long enough, and under the assumptions of inital angular momentum alignment, etc)

THAT WOULD LOOK SOOOOOOOO COOOOOL!!!!!

:---[===] *

I think we will be busier with other concerns...if humanity survives that long....which I doubt....

That is about the time, or so, that the Sun starts ballooning up to go Red Giant....


We might not have all that great of a view....

:---[===] *

flash wrote:It doesn't (shouldn't) matter how messy it is: angular momentum is angular momentum: It must be conserved. Just as with linear momentum: If you could somehow get 1million BBs to simultaneously collide with a bowling ball, their combined center of mass will move in exactly the same manner before, during, and after the collision. The same must be true of the angular momentum of the galaxies. Before, during, and after the collision. the combined angular momentum vector must remain constant. I think that means that the result will necessarily be a spiral galaxy (if you wait long enough, and under the assumptions of inital angular momentum alignment, etc)

Yes, angular momentum is conserved. But that doesn't translate into the new galaxy becoming a spiral. Spiral galaxies appear to be in a pristine state: the nature of galaxy formation produced them. Once a pair of galaxies of similar mass collide, there's no way to get back to that. The result is an elliptical galaxy. Elliptical galaxies are similar to the central bulge of spiral galaxies; the disc has been lost. Ellipticals rotate, and the final rotation characteristics will obviously be determined by the combined angular momentum after the collision.