Re: the fidelity of the simulation, the galaxy hack is a poor approximation of how a real galaxy would behave, as it doesn’t model the gravitational effects of the individual stars at all, only a single central point mass per galaxy. The stars are more like field “tracers”… My own version puts all the stars in a single plane, which I think results in a more coherent view of what’s happening in the gravitational field. I “fix” the start conditions in other ways to produce “better” near-misses that look cooler…

I have played around with Celestia a little bit.

Have you seen the program Celestia? I love it, it’s great. Once you get used to how to manipulate your camera properly, it’s wonderful.

Observed galactic collisions occur at a much less frequent rate than encrouching speeds might suggest. Why? Because current theory is wrong! Why? ask me! I think some of Halton Arp’s theories will prove to be right– after his death, similar to my own theories! se la vi.

forrest noble, forrest underscore forrest at netzero dot net.

BTW, I have to ask the obvious question: how many times have you been pondering galactic collisions and NOT had an image of galactic collisions appear in your inbox, hmm?

I have no idea what factors should be in such a formula. Can any of the astronomers here venture a guess?

Cool, unless there’s some sort of delay for the southern hemisphere, though NZ may have drifted north by then anyway. Might have to look it up sometime before then.

And Phil, many thanks for the paper. I’ll try to read it as soon as I can.

Here’s the patch in a different format; to apply just run “patch galaxy.c”, paste in this text, and hit ctrl-d exactly twice (for some reason.) Remember, this is for ver. 5.03, but I don’t think the lines in question have changed, well, forever.

To build, just “.configure;make” and then you can run “./galaxy” directly. I suggest using the flags “-count 2 -

I’m kind of surprised nobody has ever found the error…

(BTW, while looking at it again I found 1 more bug, fixed here, at line 373.

“”"”"”"”"”"”"”"”"”"”"”
[jnyberg@jnyberg-231l ~]$ diff -e galaxy.c.orig galaxy.c
410,415c
gt->vel[0] += d0 * gtk->mass;
gt->vel[1] += d1 * gtk->mass;
gt->vel[2] += d2 * gtk->mass;
gtk->vel[0] -= d0 * gt->mass;
gtk->vel[1] -= d1 * gt->mass;
gtk->vel[2] -= d2 * gt->mass;
.
405c
d = 1 / (EPSILON * sqrt_EPSILON) * DELTAT * QCONS;
.
403c
d = 1 / (d * sqrt(d)) * DELTAT * QCONS;
.
373c
d = gtk->mass / (eps * sqrt(eps));
.
371c
d = gtk->mass / (d * sqrt(d)) * DELTAT * DELTAT * QCONS;
.
“”"”"”"”"”"”"”"”"”"”"”"

Jason Nyberg

To bad it has to share the webpage with personality test ads featuring Dianetic by L Ron Hubbard…

Keep up the great work!

Some multi-tentacled life forms on a planet orbiting a mediocre star 2/3 the way out from galactic centre of the galaxy on the right in that picture, are looking at an image of our Milky Way and the Andromeda galaxy on their brain-link screens with a little narrative that reads something like:

“Astronomers observing these galaxies believe that in 1-2 billion quatlons, these galaxies will interact much like our own, and eventually coalesce into an elliptical galaxy. This would be a spectacular sight for any life in those galaxies.”

And here it is, hehe. Great explanation, as always. Thanks for that.

Just a little question: you said that the Milky Way and Andromeda will collide in 2 billion years or so. I’ve been trying to find that information, but I got numbers from 3 to even 15 billion years for that to happen… all very unclear.
So is 2 billion years the valid term now?

Apply these changes to fix it, and watch galaxies collide, merge, and get torn apart in your screensaver the way they were meant to! You’ll often see images surprisingly similar to the one heading up this thread.

(Basically, the wrong masses are used in the force calculations. I’ve known about the bugs for years, but am too lazy to push them upstream…)

The “greater-than” fixes are the right ones; these are based on ver. 5.03, the sources of which can be found here: http://www.jwz.org/xscreensaver/download.html

“”"
[jnyberg@jnyberg-231l hacks]$ diff galaxy.c ~
371c371
mass / (d * sqrt(d)) * DELTAT * DELTAT * QCONS;
—
> d = gtk->mass / (d * sqrt(d)) * DELTAT * DELTAT * QCONS;
373c373
mass * eps;
—
> d = gtk->mass * eps;
403c403
mass * gt->mass / (d * sqrt(d)) * DELTAT * QCONS;
—
> d = 1 / (d * sqrt(d)) * DELTAT * QCONS;
405c405
mass * gt->mass / (EPSILON * sqrt_EPSILON) * DELTAT * QCONS;
—
> d = 1 / (EPSILON * sqrt_EPSILON) * DELTAT * QCONS;
410,415c410,415
vel[0] += d0 / gt->mass;
vel[1] += d1 / gt->mass;
vel[2] += d2 / gt->mass;
vel[0] -= d0 / gtk->mass;
vel[1] -= d1 / gtk->mass;
vel[2] -= d2 / gtk->mass;
—
> gt->vel[0] += d0 * gtk->mass;
> gt->vel[1] += d1 * gtk->mass;
> gt->vel[2] += d2 * gtk->mass;
> gtk->vel[0] -= d0 * gt->mass;
> gtk->vel[1] -= d1 * gt->mass;
> gtk->vel[2] -= d2 * gt->mass;
“”"

Jason Nyberg

Anyway, I think I have a similar curiosity as Anse above:

The tendril from A is wrapped at least twice around the GC of B – maybe even three times, if what I’m seeing as an inner wrap isn’t formed by a different process. But when the objects stripped from A fall under greater influence from B, getting wrapped as they are, are they not also picking up B’s rotation? The tendril seems to be staying in the plane of A, instead of twisting around while wrapping as I naively suppose it should.

Also, in A there appears to be ghost spiral limb of young stars that wasn’t displaced by the close encounter – I can see how the limb was deformed into the tendril by the influence of B, but the young stars are about where I would expect to see the limb if it wasn’t deformed.