Starship Asterisk*

by **geckzilla** » Thu Oct 18, 2007 1:46 am

Hmmm, if Mr g---- is referring to me, you can call me Judy instead. Sorry, I know, my nickname is gender confused... most people think I am a male.

by **toejam** » Wed Oct 17, 2007 6:18 pm

Thanks Chris & geckzilla. Great site Mr g----,

It fair makes your head spin, don't it? My head, anyway.

by **Chris Peterson** » Tue Oct 16, 2007 11:00 pm

toejam wrote:More questions.

Oct 12 APOD

http://antwrp.gsfc.nasa.gov/apod/ap071012.html

States that the galaxies there have had close encounters in the past.
Well, now they are far apart, so gravity did not cause their motion to halt when they were close. Why? If gravity is responsible for their motion towards one another it should not allow them to "overshoot" in this way. Should they not have comingled?Or will theyfall back together again, because of gravitational attraction?

Keep in mind that nothing actually collides physically when two galaxies collide, since they are both mainly empty space. They pass through each other, and you get all sorts of interesting tidal effects, and some objects have their paths altered. Consider a much simpler example like a comet orbiting the Sun. Under the influence of gravity, it comes very close to the Sun, but then heads back out into deep space. Something similar happens on the large scale of colliding galaxies. All the velocity acquired as components head towards each other carries everything apart again. And assuming the galaxies are gravitationally bound (that is, they aren't moving so fast that their mutual gravity can't keep them together), they will continue to collide repeatedly, just like a comet keeps coming back to the Sun.

by **geckzilla** » Tue Oct 16, 2007 9:10 pm

toejam, this (relatively) simple little java applet might help you imagine it better...

http://burro.cwru.edu/JavaLab/GalCrashWeb/main.html

(click on "applet" on the left menu)

by **toejam** » Tue Oct 16, 2007 8:20 pm

toejam wrote:
Chris Peterson wrote:
toejam wrote:What I want to know is why, if the universe is expanding, are there any collisions between the galaxies? Should there not be more space available?
The expansion of space is nowhere near strong enough to overcome the local effects of gravity. That's why the Earth isn't expanding with the Universe, and it's why groups of galaxies can be gravitationally bound, allowing for some collisions. Also, even galaxies that are not gravitationally bound can still have motion paths that intersect, even in an expanding Universe.

The expansion of space is presumably uniform, but the distribution of material is not, and neither is the distribution of velocities.
Thank you Chris.

It boggles my mind that the Milky Way and the Andromeda Galaxy, some 2 MILLION light-years apart, are approaching one another at 120 km/sec owing purely to gravitational attraction at that distance. I am told (not my calculation) that they will collide in a mere 5 BILLION years. There's something to worry about!!!

More questions.

Oct 12 APOD

http://antwrp.gsfc.nasa.gov/apod/ap071012.html

States that the galaxies there have had close encounters in the past.
Well, now they are far apart, so gravity did not cause their motion to halt when they were close. Why? If gravity is responsible for their motion towards one another it should not allow them to "overshoot" in this way. Should they not have comingled?Or will theyfall back together again, because of gravitational attraction? I have not ever seen any statement that this happens, although there have been many mentions of galaxy "capture" and many of galaxies "having passed through" one another. What keeps them going? Are they going to reverse?

by **toejam** » Thu Oct 11, 2007 6:22 pm

Chris Peterson wrote:
toejam wrote:What I want to know is why, if the universe is expanding, are there any collisions between the galaxies? Should there not be more space available?
The expansion of space is nowhere near strong enough to overcome the local effects of gravity. That's why the Earth isn't expanding with the Universe, and it's why groups of galaxies can be gravitationally bound, allowing for some collisions. Also, even galaxies that are not gravitationally bound can still have motion paths that intersect, even in an expanding Universe.

The expansion of space is presumably uniform, but the distribution of material is not, and neither is the distribution of velocities.

Thank you Chris.

It boggles my mind that the Milky Way and the Andromeda Galaxy, some 2 MILLION light-years apart, are approaching one another at 120 km/sec owing purely to gravitational attraction at that distance. I am told (not my calculation) that they will collide in a mere 5 BILLION years. There's something to worry about!!!

by **Chris Peterson** » Wed Oct 10, 2007 10:13 pm

toejam wrote:What I want to know is why, if the universe is expanding, are there any collisions between the galaxies? Should there not be more space available?

The expansion of space is nowhere near strong enough to overcome the local effects of gravity. That's why the Earth isn't expanding with the Universe, and it's why groups of galaxies can be gravitationally bound, allowing for some collisions. Also, even galaxies that are not gravitationally bound can still have motion paths that intersect, even in an expanding Universe.

The expansion of space is presumably uniform, but the distribution of material is not, and neither is the distribution of velocities.

by **toejam** » Wed Oct 10, 2007 9:54 pm

What I want to know is why, if the universe is expanding, are there any collisions between the galaxies? Should there not be more space available?
Is the expansion uneven? If so, why?

by **Chris Peterson** » Mon Oct 08, 2007 9:30 pm

hermanbubbert wrote:In the enlarged version of Schirmer's image, similar complex, multiple layers are visible around another galaxy that is cropped out of view on the right side of the smaller version. Is there a possibility that this appearance is artifact due to his image processing techniques? It just seems odd that this unusual finding should show up in two galaxies in one image.

No. The same structure is seen in lots of other images of both these galaxies. The one you are referring to is NGC 467. The main object, NGC 474, is simply close enough that we can get pretty good resolution of its structure. But many galaxies show similar disruptions, presumably from tidal forces during collisions.

by **geckzilla** » Mon Oct 08, 2007 8:07 pm

Aliens have placed giant mirrors in space in order to see what their own galaxy looks like. Since the mirrors are some distance away from the galaxies they are reflecting, the "mirror" image looks slightly different rather than an exact duplicate because the light from the mirror took so much longer to reach us than the actual galaxy.

Sorry, sorry. Back on track, now...

by **Qev** » Mon Oct 08, 2007 7:43 pm

Well, considering that most galaxies are interacting with their fellows, it doesn't strike me as too unlikely to spot multiple galaxies showing this sort of structure.

It certainly doesn't seem any more unlikely than this arrangement: http://antwrp.gsfc.nasa.gov/apod/ap040815.html

by **orin stepanek** » Mon Oct 08, 2007 7:25 pm

http://antwrp.gsfc.nasa.gov/apod/ap071008.html
I wonder if it may be due to lensing. I notice there are also two distant background galaxies that appear on the enlarged view where you noticed the similarity.

Orin

by **hermanbubbert** » Mon Oct 08, 2007 6:12 pm

In the enlarged version of Schirmer's image, similar complex, multiple layers are visible around another galaxy that is cropped out of view on the right side of the smaller version. Is there a possibility that this appearance is artifact due to his image processing techniques? It just seems odd that this unusual finding should show up in two galaxies in one image.

Starship Asterisk*

NGC 474 artifact? (APOD 08 Oct 2007)

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