Wednesday, April 30, 2008


When Galaxies Collide

450 million light years away, in the Hercules Galaxy Cluster, are these colliding galaxies, #272 in Hilton Arp's Atlas of Peculiar Galaxies (NGC 6050). The photo, from the Hubble, spans approximately 150,000 light years. Apparently, collision of galaxies is common, which is strange, as there is so much unused space out there a tyro, like me, would thing collisions would be rare. The big blue things peppered along the arms of the galaxies are globular clusters of stars.

So beautiful.



If you read this, would advise what happens when galaxies collide.
Do stars actually collide w/ one another? Are the spaces between them so great that they fail to collide and just pass by one another? Does all that gravity wreck havoc?

Thanks in advance.

Space is very big; everything else is very small. The sun is about a million kilometers across (cross-sectional area of about 2.5 * 10^23 square meters), which sounds big. But the next closest star is about 1.3 parsecs away (4*10^16 meters), so the sun takes up about 5*10^-11 of the space in the local area (two-dimensional calculation). Even though there are lots of stars involved in a galactic collision, very, very few will actually be involved in a collision.

Note that this is predicated on stellar separations similar to those in the local stellar neighborhood. Stars in galactic cores are much more closely spaced, which would dramatically increase the probability of collisions.

If two stars about the size of the sun did collide, the energy release would be immense. After some back-of-the-napkin calculations, I'd put the total kinetic energy release at about 1*10^40 J in about 1000 seconds. This would be about 10^11 times the normal output of the sun. Increased fusion in the (much) hotter environment would increase that energy output.

The result would be hard to distinguish from a rather large nova, and would flash-fry everything in that system at right about the speed of light.

As to interactions without collisions, the effects would probably not be especially noticeable over any reasonable lifespan. At 100 km/sec., it would take something like 300 million years for one galaxy to pass entirely through another galaxy.
Thanks dove. "It's the stuff dreams are made of."

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