The orbit of our sun around the galaxy

[csmyth3025]

I've been reading about the nature of the spiral arms in galaxies such as the Milky Way and Andromeda. As I understand it these spiral arms are believed to be pressure waves. Like most laymen, I thought that our sun - and most stars in general - orbit their galaxies in a more-or-less circular orbit. The explanation I read in Wikipedia indicates that these orbits are generally elliptical as shown in their article here:

http://en.wikipedia.org/wiki/Spiral_galaxy#Density_waves_model

In a related article, Wikipedia says that the Sun orbits the galactic center once every 225-250 million years. Have there been any computer studies done about the proximity of other stars to our sun over these orbital periods? In particular, I believe the sun now occupies a place close to the inner rim of the Orion Arm (or Orion Spur). Is the general concentration of stars much different in these arms than in the "spaces" between them?

Chris

 

[MeteorWayne]

All orbits are elliptical to varying degrees. After all, a circle is just an ellipse with eccentricity of 0, and they are almost impossible to perfectly obtain.

I believe the proper term is "density wave" not "pressure wave". They have different meanings.

Yes the stellar density is higher in the arms, but not enough you would really notice from a planet there I suspect. Of more importance for your local view is the proper motion of the stars in your local neighorhood.

 

[csmyth3025]

All orbits are elliptical to varying degrees. After all, a circle is just an ellipse with eccentricity of 0, and they are almost impossible to perfectly obtain.

I believe the proper term is "density wave" not "pressure wave". They have different meanings.

Yes the stellar density is higher in the arms, but not enough you would really notice from a planet there I suspect. Of more importance for your local view is the proper motion of the stars in your local neighorhood.

Your right, the correct term is "density wave" (my mistake - Wikipedia also uses the term density wave). On the question of the proper motion of stars in our local neighborhood, I understand that the star Gilese 710 (about 0.5 solar mass) is expected to approach the sun at a distance of about 1.1 LY in about 1.4 million years according to calculations made in 1999. Apparently, calculations made in 2010 indicate this distance might be as close as 0.68 LY. Either distance estimate would bring Gilese 710 "fairly close" to the proposed Oort cloud which may extend from 0.32 LY to 0.79 LY from the sun

I suspect that this sort of close approach has occurred many times over the last 4 billion years to varying degrees. On a human scale it's nothing we need worry about. On a geologic scale, does it lend any credibility to those postulating periodic bombardment as a natural occurrence - or is the connection too tenuous to draw any conclusions?

Chris

 

[MeteorWayne]

Well, I'd say we are still in the data collection and analysis phase regarding past and future close approaches of other stars to our solar system. Reasonably accurate calculations of stellar motion are only about a decade old as you said, and the error bars are still quite large.

Certainly a close approach of < 1 LY in the past or future would cause some disruption of the Outer Oort cloud, and perhaps the inner as well.

The late heavy bombardment ~ 4 GY ago, seems to have been caused by internal solar system dynamics (See Nice Model ), and the evidence for periodicity or even spikes in cratering rates since is what I would describe as "thin"