Yes. You’re missing the scale of the distances between stars in most of the galaxy. The Sagan Planet Walk in Ithaca, NY, is a scale model of the solar system, with distances and sizes of each object represented on the same 1:5,000,000,000 scale. The Sun in this model is 27.8 cm in diameter, a little less than 11 inches. If they were to show the nearest star, Alpha Centauri, on the same scale, it would be somewhere in Hawaii. No kidding.
Even if the stars are exerting a gravitational force on each other, they’re just so far apart as compared to their size that they are not likely to collide.
I remembered in an earlier thread, someone had calculated a probability of two stars colliding for an Andromeda/milky Way collision. Turns out, it was me. Who knew?
(The calculation is in Post 22, with some discussion in later posts. I got a 0.16 percent chance, but that’s a very rough number.)
Is this true? I thought that two widely separated observers, each measuring a zero dipole moment in the CMB, will have a velocity relative to each other (due to Hubble expansion). Sure, you could define a rest frame that works for a single galaxy, but that’s still not “absolute”.
I concur with you that it’s very unlikely that two stars will directly collide.
Thanks ZenBeam, for posting the link to the earlier discussion on the subject.
The reason I originally posted was because I parsed dtilque’s statement to mean that there was a reason (other than the vast distances involved) that no collision(s) would occur.
How big is the “orbiting hierarchy” between the sun and the galaxy’s CM?
Somebody also said the Sun is orbiting the center of mass of the solar system (which in turn is orbiting the center of mass of the galaxy).
So we have
Luna->Earth->Sol->Solar System’s CM->?..->Galaxy CM->Local Group CM->…
How many non-trivial “?” are in there? Is there any formal hierarchy like “group of solar systems”->“cluster of solar system groups”->“group of clusters” etc in between? The terms themselves I made up, but I hope they kind of illustrate what I’m getting at.
dtilque didn’t say it can’t happen, only that it won’t. An asteroid could fall on my house today. There’s nothing in the laws of physics that says that can’t happen. But I can pretty confidently predict that it won’t happen.
Earth is not orbiting the Sun, Earth is orbiting the solar system’s center of mass, as is the Sun. The solar system’s center of mass is close enough to the Sun that saying “the Earth orbits the Sun” isn’t too far off.
In the Solar System, Venus orbits in a different direction from the other planets, and Uranus ‘rolls’ on its side as it orbits. Do any stars copy such a thing on a galactic scale? As in, orbiting against the trend or ‘rolling’ around Galactic Central Point?
There are a few stars that orbit in wild and crazy directions, though. Barnard’s Star, the next closest star after the alpha Centauri system, is a halo star that just happens to be passing through our neighborhood right now, making it the galactic equivalent of a comet.
Every galaxy has a halo component, which includes stars and globular clusters orbiting the center of mass of the galaxy in a spherical distribution (ie, not confined to the disk).
In our solar system, there are some moons that orbit their planets retrograde (the other way around from how the planets orbit the Sun). If you were looking at the solar system in such a way that the planets seemed to be going around the Sun counterclockwise, these moons would be going around their planets clockwise. We think these moons were captured into orbit around their planets- they weren’t formed in orbit around the planets, the way the Earth’s moon and most other large moons in the solar system were. Galaxies capture stars and smaller galaxies, too, so there’s no reason to think there wouldn’t be stars orbiting our galaxy the “wrong way”.
None. The Sun orbits the center of mass of the Galaxy. Note however that it’s not a simple elliptical orbit, but then you can’t appoximate the Galaxy as a point mass, which is a prerequisite for an elliptical orbit.. The Sun kind of bobs up and down “above” and “below” the plane of the galactic disk. If you plotted the path with respect to that plane, it would look like a sine wave with a period of (I think) about 80 million years. Much less than the orbital period, anyway. You can think of this bobbing motion as something vaguely like an orbit about the galactic disk.
While there are star clusters, the Sun is not in one. In general, star clusters are either extremely old (globular clusters, which pre-date the galactic disk) or very young (open or galactic clusters). In general, stars form in clusters, but those evaporate as interactions between the various members eject stars. The Sun left its natal cluster a very long time ago.
Globular clusters evaporate too, but they started with many more stars than open clusters do, so they haven’t completely evaporated yet. The stars that are left are orbiting very close to each other compared to stars in the galactic disk. And sometimes they do collide. When they do they form blue stragglers.
JBDivmstr Sorry for any confusion I caused. I suppose I should have said “almost certainly won’t collide” or something like that.
