Has there ever been an observed star collision? If there hasn't, what might occur?
Not sure, though I would think in some cases (depending on the mass of each star, velocity, etc) the stars would end up orbiting each other. Maybe someone with more knowledge can answer your questions better.
Collisions between stars are extremely rare. I remember we were asked to make a rough calculation of the frequency of collisions way back in undergraduate astronomy and from memory it was something of the order of 1 collision in a galaxy the size of the Milky Way in the total age of the universe. Correct me if I'm wrong here anyone! In any case it's very rare and I don't know of any observed collisions although the origin of some Gamma Ray Bursts are unknown so it's possible I guess (though probably unlikely, I'm hazy on this area) that the odd collision could cause one of these.
On the other hand, most stars live in multiple star systems where they orbit other stars. The most common is binary stars, two stars orbiting a common centre of mass. This is not because they have passed close to each other at some point but because they form together. Interactions between the two stars are common, since the radius of a star can change significantly during the course of the stars existance. Stars that previously were far enough apart to leave each other alone can then start to exchange mass between them and even merge together.
Binary interactions are thought to be the cause of many types of Novae and Supernovae.
So to sum up, independent stars essential do not happen to run into each other as they wiz around the galaxy but binary stars do interact with their neighbors in interesting ways. We observe this kind of 'collision' if you want to call it that all the time.
Yup, even if two galaxies collide, the probability of two stars colliding is pretty much zero. Infact, the probability of two stars even 'strongly interacting' is very close to zero. It's more like two gases passing through each other than two solid bodies.
I've never heard of binary or trinary systems with collisions, if they did collide, you would have a very rapidly rotating body left over.
Any references where the probability of collapsing stars is worked out?
There is this which shows why stars do not collide in normal circumstances. It points out that collisions would be more common in dense regions, like galactic centres, but this is still time scales of millions of years at best for extremely dense regions where we would have little hope of observing anyway.
That is not the kind of reference I was looking for.
It seems that you can conclude a lot more than I from that reference.
Well this is completely wrong... Although I don't know the exact probability of a star collision taking place, in an article I read they believed they were happening every 10 seconds. You have to realize how big the universe is, and how little we can observe. The only star collisions visible to man would have to be in our Galaxy, yet there are biliions of Galaxies... Not only that we can't see every star in our own Glalaxy. And the collision have to do more with the proper motion or orbit of the star, stars have elliptical orbits and dependig on their distance to the center of our galaxy, being the center of it's orbit, and their position in their orbit, the stars would more or less run into each other. Gravity plays a role but it is more their orbits that cause the collisions. This obviously is more evident at the galactic center where there is a high density of stars. We allso don't take into affect our time scale as humans, and the actual age of the universe. But you were correct in that they can form novae and supernovae.
One hypothesis which explains the existence of the "blue stragglers" found in star clusters is that they formed by the merging of a binary system.
This very recent Nature abstract gives evidence for star collisions forming blue stragglers in globular clusters. But no one has directly seen such a collision.
Echoing other posts, stellar collisions are virtually unheard of. The average distance between stars is too great. Collisions between inspiralling binary stars are far more common, yet still very rare.
OK, I think you're missing the point here.
When one asks how common stellar collisions are, one does not talk about the sum-total in the whole universe, one talks about their frequency per unit volume.
Think about this:
How common is albinism?
Us: "Quite rare, only one in 17,000 people are albino."
You: "Pah! They are very common. You have to remember there are 6 billion people on Earth. That's 350,000 albinos."
V838 Monocerotis is (as far as I know still) a candidate for a stellar collision between two main sequence stars, although the issue isn't settled. The major alternative theory is a planet + star collision. http://en.wikipedia.org/wiki/V838_Monocerotis and HubbleSite have some spectacular pictures of the light echo.
FWIW, considering stars not bound to each other, orbiting the galaxy in the Sun's neighborhood, the Sun has about a 71% chance of randomly passing within 1 solar diameter of another star once every 10^20 years. Here's a link to an online calculator I made based on a formula included in the paper entitled "Stellar Encounters with the Oort Cloud Based on Hipparcos Data". http://orbitsimulator.com/formulas/cse.html
Of course, as others have mentioned, stars near the cores of galaxies have a higher probability, as do stars in dense clusters, and stars bound in double or triple systems.
You do realize that 10^20 years is orders of magnitude longer than the current age of the Universe, right?
Yeah, 10 orders of magnitude in fact. Or 10 billion times older than the universe is now. Or 10 billion times longer than an average sun-like star lives.
Also, stars don't randomly pass each other. They are gravitationally constrained, which makes those calculations of yours inapplicable.
There's the bizarre 'blue stragglers' as mentioned, and there's also contact binaries, which may share an envelope and evolve in unison, or the larger may go red-giant and swallow the smaller. What happens then may be fireworks...
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