[greensleeves (OP)]

There are more than 110 stars in about 80 star systems within 20 light years of our Solar System. Of all of these stars only one - alpha Centauri A - is in the same spectral class as our Sun, namely G2. Almost all other stars are much dimmer red dwarfs. A few are much brighter white stars. Alpha Centauri A is perhaps the most similar star to our Sun in all this region, and alpha Centauri B is also not much different - just a little dimmer.

Alpha Centauri A and alpha Centauri B also happen to be (along with Proxima Centauri, the third member of that system) the closest stars to our Sun.

Is this purely coincidental, or is it possible that alpha Centauri's close similarity as a 'sister star' to our Sun, and its close association in space, are in some way linked by a common origin? I believe (though am not certain) that stars evolving from the same nebula at the same time are often of a similar spectral class, so is it possible that the Sun and alpha Centauri are genuine sisters in space born from the same nebula nearly five billion years ago?

I suspect the answer is no, it is purely coincidental, but I'd like to know for sure.

[Pikaia]

Stars which formed from the same nebula as the Sun can be identified by their motion, which will be at the same speed and direction as the Sun. Such stars are known (although I don't know any examples). but Alpha Centauri is not one of them - it has a substantial velocity relative to the Sun, so it would not have been nearby when the Sun formed.

Stars formed from the same nebula have a large range of sizes (eg Alpha centaur A,B and C), and solar-mass stars are fairly common, so any similarity is just a coincidence.

[yor_on]

Cool, anyone that know where those possible 'sisters' to our sun is today? and yesterday
Like 'backtracking' them to a origin if so?

[greensleeves (OP)]

Thanks for your reply Pikaia. It seems if you're right, that the basic question has been answered by you. Clearly if alpha Centauri at only 4.3 light years distant, has a radically different velocity, or a different point of origin, then it cannot be a sister star of ours.

Would a star necessarily however have to also be moving in the same direction as our Sun in order for it to have originated in the same nebula? I would have thought that gravitational interactions / sling-shot effects etc, might have over-ridden the general direction of movement of the nebula, and sent some stars off in different directions (albeit of course with the same point of origin).

But in dismissing alpha Centauri, I also echo the other question - which other stars are known to have formed in the same nebula as the Sun, and how far and in what direction was this point of origin? Also, from our limited knowledge of these stars (their numbers, location or spectral classes), would there be any way of assessing the size or other characteristics of the mother nebula?

[solaris]

but Alpha Centauri is not one of them - it has a substantial velocity relative to the Sun, so it would not have been nearby when the Sun formed.

Along this line, I wonder whether finding the same velocity and direction would be enough to identify the common origin of stars. I guess, one can always find such instances just by chance. Are there any additional requirements?

[kname2]

A sister star to the Sun has likely been identified. The star, HD 162826, was recently identified by Ivan Ramirez and his team at the University of Texas at Austin. It's located 110 light-years away in the constellation Hercules, is about 15% more massive than our Sun, and is not visible to the naked eye. But, chemical and orbital analysis indicate it was born from the same nebula as our star.

[Ophiolite]

My understanding is that any sister stars to the sun would long ago have been separated through gravitational interactions with "the rest". For the same reasons, after 4.5 billion years, there would be no discernible similarities between sister velocities. I should be interested to see any peer reviewed research that effectively counters this understanding.

Edit: I posted this before noting the reply from kname2. I have no problem accepting an identification through compositional identities. That makes sense, whereas looking for dynamics parallels seems a bit silly.

[evan_au]

I wonder whether finding the same velocity and direction would be enough to identify the common origin of stars?

Stars forming in a single dust cloud in the bulk of the galactic disk will have a common overall motion, but individual small motions, which will separate them over time. They will bob up and down around the central plane of the galactic disk, and will have slightly different orbital periods around the center of the galaxy, and will pass close to different stars, which will scatter them in slightly different directions.

Identifying stars forming in the same dust cloud as the Sun could get hints from their chemical composition and general band within the galactic disk. These would need to be distinguished from the huge number of stars in the same general band within the galactic disk.

However, there are some streams of stars that can be easily identified as belonging together, because they are well outside the galactic disk, with orbits tilted at a large angle from the galactic disk, of a similar age, and traveling at high velocity compared to stars in the galactic disk. These star streams are thought to have originated in small galaxies which have been swallowed by the Milky Way, and slowly torn apart as they orbit the center of the galaxy.
See: http://en.wikipedia.org/wiki/List_of_stellar_streams