# Which stars did the Sun form with?

The Sun formed 4.5 billion years ago, in a molecular cloud. I assume that there were other stars in the vicinity (as is common in molecular clouds). Which stars are they? Where are they now? Are they similar to the Sun?

Most stars form in clusters, so it is very likely that the Sun was part of a star cluster when it formed.

But in On the Dynamics of Open Clusters, the relaxation time of a cluster is calculated to be in the order of $$\tau=4\times10^7 \textrm{yr}$$. During that time, about one hundredth of the stars will escape from the cluster (i.e. reach escape velocity). The dissipation time of a star cluster is therefore in the order of a few billion years. These figures were calculated for an idealised star cluster containing stars of equal mass. For more realistic clusters the dissipation time may be much shorter. Especially dwarf stars seem to dissipate faster.

It is, therefore, likely that after 4.5 billion years, the Sun has escaped from its star cluster. It would be very difficult to identify the stars with which it was formed, unless you can trace back the position of the Sun and other stars before dissipation, which is impracticable.

As far as I am aware, no sibling stars have been positively identified. The sibling stars should be similar in composition (element abundances), although they do not have to be solar twins as the masses of these stars may vary widely.

EDIT: @adrianmcmenamin mentions an article in Sky & Telescope, where a sibling has been proposed. This refers to an article on arXiv by Ramirex et al. in which the element abundances have been analysed of stars that were earlier identified as possible siblings using galactic dynamics (i.e. tracing back the movement of these stars). Of the 30 stars identified as possible solar siblings using dynamical methods, only 2 stars had similar element abundances, which in itself says something about the accuracy of these dynamical methods...

• Most star clusters live for less than 10 million years. They are not formed in a gravitationally bound state. Nov 22, 2019 at 6:51

Often stars can form when a supernova compresses surrounding clouds into higher densities with interstellar shock waves, and it can also send the stars outwards from the point of the supernova.

Two days after seeing this topic, I read a report that identifies the imprints of a low mass supernova in meteorites of the solar system, by measuring which radioactive isotopes are in meteorites:

"Identifying these 'fingerprints' of the final supernova is what we needed to help us understand how the formation of the solar system was initiated," Professor Heger said.

"The fingerprints uniquely point to a low-mass supernova as the trigger."

"The findings in this paper have opened up a whole new direction of research focusing on low-mass supernovae," he said.

In addition to explaining the abundance of Beryllium-10, this low-mass supernova model would also explain the short-lived nuclei Calcium-41, Palladium-107, and a few others found in meteorites.

The speed at which star clusters diverge is relatively high, Teegarden's star for example is 11,502 Million years old, 12 ly away, and it's group's movement has been traced back roughly to a zone of origin, and the group's stars are twice as distant from each other as they were at their formation, the group spans at least 50 light years.

I don't want to get in over my head here, but after 4 billion years, stars of a group can be millions of light years apart. High velocity stars can move at 200 light years per million years (70km/s), so that's 200.000 light years per billion years. Teegarden's star is moving that fast and it's 0.2 times the mass of the sun.

Teegarden's group are distancing themselves by about .5 light years per million years, and they are currently 10-60 light years away from the sun.

It will be a steep task to find where the Sun came from and where its neighbors are.

Proxima Centauri currently moves toward Earth at a rate of 22.4 km/s. Ηowever, after 26,700 years, when it will come as close as 3.11 light-years, it will begin to move farther away.

That's why I am surprised by the journal suggesting a stellar neighbour at 110 ly away, seeing as some of its group can potentially have travelled 200,000 ly although not in a straight line. There are proper motion simulations on the web which are interesting to see.

• Except in the rare case of a star reaching the escape velocity of the galaxy due to a close encounter with another star (or in the case of violent galaxy-galaxy merger), stars don't in general leave their host galaxy, and so can't be million of lightyears apart. Stars from the cluster in which the Sun was born drift apart but will still orbit the center of the Galaxy, and won't end up more than 50,000 lightyears from each other (the diameter of the orbit of stars at the distance of the Sun from the Galactic center), or at the very most 100,000 lightyears (the diameter of the Milky Way)
– pela
Dec 5, 2016 at 19:15
• Hi, sorry of course you are right. When a star travels 23km/s from us, i.e. proxima centauri i.e. light years per million years, it can still have travelled vast distances through the galaxy. I was busy being embarassed that i can't find the journal article i read on Teegarden's star to cite, that i didn't phrase it properly. Dec 5, 2016 at 21:17
• A typical dispersion speed for a stellar cluster is more like1 km/s. Dec 5, 2016 at 22:14
• @RobJeffries: Really? Isn't it more like 10 km/s?
– pela
Dec 6, 2016 at 11:00
• @pela 10 km/s would be a globular cluster. Which is not typical. In fact, forming in a bound cluster at all is not typical. The velocity dispersion of typical star forming region is 1 km/s and they are unbound. Dec 6, 2016 at 11:08