The Sun was born 4.57 billion years ago from a giant molecular cloud, as one of probably hundreds or even thousands of members of a freshly-minted open star cluster. So where are all these brothers and sisters of the Sun? Well, stars in an open cluster scatter across the galaxy over billions of years, so they’re probably all thousands of light-years away where we wouldn’t notice them.
Or are they?
Astronomers at UT Austin’s MacDonald Observatory have announced that they have found a star that they believe was born in the same cluster as our Sun. This star is called HD 162826, and it’s only 110 light-years away in the constellation Hercules. You can read the details of their discovery in the scientific paper here.
I’ll admit, when I first saw this story, I was very skeptical. After all, stars in open clusters scatter into the galaxy as something like a few percent of their orbital speed around the galactic center. (This behavior varies with distance and is describe by something called the Oort constants.) Since the Sun has made about 20 orbits around the galaxy since its birth, its had time to drift to the far side of the galaxy from the place of its birth.
On the other hand, it is possible for star clusters to hang together for a long time. For example, M67 is an open cluster that it estimated to be about 4 billion years old, nearly as old as the Sun, and it still has well over 500 stars. Moreover, the way the gravitational interactions work, small stars get thrown out of clusters fastest and farthest, while big stars huddle closer together for longer. Since the Sun is bigger than 85% of the other stars in the galaxy, it might have been drifting away much more slowly.
So which is it? Did the Sun wind up on the far side of the galaxy from its birthplace, or did it stay closer to home? It would appear that the answer is the second one. The astronomers at MacDonald observatory looked at two crucial properties of the stars around us to see if any of them could be the Sun’s long-lost siblings. First, they had to be on a reasonably similar orbit, not some weird elliptical one where it couldn’t have come from anywhere near here. Second, it they had to have the same chemical composition. Every giant molecular cloud has a different chemical signature–more of one element and less of another–so only stars with almost exactly the same chemical abundances as the Sun could possibly be siblings.
For the star HD 162826, only one element, Samarium, was out of place, and that is strong evidence that it really did come from the same stellar nursery!
HD 162826 is 15% more massive than the Sun and slightly hotter and brighter. Since bigger stars usually form before smaller ones, it is the Sun’s big brother in both senses of the term. Finding a solar sibling has some astronomers excited, since their proximity in their youth means that there is a greater possibility that life could have been carried between one solar system to another by asteroids and comets. Personally, i don’t put much stock in these panspermia-type theories, but seeing what kinds of planets might be around stars similar to the Sun does tell us about how they form and how many there might be out there. Sadly, HD 162826 doesn’t have any planets that we can see yet, but so far, we’ve only ruled out the biggest and hottest ones.