Astronomers have confirmed that a star in the constellation Hercules is a dead ringer for one of the Sun’s long-lost brothers.

Sun's brother in Hercules
The star HD 162826 in Hercules potentially was born in the same cluster as the Sun was. This chart is from S&T's Pocket Sky Atlas.

Stars like our Sun form in groups. We see evidence for this throughout the Milky Way, most famously in the Trapezium Cluster in the Orion Nebula. But after stars begin to emit their nascent light, their gravitational interactions with other nearby "siblings" send them out from their birth cluster and into the expanses of our galaxy.

Even though they travel thousands of light-years from where they first formed, stars carry the signature of their birthplace in the detailed chemical composition contained in their atmospheres and their motions through space. Astronomers have been searching for the lost siblings of the Sun for some time, and they’ve found several candidates. Now, using these lines of evidence, they think they’ve confirmed one.

Ivan Ramírez (University of Texas, Austin) and his team used high-resolution spectroscopy obtained at the McDonald Observatory in Texas and Las Campanas Observatory in Chile to inspect the atmospheres of 30 suspected solar siblings. (These stars are not true "solar twins," stars that appear similar to the Sun with respect to virtually every observable property — including mass, luminosity, and composition — regardless of origin.) The team chose these 30 stars from previous studies that had highlighted them as potential solar siblings, based on motions, ages, and compositions.

Typically, star formation results in the formation of an "open cluster,” a group of young stars that have formed from the same gas cloud. The detailed chemical abundances of this gas cloud, as measured by traces of elements heavier than helium, are preserved within the young stars.

Open clusters only last for a few hundred millions years, their stars spreading out throughout the galaxy over time. The Sun itself is about 4.57 billion years old, so it’s had plenty of time to get lost.

star cluster M67
Several astronomers have suggested that the open star cluster M67 could be the Sun's birthplace. But a recent analysis suggests that the Sun and M67 were born in different giant molecular clouds.
ESO / Digitized Sky Survey 2 / Davide De Martin

Fortunately, astronomers don’t need a home address to identify solar siblings. By measuring the motions of stars through space, astronomers can "reverse" their motions and see which stars were near the Sun when it formed. You can imagine watching a video of an explosion in reverse: as you play the movie, things that are initially far apart begin to move closer to one another.  The same method works here. However, it is important to model the Milky Way's gravitational field correctly, since it influences the motions of these stars. Ramírez and his team measured motions for 30 suspected siblings and were able to "rewind the tape" on each of them.

Next, the team measured the detailed atmospheric composition of each suspected sibling. In order to be a match, the star needs to not only have been close to the Sun about 4.6 billion years ago, but it also needs to have the same age as the Sun and have similar abundances of iron, silicon, oxygen, and other heavier elements.

After this test, only two of the 30 candidates showed a match to the Sun's chemical composition, and only one, HD 162826, was close to the Sun at the time it formed. Thus, this makes HD 162826 the best "solar sibling" candidate to date. This star is about 15% more massive than the Sun, making it one of the Sun's big brothers. At 110 light-years it is also relatively nearby, shining at magnitude 6.7 in the constellation Hercules.

The team found that, instead of painstakingly looking at as many elements as possible, the most useful tactic is to measure the abundances of a handful of elements that vary greatly among stars that otherwise have similar compositions. One of these elements, barium, should be easily observable with medium-resolution spectra, the team says.

Although HD 162826 is slightly more massive than the Sun, Ramírez’s team notes that many of the solar siblings are likely to be low-mass M dwarfs, since these are the most common type of star made during star formation. Current capabilities likely won’t be able to identify these little brothers — M dwarfs have crowded spectra that are difficult to analyze, and they’re also inherently dim. But with the launch of ESA’s Gaia, astronomers will have precise measurements on the motions of millions of nearby M dwarfs, which will help isolate which of these were formed alongside our Sun.


Reference: I. Ramírez et al. "Elemental Abundances of Solar Sibling Candidates." Astrophysical Journal, in press.


Image of Tom Hoffelder

Tom Hoffelder

May 14, 2014 at 12:07 pm

Dr. Bochanski, during my Adult Ed Basic Astronomy class last Thurs evening, which covered the Milky Way, I presented two well known numbers included in your article: 1) the Sun is 4.5 billion years old and 2) when viewing open clusters we are looking at young stars since the clusters are torn apart by gravitational forces in a few hundred million years.

Considering those two numbers, the M67 photo caption is confusing. Since we can see it, the open cluster can't be more than a few hundred million years old. Wouldn't that rule it out as the Sun's birthplace? It would not seem necessary to rule it out on the basis of molecular analysis. On what basis did some astronomers consider it a candidate?

Any comment would be appreciated. Thank you and thanks for the interesting article. Next time I have one of our telescopes out, and Hercules is up, I will be checking HD 162826!

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Anthony Barreiro

May 14, 2014 at 6:16 pm

M67 has been proposed as a possible birth cluster for the Sun because the stars in M67 all have approximately the same age and chemical make-up as the Sun.

It is true that most open clusters lose all their stars within a few hundred million years, but M67 is an exception that proves the rule. Most open clusters orbit the center of our galaxy in the plane of the spiral arms. M67's orbit is highly inclined, so the cluster spends most of its time far north or south of the galactic plane, out in the halo where there's a lot less stuff to gravitationally perturb the cluster. Thus M67 has been able to hold onto its stars for billions of years. This makes it an excellent natural laboratory for studying stellar evolution.

M67 is also a lovely object to observe through a small telescope. In the northern hemisphere during the Spring, while you're waiting for Hercules to rise, Cancer will be high in the south-southwest.

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John Bochanski

May 16, 2014 at 10:50 am

Anthony is correct. While most open clusters dissolve after a few hundred million years, a few have persisted for a very long time, which make them important age benchmarks (since it is easier to estimate the age of a star in a cluster than one in the field). Thank you for your great question!

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May 14, 2014 at 4:11 pm

Messier 67 is an odd ball amongst the open clusters of the Milky Way. Its age is estimated to be between 3 and 5 billion years old. Why this open cluster has persisted when others as you mentioned fall apart due to gravitational interactions of it members is not well understood. NGC 188 in Cepheus and NGC 6791 in Lyra are the two oldest open clusters I know of. They are 5 billion and 7 billion years old respectively.

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Tom Hoffelder

May 14, 2014 at 6:21 pm

Wow, thanks! So M67 is 4.5 billion years old! Amazing, totally amazing!!! I have seen M67 many times, as I have done 20 Messier Marathons, and seen it during every one of them. I have also observed it, as opposed to "seen," on a number of occasions in our C14.

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May 16, 2014 at 6:26 pm

This article is purely speculative, and full of personal theories based on preconceived/learned biases.

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May 18, 2014 at 8:37 am

Back to 1980s, I was pleased to help Dr. Junqueira here in Brazil who developed a revolutionary vision of the Origin of our Solar System. That was a major breakthrough of the theories at that moment - and still is. Basically Dr. Junqueira realized that we can understand our Solar System easily if we assume that the primordial cloud started with 2 stars and not only one. The Lagrangian Points and the hyperbolic orbit between these 2 stars generated the appropriate accretion of matter that ignites the stars and create planets and moons. More amazing, we were able at that moment to simulate the dynamics of it, creating a Solar System with the same family of planets that we know today. And we also predicted what observations would discover after, like for instance the small and awkward orbit of exterior planets. That's amazing how nobody gave the appropriate credit to Dr. Junqueira whilst more and more evidences of its theory appears. This article is just another example of how close we are coming to realize that our Solar System was a binary system and that is a very natural genesis of solar systems and planets in the Universe. Dr. Junqueira passed away and we have been keeping his legacy alive at: (in Portuguese)

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