Searching for Nemesis

The Sun's Long-Lost Twin

Bok globule
This is a Bok globule, a place where stars begin to form.Astronomers think that newborn stars inside nebulae such as these might form in pairs and multiple systems and then separate after birth. Wikimedia Commons

Astronomers surveying distant stellar birth clouds in other galaxies think that most stars are born in pairs. This means that the Sun could have had a twin sibling born at the same time some 4.5 billion years ago.If so, where is that star?

Looking for Nemesis

Astronomers have long searched for the Sun's twin—which has been nicknamed Nemesis, but so far haven't found it among the nearby stars. The nickname comes from a theory that a passing star perturbed an asteroid into a collision course with Earth. When it hit, it supposedly contributed to the death of the dinosaurs some 65 million years ago. 

Astronomers study distant clouds where star formation takes place, including the Orion Nebula star birth region. In some cases, they look at these stellar nurseries using radio telescopes which can peer into these créches and make out more than one star in a birthplace. Sometimes these stars are spaced pretty far apart, but they're clearly in orbit with each other around a common center of gravity. Such stellar pairs are called "binaries." After the star birth process is done, some binaries break apart and each star wanders off into the galaxy.

The Sun's Possible Twin

Astronomers who study how stars are born and evolve made a computer model to see if a star like our Sun could have had a twin at one time in the distant past. They know that the Sun formed in a cloud of gas and dust and that the birth process likely was begun when a nearby star exploded as a supernova or perhaps a passing star stirred up the cloud. That got the cloud "stirred up" and moving, which eventually led to the formation of young stellar objects. How many were formed is an open question. But, it's like that at least two were, and maybe more.

The search for understanding the Sun's formation with a twin is part of studies that astronomers are doing to figure out how binary and multiple star systems form in their birth clouds. There has to be enough material to form multiple stars, and most young stars are created inside egg-shaped cocoons called "dense cores." These cores are scattered throughout clouds of gas and dust, which are made of cold molecular hydrogen. Although regular telescopes can't see "through" those clouds, the young stellar objects and the clouds themselves do emit radio waves, and those can be detected by radio telescopes such as the Very Large Array in New Mexico or the Atacama Large Millimeter Array in Chile. At least one other star birth region has been observed this way. At least one cloud, called the Perseus Molecular Cloud, appears to have multiple dense cores containing binaries and multiple star systems all being born. Some of them are widely separated but still orbiting together. In the future, those systems will break apart, and the stars will wander off.

So, yes, it's entirely possible that a twin to the Sun formed along with it. Chances are very good that the Sun and its twin formed fairly far apart, but close enough to be bound together by gravity, at least for a while. The "Nemesis" star was quite far away—probably about 17 times the distance between Earth and Neptune. So, it's not surprising that the two young stars separated not long after birth. Nemesis could be halfway across the galaxy by now, never to be seen again.

Starbirth is a complicated process that astronomers are still working to understand. They know stars are born in our galaxy (and in many others), but the actual birth is hidden from view behind clouds of gas and dust. As the young stars in a créche grow and start to shine, they gobble up the birth cloud and their strong ultraviolet light destroys what's left. The stars then travel through the galaxy, and can lose gravitational "touch" with each other after a few million years.

What if we Could Find Nemesis?

About the only way to tell Nemesis from any other star in the galaxy would be to look at its chemical composition and see if it has the same ratios of chemical elements that the Sun does. All stars have a lot of hydrogen, so that wouldn't necessarily tell us anything about a possible sibling. But, many stars born in the same birth cloud can have very similar amounts of trace elements heavier than hydrogen. These are called "metal" elements.

So, for example, astronomers could take a census of the Sun's trace elements and compare its metallicity with other stars to see if any are a close match. Of course, it would help to know in which direction in the galaxy to look for those stars. By now, Nemesis could be in any direction, since it's not clear which direction it went. Whether or not Nemesis is actually found, studying regions of starbirth for other binaries and triples that are gravitationally bound will tell astronomers more about our own Sun and its early history.