Finding the Sun’s Lost Siblings

　　Our sun once had a group of brothers and sisters who were born in the same nebula and spent a brief childhood together, but some of them got lost. Now they are in the vast sea of ​​stars, and their whereabouts are unknown.
　　But just as DNA testing can uncover unknown members of the human family, astronomers think it may be possible to find the sun’s lost siblings by examining the stars, and they may have found some clues.
stars are born in nebulae

　　Although the sun was born billions of years ago, we can get a rough idea of ​​how this process happened by studying the nebula we see today.
　　We know that stars like the sun are born in nebulae. The nebula is composed of interstellar gas, mainly hydrogen and helium, with trace amounts of other elements. Many nebulae are inert, have no stars forming in them, and only give away their presence because they block light from more distant stars—in fact, the Milky Way would be brighter in the night sky if they weren’t there. Only those nebulae with stars in them glow. For example, the Large and Small Magellanic Clouds are visible to the naked eye all year round in the southern hemisphere, and the Orion Nebula is visible to the naked eye in the northern hemisphere in winter.

Hundreds of millions of years later, the Big Dipper’s “Seven Sisters Family” will no longer exist.

The Orion Nebula contains more than 2,700 stars in various stages of formation.

　　However, just as water vapor needs to have condensation nuclei in order to turn into small liquid droplets at high altitudes, there must be a condition for the birth of stars in nebulae: disturbance. So, millions of years before the sun formed, something must have disturbed the nebula that eventually gave birth to the sun. So who is playing this role? Astronomers thought it was a supernova.
　　After the shock wave from the supernova explosion passed through the nebula, the dust and gas began to collapse due to gravity; more and more matter gathered together to form a dense core called a protostar (which would form the sun), and A protoplanetary disk (which will eventually form the rest of the solar system).
stars tend to be born in groups

　　However, a disturbance can act in many places in a nebula, so a large group of stars are often born in the same nebula. This has also been confirmed by observation.
　　For example, astronomers have carefully studied several nebulae where stars are being formed. One is the Orion Nebula (M42). Although the Orion Nebula is more than 1,300 light-years away, there are many stars forming there. Astronomers have observed about 2,700 stars in various stages of formation in it.
　　Despite the high activity in “stellar nurseries” like the Orion Nebula, we don’t expect this phase to last long. Shock waves from the formation of massive stars blow away the remaining gas in the nebula. Simulations estimate that in 100,000 years, the Orion Nebula will be cleared of gas.
　　After the dull gas had been blown away, a young cluster of stars glowing brilliantly remained, like a handful of pearls washed away from mud. An example of this is the Pleiades star cluster, visible to the naked eye. Located in the constellation Taurus, the star cluster is mainly composed of young stars formed within the past 100,000 years.
　　From this process, it can be seen that a star does not start its career in isolation. Our sun should be no exception.
How did stellar families fall apart?

　　But if the sun once had thousands of siblings, what about the rest of the stars? In the vast sea of ​​stars, can we still recognize these long-lost members?
　　The separation would start like this: The sun and its siblings were born as a star cluster. But almost from the moment they were born, they were pulled in different directions by other stars in the Milky Way and began to drift away. Within a few hundred million years, the tight cluster will become a loose mass, which will become more and more fragmented over time. Ultimately, you won’t know that these stars were once members of the same cluster.
　　A perfect example of this process is found in our night sky – the Big Dipper. In the Big Dipper, astronomers found that six of the eight stars (one of which is a binary star) are moving in the same direction. Over time, they will become farther and farther apart from the other 2. Hundreds of millions of years later, this “seven sisters family” will cease to exist.
　　On longer timescales, stars are pulled apart not only by nearby stars, but also by the rotation of the Milky Way itself. The Milky Way is shaped like a disk with spiral arms and a bar-shaped center around which all the stars orbit. But we need to know that the celestial bodies in the galaxy are not like a point on a solid plate, they cannot move, they have relative freedom. When the “plates” are turned, they also move back and forth, side to side. Our sun has a roughly circular orbit, and it takes about 220 million years to go around the center of the galaxy. The sun is 4.5 billion years old, so it has orbited the center of the galaxy about 20 times. For stars that have been drifting away, this time is enough to make them distant.
　　So, neighboring stars in space distance are not necessarily siblings. Conversely, members of a star cluster will eventually be dispersed over a large area, and may even appear on the other side of the Milky Way.
Find method one:
Check trace element abundance

　　However, since they come from the same family, members must have common characteristics. For example, you and your brothers and sisters both have the genes of your parents, and even retain the same living habits. Stars are similar to people in this respect. Although the composition of stars is mainly hydrogen and helium, other trace elements (elements other than hydrogen and helium) from the proto-nebula are also present. The abundance of these trace elements is different for different nebulae, but it is the same for stars born in the same nebula. So, if you take the spectra of the stars in the Orion Nebula (or the Pleiades) you’ll find that the trace element abundances of these stars are consistent.
　　We now have detailed spectra of hundreds of thousands of stars in the Milky Way, so astronomers can compare individual star-by-star trace element abundances with those of the sun. Using this method, astronomers have identified some possible siblings of the sun.
　　Still, some astronomers are skeptical, fearing that the differences between nebulae are so small that examining the abundance of trace elements alone is not enough to identify the closeness of stars.
Look for method two:
Restoration of stellar trajectory

　　So, the second way to solve this problem comes into play. In 2013 the European Space Agency launched the Gaia space observatory to map the precise positions of the 1.3 billion stars in our galaxy. In 2018, Gaia released the most accurate 3D map of the Milky Way ever created.
　　In addition to describing the position of stars, Gaia can also monitor a star for many years and distinguish the movement of the star; from its current position and movement, astronomers can infer its past movement trajectory.
　　In this way, with the Gaia data, astronomers can trace the trajectory of each star that may be a “sibling” of the sun to see if it intersects with the trajectory of the sun when searching. If they are from the same family, then naturally there should be a common beginning.
　　Using this approach, astronomers ruled out many stars that are spatially close to the sun, as well as stars with similar trace element abundances to the sun. For example, astronomers have speculated for years that the open star cluster M67 in the constellation of Cancer might be the sun’s “mother” because its 100 stars have similar trace element abundances and are roughly the same age as the sun. But simulation studies show that the past trajectory of the open star cluster M67 is unlikely to intersect with the sun.
　　However, astronomers have discovered a new candidate that not only has a similar abundance of trace elements to that of the sun, but also has a trajectory that overlaps with that of the sun. It was called SS1, which means “Sibling Sun 1”. SS1 is located about 1100 light-years away in the constellation Cygnus. You would never have guessed that this humble star might be a sibling of the Sun unless you took the clues that modern astronomy provides.
　　Of course, we can’t make a conclusion yet. But as the accuracy of Gaia’s data improves, it should have no problem finding the sun’s lost siblings by then.
　　Searching for the “siblings” of the sun is of great significance to our understanding of the structure and evolution of the Milky Way.