OUR sun is a lonely star. Its light takes more than four years to travel through space to its nearest neighbour, Alpha Centauri. Yet this wasn’t always the case. Turn the clock back 4.6 billion years and the same cloud of gas and dust that formed the sun also gave birth to a bunch of other stars: the sun’s siblings.
The interstellar cloud that spawned the sun has long since vanished, but most of the stars born with it must still shine somewhere in the Milky Way. Now astronomers in Australia are gearing up to search the skies for them. “I’m utterly fascinated by anything historical,” says Joss Bland-Hawthorn, a member of the search team at the University of Sydney, New South Wales. “I would love to know whether we can identify stars that were born with our sun.” At stake is one of the key questions of our existence: how did the solar system produce a planet like ours?
Evidence that the sun was once part of a brood comes from the rubble leftover from the formation of the solar system. These primordial meteorites contain compounds that can only have formed from the decay of radioactive isotopes produced when a star explodes in a supernova. The abundance of such compounds tells us that the sun grew up less than 1 light year away from a massive star that exploded.
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It gives other information, too. Astronomers have observed that stars form either in loose groups of a few dozen similar in mass to the sun and smaller, or in large clusters containing hundreds or thousands of stars spanning a wide range of masses. That meteorites contain the remains of a massive star therefore points to the sun’s nursery being a large star cluster.
In 2010, Fred Adams at the University of Michigan in Ann Arbor reviewed all the properties that constrain the solar system’s history and concluded that the sun’s star cluster probably had at least 1000 members ().
Sibling-hunters face a daunting task, though. The sun’s brothers and sisters are a tiny minority in a galaxy with hundreds of billions of stars. And history has not been kind to them. Once a star cluster is born, it emerges into a galaxy that tries to tear it to shreds. Ironically, one of the first threats comes from the same giant clouds of gas and dust that create stars. The largest such clouds are millions of times more massive than the sun, and their gravity can hurl stars into the galaxy at large.
Exploding stars only add to the problem. A cluster is held together by gravity, so when an exploding star catapults material into space, the loss of mass weakens the cluster’s grip on its members. That leaves it vulnerable to the galaxy’s gravity, which pulls hardest on the stars closest to the galactic centre. Passing stars also tug at the cluster, their pull luring individual stars away. Finally, interactions among cluster stars can eject the lightest ones.
After 4.6 billion years, the forces of galactic erosion have abraded the sun’s birth cluster and scrambled its stars. They could be anywhere – including on the other side of the galaxy. Throw a drop of red dye into the ocean and look for the colour an hour later, and you have an idea of the size of the challenge.
The good news is that not all of the sun’s siblings might be so far away. In 2009, Simon Portegies Zwart of Leiden University in the Netherlands carried out computer simulations of large star clusters to see where the sun’s siblings might have ended up. He estimates that between 10 and 60 of them still reside within just 330 light years of us (). From that distance, a star like the sun is visible through binoculars.
“Up to 60 of the sun’s siblings could reside close enough to us to be visible through binoculars”
Not everyone is convinced by this tally. Yury Mishurov and Irina Acharova of the Southern Federal University in Rostov-on-Don, Russia, believe it is too optimistic. In 2010, they argued that gravitational interactions with the Milky Way’s spiral arms can also scatter a cluster’s stars. Their own simulations suggest that just a few siblings might remain nearby ().
To find them, Bland-Hawthorn and his colleagues are pinning their hopes on a new instrument, called . Attached to the Anglo-Australian Telescope at Siding Spring Observatory near Coonabarabran, New South Wales, HERMES will look at the chemical make-up of more than a million stars up to 20,000 light years away.
Starting next year, HERMES will measure the abundance of 25 chemical elements in every star it looks at, each of which tells a story about its star’s origins. By using so many chemical elements, Bland-Hawthorn and his colleagues hope to find stars with a chemical make-up matching the sun’s. “I wish I could tell you that it’s going to be easy,” says Bland-Hawthorn. “It’s going to be a 10-year mission to try to crack this.”
Even the results from HERMES may not be enough. According to the laws of motion, the sun’s brothers and sisters should follow a similar course around the galaxy as the sun, despite galactic forces having tossed the stars around so much. To identify a true sibling, researchers will need to combine the chemical information about stars with measurements of their velocity made by the European Space Agency’s spacecraft, which is set to launch next year.
Before then, a smaller survey called at the Apache Point Observatory in New Mexico might luck out. The project’s goal is to decipher the Milky Way’s history by examining 100,000 stars. It will measure the abundances of 15 chemical elements and although it is not looking explicitly for the sun’s siblings, “it’s very possible that these stars will be in the sample”, says of the University of Virginia in Charlottesville.
One advantage of APOGEE is that 10 per cent of the stars it surveys are shared by spacecraft, whose observations of flickering starlight will tell us the ages of these stars. This is crucial, says Majewski, because any solar sibling must be the same age as the sun, as well as share its chemical composition and motion.
Finding them could help explain how a planet like ours came into existence. Perhaps a massive star in the sun’s cluster exploded at just the right time and in just the right place to produce the conditions necessary to build a life-supporting planet. Other stars in the sun’s cluster may have helped too, their radiation and gravitational tugs sculpting the nascent solar system.
The sun may have returned the favour. If its cluster had a thousand stars, then about 40 should be similar in size, temperature and luminosity to the sun. Perhaps they have planets supporting intelligent life, too. Residents of a distant planet may have already recognised their kinship with our sun, whose light and gravity could have helped their solar system prosper 4.6 billion years ago.
