Travels with the Sun
When three astronomers discovered a small Sun-orbiting object far beyond the orbit of Pluto in 2003, they called it Sedna, for an Inuit goddess living at the bottom of the Arctic Ocean.
A very fitting name, it turns out, for Sedna has since been telling tales of visits to the deep: to the inner reaches of the Milky Way galaxy. Her ride: the whole Solar System, according to an article in this month’s issue of the journal of planetary science Icarus.
The travels of Sedna around the Sun, and of the Sun around the centre of the galaxy cosmos, can be combined nicely to solve a pair of puzzles in celestial dynamics. For Sedna itself, the riddle is the shape of her orbit. When the Solar System was formed, a dusty disk around the young sun clumped into planets, going around her in roughly circular orbits.
Sedna is presumably one of them, a dwarf planet cosmos, but her orbit is elongated. This suggests that one of the large planets, probably Neptune, once gave her a gravitational push.
The physics of such encounters predicts that the resulting orbit should bring Sedna regularly near Neptune again. But calculations show that she will give the god of the seas a wide berth forever. That means some heavenly body must have pushed her around some more. And rather than some planet, another star is generally thought to be the most likely culprit.
To this picture, now add another emerging view of the Sun’s past and present environment: that she may not always have followed the same path within the disk of the Milky Way galaxy. In this disk, most of its stars are born, after which they are supposed to go around the center of gravity of the whole thing in roughly circular orbits.
For years, cosmos astronomers have struggled with a contradiction here. If stars at a certain distance of the centre of the galaxy always stay at this distance, this ring of stars forms a distinct family. The stars convert hydrogen and helium into heavier elements – ‘metals’ in astronomical parlance. Because they eventually explode, the mix of gas from which each next generation of stars is made becomes steadily more ‘metal-rich’.
Analysis of the colours of starlight will tell you how ‘metallic’ a star is. And for the Sun’s local neighbourhood this is where the puzzle appears: old stars should have relatively low metallicity, because they formed from virgin interstellar material. And young stars should appear to be formed from older gas that contains more star-leftovers and is thus more ‘metallic’. But the actual picture is far more jumbled.
One explanation could be that stellar mixing occurs: stars might move inward or outward from their original level into regions of the galaxy where the metallicity of stars and gas is different, with a different history, thus spoiling the tidy picture the astronomers expected to see.
This migration in the radial direction could be accomplished by close encounters between stars or by the passage of one of the galaxy’s spiral arms – in fact a wave that ripples through the stars, temporarily packing them closer together like a sound wave does with air molecules.
Until recently, that solution, attractive as it is, had a snag of its own: stars that were thrown off course in that way should have elongated orbits – just like Sedna’s is around the Sun. Elongated stellar orbits do exist, but they are not common enough to provide all the stellar mixing that seems to be going on.
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