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Why the Sun feels the force of the galactic bar
Posted: 07 January 2010

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Our Sun has wandered into the outskirts of the Milky Way thanks to the powerful interaction of our Galaxy’s spiral disc with the bar that runs through its centre, say a pair of astronomers from Strasbourg University.

An artist’s impression of our Galaxy, with the bar running through the centre. Image: NASA/JPL–Caltech/R Hurt (SSC–Caltech).

Our Galaxy is what is known as a barred spiral; that is, a spiral galaxy with a 27,000-light year long stream, or bar, of stars that crosses the galactic centre. This bar funnels gas and stars from one side of the Galaxy to the other, and very occasionally into the supermassive black hole at the core of the Galaxy (see related news story here). However, Ivan Minchev and Benoit Famaey have discovered that the bar can also have an effect on stars in the disc, including our Sun, mixing them all up in a process called radial migration. Astronomers already knew about this process – our Sun has a chemical composition suggesting that it formed thousands of light years closer to the galactic centre – but until now they were never clear as to why it was happening.

Minchev and Famaey put it all down to resonances between the bar and the disc. At certain distances from the galactic centre, the period of radial oscillation (how close or far away from the centre of the galaxy a star moves during its orbit) matches the rotation frequency of the bar and spiral disc. Consequently, stars in these resonant orbits feel an extra gravitational push that sends them migrating through the spiral arms.

Corroborating this idea of radial migration are observations of other galaxies with bars, which all display a smooth distribution of stellar chemical compositions throughout their spiral arms, implying radial migration is at work in these galaxies too. The most important consequence for our understanding of galactic evolution, say Minchev and Famaey, is that we must appreciate that the bar and the spiral disc are more tightly linked than they first appear.