The stellar superhighway in the Milky Way

Conventional wisdom suggests that, like planets round the Sun, stars
follow approximately circular orbits which cross the spiral arms, and
that the Sun presently lies in a spur rather than a major spiral arm.

A new study published in Monthly Notices of the Royal Astronomical
Society (MNRAS) shows that contrary to this belief, nearly half of
nearby stars, including the Sun, have orbits aligned with the Orion
arm, one of two major spiral arms.

Of the remainder, one third have orbits aligned with the Centaurus
arm, and are currently crossing the Orion arm on their way outwards to
rejoining the Centaurus arm later in their orbit. Another third have
more eccentric orbits straddling both arms.

According to this research spiral arms are highways for the stars.
Stars join the arms in the furthest part of their orbit from the
Galactic centre, attracted by the gravity of the arm. They then follow
the arm inwards, picking up speed as they go, analogous to marbles in
a marble run. Eventually they pick up so much speed that they break
free of the gravity of the arm, cross outwards over the other arm and
rejoin their original arm to repeat a similar orbit.

The research demonstrates that the Milky Way is a more tightly wound,
more extensive, and more regular, two armed spiral than has been
thought. The researchers have traced the spiral pattern in the
distribution of stars found by the Two Micron All-Sky Survey (2MASS),
finding that it extends inwards to about 2.1 kpc from the Galactic
centre -- the length of the short bar, or 'bulge', originally
observed by Cosmic Background Explorer satellite (COBE) in the
1990s. This is about half the length of the so called 'long
bar', seen now as an artefact due to increased star formation in
the spiral arms near the ends of the short bar.

The paper is available at
http://onlinelibrary.wiley.com/doi/1...693.x/abstract
http://arxiv.org/abs/1202.1375

[Mod. note: non-ASCII characters removed. Please post in plain 7-bit
ASCII only -- mjh]