What is a Barred Spiral Galaxy?
A barred spiral galaxy is a spiral with a straight bar of stars running across its centre, with the arms trailing from the ends of the bar rather than from the nucleus. About two thirds of spirals have one — and the Milky Way is among them, a fact only firmly established in the 1990s and 2000s.
A bar is not something added to a disc; it is something a disc does. A rotating, self-gravitating disc of stars is dynamically unstable to exactly this pattern, and left alone it will grow one. What makes it a bar rather than a smear is that the stellar orbits in the inner region stop precessing and lock into alignment with each other — every ellipse pointing the same way, so their union is elongated. The model does precisely that: the ellipse orientation advances with radius outside the bar, producing arms, and is frozen inside it, producing the bar. One rule, two structures.
Bars matter because they move gas. A rotating bar is not axisymmetric, so it torques the gas in the disc, robbing it of angular momentum and funnelling it inward toward the centre. That drives bursts of star formation in the nuclear region and can feed the central supermassive black hole. Bars are, in effect, the galaxy's own plumbing, and they can dissolve and re-form over its lifetime.
Working out that we live in one was awkward, because we are inside it. Nobody can photograph the Milky Way from outside; its shape has been reconstructed from the motions and distances of stars and gas measured from a single point 26,000 light years off centre. Infrared star counts in the 1990s and later surveys settled it: there is a bar roughly 27,000 light years long, tilted about 25 degrees to the line joining the Sun to the centre.