Our galaxy's central bar may be a phase that comes and goes
The Milky Way has a bar of stars across its middle, like about two thirds of nearby spiral galaxies. Such bars appear to be temporary. Simulations suggest they grow, buckle under their own weight and fade, and that a spiral may cycle between barred and unbarred forms over roughly two billion years.
A bar is thought to start with a density wave spreading out from a galaxy's centre, bending the orbits of inner stars. The effect gradually reaches stars farther out, and the elongated pattern begins to sustain itself. Once in place, it works like a funnel, steering gas inward from the spiral arms through orbital resonances and feeding bursts of star birth near the core. That inflow may also explain why so many barred spirals, such as the Southern Pinwheel Galaxy, host active galactic nuclei.
Bars cannot grow forever. Past a certain size, the mass piled into them undermines their stability, and simulations show many undergoing a buckling event in which the bar collapses inward, becoming thicker and shorter. Exactly how that instability works is still hotly argued. A supermassive black hole at the centre delays and softens buckling but does not prevent it, and galaxies with heavy cores tend to have short, stubby bars.
Timing tells a story too. A 2008 study found that only about 20 percent of spirals in the distant past had bars, compared with around 65 percent today, supporting the idea that a bar marks a galaxy leaving its formative years behind. Models had not expected early galaxies to settle down enough to form bars, though recent observations have turned up many spirals in the young universe.
Edwin Hubble gave these galaxies the label SB, for spiral barred, and graded them by their arms, from tightly wound SBa through SBb to loose SBc. Later an SBm class was added for ragged barred spirals such as the Magellanic Clouds, once dismissed as merely irregular.
Source: Barred spiral galaxy
