The lights of a galactic bar

This week, an image of the broad and sweeping spiral galaxy NGC 4731 is the Hubble Picture of the Week. This galaxy lies among the galaxies of the Virgo cluster, in the constellation Virgo, and is located 43 million light-years from Earth. This highly detailed image was created using six different filters. The abundance of colour illustrates the galaxy's billowing clouds of gas, dark dust bands, bright pink star-forming regions and, most obviously, the long, glowing bar with trailing arms.

Barred spiral galaxies outnumber both regular spirals and elliptical galaxies put together, numbering around 60% of all galaxies. The visible bar structure is a result of orbits of stars and gas in the galaxy lining up, forming a dense region that individual stars move in and out of over time. This is the same process that maintains a galaxy's spiral arms, but it is somewhat more mysterious for bars: spiral galaxies seem to form bars in their centres as they mature, accounting for the large number of bars we see today, but can also lose them later on as the accumulated mass along the bar grows unstable. The orbital patterns and the gravitational interactions within a galaxy that sustain the bar also transport matter and energy into it, fuelling star formation. Indeed, the observing programme studying NGC 4731 seeks to investigate this flow of matter in galaxies.

Beyond the bar, the spiral arms of NGC 4731 stretch out far past the confines of this close-in Hubble view. The galaxy’s elongated arms are thought to result from gravitational interactions with other, nearby galaxies in the Virgo cluster.

[Image Description: A close-in view of a barred spiral galaxy. The bright, glowing bar crosses the centre of the galaxy, with spiral arms curving away from its ends and continuing out of view. It’s surrounded by bright patches of light where stars are forming, as well as dark lines of dust. The galaxy’s clouds of gas spread out from the arms and bar, giving way to a dark background with some foreground stars and small, distant galaxies.]