The speed of a star as it circles the central region of a galaxy is usually not the same as that of the spiral pattern. This is because the orbital speed of the gas molecules, dust grains and stars is relative to their distance from the middle of the galaxy- material in the outskirts travels much slower than the matter situated near the center. If this were not true then, over time, the spiral pattern would slowly tighten upon itself and eventually disappear. However, this does not match the measurements that have been made of spiral structures. Instead, the spiral pattern is the physical manifestation of gravitational density waves that wind outward from the inner region. These waves travel at a different speed than the stars and other elements that make up a galaxy.
When a density wave encounters molecules of gas and granules of dust, it herds and compresses them into vast, dense clouds as it ripples past. These billows can become exceptionally enormous and often span hundreds of light-years. They are incubators that spawn hot, bright blue-white stars- these then illuminate the wave and make it visible. Because these stars are massive, they have relatively short lives- most ignite and burn through their nuclear fuel before completing a single revolution around the galaxy. In fact, their stellar existence is often limited to as little as five percent of their first orbit. As a result, the trailing edge of the wave appears darker because the stars have already been extinguished when the wave fully moves beyond their location. Therefore, even though stars do not remain in a fixed location, they also do not follow the arms. The arms simply appear to pass through the stars as they travel in their orbits.
For example, the Sun has been in and out of a spiral arm during the roughly twenty times it has orbited the Milky Way's center. Even though there is no sensation of the Sun's velocity, it is traveling at about half a million miles per hour through our Galaxy with the solar system, and Earth, in tow. On average, the Sun and its planets spend about forty million years inside a spiral arm and approximately eighty million years on the outside. About ten million years ago, we emerged from our last encounter with a spiral arm.
The source of the density waves is still being investigated- some astronomers believe they arise from the gravitational tug of nearby galaxies or are given a push from supernova explosions while other researchers are now pointing their fingers to the newly discovered presence of a super massive black holes at the heart of most galaxies. Most likely, it is due to all of these and other, yet, unrecognized reasons.
About two-thirds of all spiral galaxies have a river of gas, dust and stars running through their inner most area that bridges the spiral arms located on either side. These are called barred spiral galaxies and even our Milky Way is now thought to possess one. The bar is also thought to be caused by a density wave- one that extends laterally from the galaxy's center. At first, the wave changes the orbits of interior stars but, over time, it affects stars farther out. As more time passes, it enlarges and creates this unusual, bright structure. Bars cause the motions of interstellar dust clouds and a large number of stars within the galaxy to become chaotic. Collisions between the clouds become frequent and violent and this causes the clouds to move all over the galaxy's disk thus radially redistributing the gas and dust within an Island Universe. In particular, a large quantity of gas will fall along the bar and eventually reach the galaxy's central region. This is why it is believed that bars could be an important source of material that feeds the supermassive black hole found at the heart of most galaxies.
This new picture is of a barred spiral galaxy located about 25 million light-years away in the direction of the constellation Leo. It is only known as NGC 2903 because Charles Messier overlooked it when preparing his list of bright, comet-like objects- this one is bright enough for him to have spotted. It shines at approximately magnitude 10, thus making it an easy target to glimpse with a modest telescope under a relatively dark sky. The width of the galaxy is roughly 80,000 light years from edge to edge and this makes it slightly smaller than our own Galaxy.
Exceptional atmospheric conditions enabled the exposure of unusually clear images used to construct this picture. Looking through a curtain of stars that are actually in our own Galaxy, this new image shows amazing, cataclysmic activity as we peer down into the bar and across the disc of this much more distant galaxy! NGC 2903 has been known to contain numerous hot spots of violent stellar activity and this image clearly shows the results of that description: spectacular jets, looping and arching dust lanes and huge winding partial rings of gas and dust clouds rising up from the galaxy's plane. In short, this galaxy is experiencing multiple, simultaneous, explosive events on an unimaginable scale! This galaxy has been described as being similar to our own, but I believe this picture casts some doubts on taking that analogy too literally.