One explanation centers on the fact that gravitational systems act to increase their central binding energy. Spiral arms remove angular momentum from the center of the galaxy, allowing it to achieve a state of higher binding energy. There are two main versions of the theory of spiraling: one in which the waves are steady and long-lived, the other in which spirals are transient features that come and go. The natural, but not very easy, test is to observe spiral galaxies for a few hundred million years and see what happens.
Elmegreen, staff scientist at the IBM T. Watson Research Center, have extensively studied this question. Here is their response: "Most spiral arms in galaxies are density waves, which are compression waves like sound that travel through the disk and cause a piling-up of stars and gas at the crest. The wave is temporarily sustained by the force of its own gravity, but it eventually wraps up or gets absorbed at orbital resonances, places where random stellar oscillations have the same period as the local wave.
In all cases, the stars and gas rotate around the galaxy's center faster than the wave in the inner parts of the disk, and slower than the wave in the outer parts. This differential rotation forces gas to enter the wave at a high speed in the inner regions, causing it to shock and form long, thin dust lanes in each spiral arm. Some density-wave galaxies, like M81, have highly symmetric spiral arms; others, like M, have several arms and less overall symmetry.
The difference between these two cases is related to the symmetry of the perturbation that formed the arms in the first place, and to the relative importance of the standing wave pattern, which tends to be symmetric. A large central bar, such as is seen in NGC , may drive a two-arm density wave for a relatively long time, eventually causing the gas in the outer disk to move outward and wrap into a giant ring at the edge of the galaxy's disk.
A companion galaxy can also generate a two-arm spiral by tidal forces. Such tidal arms probably last only for several rotations before they either wrap up and disappear or initiate a longer-lived standing wave. The Whirlpool galaxy, M51, has companion-triggered spirals. Galaxies that appear in visible light to have neither bars nor companions can still have spiral waves. These galaxies may have hidden weak bars or small companions that trigger the spirals, or they may be excited entirely by small asymmetries and perturbations within their disks.
These arms are probably not density waves at all, but are short-lived star-forming regions that are sheared into spiral-like pieces by differential rotation of the galaxy. Such star-formation features last only as long as the bright, high-mass stars that dominate their light--about a hundred million years, less than a single rotation period of the galaxy.
So, in the time it takes an inner star to complete one revolution around its galaxy, an outer star might have only finished half a revolution. Spiral arms show the same structure whether composed of billion-year-old stars or million-year-old stars. This indicates that the arms are the result of a persistent pattern of stars rather than particular stars causing the structures.
That pattern is caused by a density pressure wave that spirals from the edge of the disk to the center and back out again, creating the visible spiral arms of the galaxy. Essentially, as stars and gas move through the pattern, they bunch up in the wave crests, like a stellar traffic jam, and then eventually break past the crest and continue on their orbit.
Both planetary rings and protoplanetary disks can have density waves and spiral structure. Planetary rings are made of small amounts of debris trapped in a particular orbit. Sometimes they are perturbed by moons that cause waves. According to computer simulations, observed spirals in protostellar disks are from density waves driven by planets forming in the disk. Receive news, sky-event information, observing tips, and more from Astronomy's weekly email newsletter.
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Spiral galaxies come in a wide range of sizes, from 5 to kiloparsecs across, have masses between 10 9 and 10 12 solar masses , and luminosities ranging from 10 8 to 10 11 time that of the Sun. The majority of spiral galaxies rotate in the sense that the arms trail the direction of the spin.
By measuring the rotation curves of spiral galaxies, we find that the orbital speed of material in the disk does not fall off as expected if most of the mass is concentrated near the centre. From this it is clear that the visible portion of spiral galaxies contains only a small fraction of the total mass of the galaxy, and that spiral galaxies are surrounded by an extensive halo consisting mostly of dark matter.
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