Milky way how many spiral arms

Orion Spur: The Sun is located in the Orion Spur, which is a minor spiral arm located between two other arms. In this diagram, the white lines point to some other noteworthy objects that share this feature of the Milky Way Galaxy with the Sun.

Instead, the way individual objects turn around the center of the Galaxy is more like the solar system. Objects farther from the center take longer to complete an orbit around the Galaxy than do those closer to the center.

In other words, stars and interstellar matter in larger orbits in the Galaxy trail behind those in smaller ones. This effect is called differential galactic rotation. Differential rotation would appear to explain why so much of the material in the disk of the Milky Way is concentrated into elongated features that resemble spiral arms.

No matter what the original distribution of the material might be, the differential rotation of the Galaxy can stretch it out into spiral features.

Figure 3 shows the development of spiral arms from two irregular blobs of interstellar matter. Notice that as the portions of the blobs closest to the galactic center move faster, those farther out trail behind. Figure 3. Simplified Model for the Formation of Spiral Arms: This sketch shows how spiral arms might form from irregular clouds of interstellar material stretched out by the different rotation rates throughout the Galaxy.

The regions farthest from the galactic center take longer to complete their orbits and thus lag behind the inner regions. If this were the only mechanism for creating spiral arms, then over time the spiral arms would completely wind up and disappear.

Since many galaxies have spiral arms, they must be long-lived, and there must be other processes at work to maintain them. But this picture of spiral arms presents astronomers with an immediate problem. But did the Milky Way actually have spiral arms when it formed 13 billion years ago? Their observations focused on clouds of gas in the Galaxy in which new stars are born, revealing four major spiral arms. They revealed about million stars, but only evidence of two spiral arms.

The astronomers behind the new study used several radio telescopes to individually observe about 1, massive stars in the Galaxy. The distances and luminosities of these stars were calculated, revealing a distribution across four spiral arms. Massive stars are much less common than their lower mass counterparts because they only live for a short time — about 10 million years.

The shorter lifetimes of massive stars means that they are only found in the arms in which they formed, which could explain the discrepancy in the number of galactic arms that different research teams have claimed.

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.