The latest picture of the Milky Way Structure

Initially, the Milky Way is depicted as a spiral structure with four arms are composed of stars, each is the arm of Norma, Scutum-Centaurus, Sagittarius and Perseus. There were also bands of gas and dust in the central regions of galaxies. Our sun is situated on a small arm called the Orion arm, which is located between the Sagittarius and Perseus arms.

Model based on radio observations of the 1950s gas-gas in this galaxy has survived until the revision of 1990. Based on results from the Large infrared sky surveys, found the existence of a large band consisting of stars in the Milky Way. Infrared rays can penetrate through dust, and thus the telescope designed to collect infrared light can be seen more clearly into the galactic center filled with dust and various objects.
Later, in 2005, astronomers began using infrared detectors on Spitzer space telescope to obtain more detailed information about these bands. A group of astronomers led by Robert Benjamin of the University of Wisconsin found that the ribbon that stretched from the galaxy center outwards is wider and longer than previously thought.
They obtained a new infrared image from the show Milky Way galaxy stretching 130 degrees across the sky and one degree of stretching of field galaxies toward the top and bottom. This mosaic consists of 800 000 pictures taken by and featuring more than 110 million stars.
Benjamin then develop special software to count the stars and measure its density. The calculation is done in the Scutum-Centaurus arm showed improvement compared to the number of stars that should have been in a spiral arm. While the measurement of Sagittarius and Norma arms, there was no increase in the number of stars. Arm to-four, namely Perseus arm which envelops the outer portion of the Milky Way, can not be viewed in the latest images taken by Spitzer.
These findings suggest that the Milky Way galaxy has two spiral arms, as the structure of galaxies banded in general. Major arms, the Scutum-Centaurus arm and Perseus arms, have the greatest density of stars is composed of young, bright stars are older, known as red giants (red-giant stars). Benjamin claimed that the two major arms are visible in touch with the nearest and farthest from the main ribbon.
"Now, we can unite the two arms with the main bands, such as drawing up a puzzle," explained Benjamin. Previous infrared observations found hints of both of these arms. But those results were unclear because the position and width of arms is still unknown.
Though galaxy arms appear to be feature complete, but in fact star in it are constantly moving in and out of these arms. This is caused by movement of the stars as they orbit the galactic center.
The sun might have once resided in a different arm. And since it was formed four billion years ago, it has around the galactic center as much as 16 times.

Source : spitzer.caltech.edu