Astronomy

According to estabilish concept, our Milky Way galaxy is a spiral structure with four major star-forming arms, called Norma, Scutum-Centaurus, Sagittarius and Perseus. In addition to arms, there are bands of gas and dust in the central part of the galaxy. Our sun lies near a small, partial arm called the Orion Arm, or Orion Spur, located between the Sagittarius and Perseus arms.

But large infrared sky surveys in the 1990s led to some major revisions of these models, including the discovery of a large bar of stars in the middle of the Milky Way. Infrared light can penetrate through dust, so telescopes designed to pick up infrared light get better views of our dusty and crowded galactic center. In 2005, astronomers used Spitzer’s infrared detectors to obtain detailed information about our galaxy’s bar, and found that it extends farther out from the center of the galaxy than previously thought.

Recently, new infrared imagery from Spitzer shows an expansive swath of the Milky Way, stretching 130 degrees across the sky and one degree above and below the galaxy’s mid-plane shows an. This extensive mosaic combines 800,000 snapshots and includes over 110 million stars.

By counting the stars and measuring stellar densities in the direction of the Scutum-Centaurus Arm, astronomers noticed an increase in their numbers, as would be expected for a spiral arm. But, when they looked in the direction where they expected to see the Sagittarius and Norma arms, there was no jump in the number of stars. The fourth arm, Perseus, wraps around the outer portion of our galaxy and cannot be seen in the new Spitzer images.

The findings make the case that the Milky Way has two major spiral arms, a common structure for galaxies with bars. These major arms — the Scutum-Centaurus and Perseus arms — have the greatest densities of both young, bright stars, and older, so-called red-giant stars. The two minor arms, Sagittarius and Norma, are filled with gas and pockets of young stars. The two major arms seem to connect up nicely with the near and far ends of the galaxy’s central bar.

As shown on the picture above, the galaxy’s two major arms (Scutum-Centaurus and Perseus) attach to the ends of a thick central bar. Two minor arms (Norma and Sagittarius) appear less distinct. The major arms host the highest densities of both young and old stars. The minor arms are primarily filled with gas and pockets of star-forming activity. A new feature called the Far 3-kiloparsec Arm was discovered by radio surveys of galactic gas. Our Sun lies near a small patch called the Orion Spur, located between the Sagittarius and Perseus arms.

Picture credits: NASA/JPL-Caltech

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