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| Credit: NASA/JPL-Caltech/R. Hurt (SSC) |
The above illustration, published on August 2005 on the Spitzer Space Telescope website, was accompanied by the following caption:
This artist's rendering shows a view of our own Milky Way Galaxy and its central bar as it might appear if viewed from above. An arrow indicates the location of our Sun. Astronomers have concluded for many years that our galaxy harbors a stellar bar, though its presence has been inferred indirectly. Our vantage point within the disk of the galaxy makes it difficult to accurately determine the size and shape of this bar and surrounding spiral arms.
New observations by the GLIMPSE legacy team with NASA's Spitzer Space Telescope indicate that the bar-shaped collection of old stars at the center of our galaxy may be longer, and at a different orientation, than previously believed. The newly-deduced size and angle of the bar are shown relative to our Sun's location. Our Milky Way galaxy may appear to be very different from an ordinary spiral galaxy.
But only three years later, the image of the Milky Way inferred by the astronomers was already changed. A new illustration published on the same website showed clear differences.
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| Credit: NASA/JPL-Caltech/R. Hurt (SSC/Caltech) |
The new image was accompanied by the following caption:
Like early explorers mapping the continents of our globe, astronomers are busy charting the spiral structure of our galaxy, the Milky Way. Using infrared images from NASA's Spitzer Space Telescope, scientists have discovered that the Milky Way's elegant spiral structure is dominated by just two arms wrapping off the ends of a central bar of stars. Previously, our galaxy was thought to possess four major arms.
This artist's concept illustrates the new view of the Milky Way, along with other findings presented at the 212th American Astronomical Society meeting in St. Louis, Mo. The galaxy's two major arms (Scutum-Centaurus and Perseus) can be seen attached to the ends of a thick central bar, while the two now-demoted minor arms (Norma and Sagittarius) are less distinct and located between the major arms. The major arms consist of the highest densities of both young and old stars; the minor arms are primarily filled with gas and pockets of star-forming activity.
The artist's concept also includes a new spiral arm, called the "Far-3 kiloparsec arm," discovered via a radio-telescope survey of gas in the Milky Way. This arm is shorter than the two major arms and lies along the bar 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.
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