Only Two Spiral Arms for our Milky Way

We will never know for sure how our galaxy looks like from the outside because we are stuck inside.

In the 1950s, astronomers suggested that our galaxy has a spiral structure with four major arms, namely Norma, Scutum-Centaurus, Sagittarius and Perseus Arm. Our Solar System resides near a partial arm called the Orion Arm, between the Sagittarius and Perseus Arm. This early models were built based on radio observations of gas in our galaxy.

In 1990s, astronomers discovered a large bar of stars in the middle of Milky Way through large infrared sky surveys. Infrared light can penetrate through dust, so by using infrared telescopes we can get “clearer” views of our dusty and crowded galactic centre.

In 2005, astronomers discovered that our galaxy’s central bar extends farther out from the centre of our galaxy than previously thought.

And today, astronomers discovered that our Milky Way has only two major arms instead of four, a common structure for galaxies with bars.  The result is based on the new infrared imagery from Spitzer of an expansive swath of the Milky Way, stretching 130 degrees across the sky and 1 degree above and below the galaxy’s mid-plane, which includes over 110 million stars,

So, from this:

MW_no bar

we revised our model to this:

MW with bar

and now to this:

MW with bar & 2 arms. Click for annotation.

When astronomer (I mean the software developed by the astronomer) counts the stars in the Spitzer’s images, they found an increase in density of stars in the direction of the Scutum-Centaurus Arm, as 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.

These major arms, the Scutum-Centaurus and Perseus Arms, have the greatest densities of both young, bright stars, and older red-giant stars. The two minor arms, Sagittarius and Norma, are filled with gas and pockets of young stars. Astronomer said the two major arms seem to connect up nicely with the near and far ends of the galaxy’s central bar.

Source: Spitzer Newsroom


~ by thChieh on June 4, 2008.

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