The missing arms of our galaxy

After 12 years of observations, a research team led by an astronomer at the Max Planck Institute confirmed that the Milky Way has four spiral arms, contrary to what had been stated in 2008 by NASA’s Spitzer Space Telescope. The study belongs to the Monthly Notices of the Royal Astronomical Society.



A battle with strokes images and theories, one built around the arms of the Milky Way: their number has increased from two to four, and then to be put back into question over the years. Today, the game seems to have been finally closed. For the first time you have taken all the outer arms of our galaxy: the verdict is that they are just four, as had been assumed at the beginning.

The confirmation comes from a study coordinated by James Urquhart at the Max Planck Institute, based on 12 years of observations and analysis. Urquhart and his team went through about 1,650 giant stars of our galaxy, and then calculate the shape of the Milky Way from the distances and brightness of the stars.

Make deductions of this kind is not easy, since we are immersed in the Milky Way. Astronomers have begun to try in the 50s, using radio telescopes to construct a map of our galaxy. Observations have shown gas clouds in which new stars were born, revealing four arms of the main galaxy. Over the years this theory has been repeatedly challenged, until the complete denial in 2008: NASA’s Spitzer Space Telescope, based on observations in the infrared, identified about 110 million stars and it was confirmed only two spiral arms.

In the study by Urquhart and colleagues , published in the Monthly Notices of the Royal Astronomical Society , have been used several radio telescopes in Australia, the United States and China to observe individual stars that eventually led to the conclusion that there are four arms.



“You cannot say that our results are correct and those of Spitzer’s wrong ,” said Melvin Hoare School of Physics and Astronomy, University of Leeds and co- author of the article . “The two surveys were trying different things: Spitzer observed only stars cooler and smaller mass, like the Sun, which are more numerous than the giant stars in which we have focused ourselves.”

Stars, the latter are more difficult to observe than their smaller counterparts, because they live less – “only” 10 million years. But there is an advantage: their short life means that they are found only in the arm in which they originally formed, and thus can be used for a most faithful of the space that surrounds them. This may explain the discrepancy with the results of Spitzer that evidenced only two of the four arms but there is enough gravitational energy to hold small stars, which in contrast to those giants move several times in the galaxy. But in all four arms is possible the formation of giant stars, which are monitored on a selective group of Urquhart, that confirmed the structure of the Milky Way.


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