One question raised in the thread in the Lounge is whether it is possible to identify the spiral structure of the Milky Way from the Gaia dataset. I have tried to do this by marking the OB stars: massive, short-lived stars that do not live long enough to move very far from their birthplaces, which should be concentrated in the spiral arms. With the dataset as it stands, this is not possible due to a large number of stars that are misidentified as OB-stars based on their B-V colour index. I am working on further updates to the dataset that may help correct this.

Watch this space!

The astronomers behind the new study used several radio telescopes in Australia, USA and China to individually observe about 1650 massive stars that had been identified by the RMS Survey. From their observations, the distances and luminosities of the massive stars were calculated, revealing a distribution across four spiral arms.
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NASA’s Spitzer Space Telescope, on the other hand, scoured the Galaxy for infrared light emitted by stars. It was announced in 2008 that Spitzer had found about 110 million stars, but only evidence of two spiral arms.

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“Spitzer only sees much cooler, lower mass stars – stars like our Sun – which are much more numerous than the massive stars that we were targeting.”

Massive stars are much less common than their lower mass counterparts because they only live for a short time – about *10 million years*. The shorter lifetimes of massive stars means that they are only found in the arms in which they formed, which could explain the discrepancy in the number of galactic arms that different research teams have claimed.
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Since lower mass stars live much longer than massive stars and thus rotate around our Galaxy many times, they have time enough for spreading out in the MW disc. The gravitational pull in the two stellar arms that Spitzer revealed is enough to pile up the majority of stars in those arms, but not in the other two. However, the gas is compressed enough in all four arms to lead to massive star (MYSO) formation.

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