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The structure of nearby galaxies such as Andromeda is relatively straightforward to see. But the Milky Way presents an entirely different kind of challenge.

The problem is that we see the Milky Way edge on, so that nearer stars and clouds are superimposed on more distant ones. Telling these apart is tricky because working out the distance of any astronomical object is hard. And that makes the overall structure a real head scratcher.

That’s not to say astronomers haven’t got a few tricks up their sleeves to help. The conventional way to work out the structure is a two step process. Astronomers first create a model of the galaxy and work out how each part of ought to be moving relative to us.

Then they scour the Milky Way for clouds of ionised hydrogen. Astronomers can work out the velocity of these clouds by studying the emission spectra and looking for the tell tile shifts in spectral lines that movement causes.

By matching this measured velocity to the calculated values, astronomers can work out where in the galaxy any cloud should be.

But this method is notoriously ambiguous, not least because nobody is quite sure how fast the galaxy is rotating, so the model probably has all kinds of errors. Another problem is that stars orbiting the centre of the galaxy at the same distance as us (a large portion of the galaxy, as it turns out) all have a similar velocity. So working out where they are is tricky.

It’s no surprise, then, that there is little consensus on the exact structure of the Milky Way’s spiral arms.

Today Jaques Lepine at the University of Sao Paulo in Brazil and a few buddies add a little spice to this mix.

They’ve studied the spectra produced by clouds of carbon monosulphide, a relatively common component of our galaxy, rather than ionised hydrogen. This gave them velocity information for 870 regions of the Milky Way which they’ve used to create a new map of the galaxy with detail never seen before.

One conclusion is that the Milky Way has an additional spiral arm, not seen in previous surveys of the galaxy. The new arm is about 30,000 light years from the galactic core at a longitude of between 80 and 140 degrees.

But a bigger surprise is their conclusion that some of the arms in the Milky Way are not curved in the traditional way, but are straight instead. This gives the Milky Way a distinctly squarish look.

That’s not as outrageous as it sounds. Astronomers know of many galaxies with straight arms, such as M101, the Pinwheel Galaxy, shown above.

So according to Lepine and co, anybody looking at us from M101 will see a similar kind of squarish structure. Fascinating stuff!

Ref: arxiv.org/abs/1010.1790: The Spiral Structure Of The Galaxy Revealed By CS Sources And Evidence For The 4:1 Resonance

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