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Milky Way spreads its arms

Diagram showing possible new Milky Way spiral arm

The Milky Way's basic structure is believed to involve two main spiral arms emanating from opposite ends of an elongated central section. But only parts of the arms can be seen—grey segments indicate portions not yet detected. (Other known spiral arm segments, including the Sun's own spur, are omitted from this diagram for clarity.)

OUR MILKY WAY GALAXY, like other spiral galaxies, comprises a main flattened body (or “disc”) with sweeping arms of stars, gas, and dust that curve around the galaxy like the arms of a huge pinwheel.

Our Solar System is located in a “spur” or offshoot that lies between two of the spiral arms, collectively orbiting around the galaxy about 25,000 light-years from its centre.

But because the Milky Way contains huge amounts of dust that blocks our view at normal optical wavelengths, it is extremely difficult to gauge the shape of the galaxy from our vantage point within the disc. It’s like trying to determine the overall shape of forest when you’re stuck in the middle.

This means that our knowledge of our galaxy’s spiral arms is much less certain than that of other galaxies such as Andromeda … because even though Andromeda is million light-years away, we have the advantage of seeing it from the outside.

Radio telescopes can peer through the dust, however, and molecules like carbon monoxide that emit radio wavelengths and concentrate in the Milky Way’s spiral arms, are particularly good “tracers” of the arms’ structure.

Using a small 1.2-metre radio telescope on the roof of their science building in Cambridge, Massachusetts, Harvard-Smithsonian Centre for Astrophysics astronomers Tom Dame and Pat Thaddeus used carbon monoxide emission to search for evidence of spiral arms in the most distant parts of the Milky Way, and discovered a large, new spiral arm peppered with dense concentrations of molecular gas.

They suggest that the new spiral is actually the far end of the Scutum-Centaurus Arm, one of the two main spiral arms thought to originate from opposite ends of our galaxy’s central section.

If their findings are confirmed, it will demonstrate that the Milky Way has a striking symmetry, with the new arm being the counterpart of the nearby Perseus Arm.

Adapted from information issued by the Harvard-Smithsonian Centre for Astrophysics. Image courtesy T. Dame.

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