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In 1852, Stephen Alexander, an astronomer at the College of New Jersey, put forward the radical suggestion that the Milky Way galaxy is a spiral.

But while today’s astronomers agree on this general shape, they disagree over the precise structure of the spiral and in particular on the number of arms.

Astronomers have named at least 6 arms and in the 1990s, evidence emerged that the galaxy had a central bar. The uncertainty is easy to understand. Our view of the galaxy shows the nearer stars superimposed on the ones that are further away. And much of the opposite side of the Milky Way galaxy is obscured entirely by the central mass of stars at the centre.

Recently, however, a clearer picture has begun to emerge. The growing consensus is that the Milky Way has a central bar with two main arms, called the Perseus Arm, which passes with a few kiloparsecs of the Sun, and the Scutum-Centaurus Arm. (The other arms are now thought to be minor structures made up largely of gas.)

Today, Thomas Dame and Patrick Thaddeus at the Harvard-Smithsonian Center for Astrophysics in Cambridge provide further evidence of this 2-arm structure but with a twist that explains why astronomers have previously been unable to see it clearly.

Astronomers traditionally study the Milky Way’s structure by measuring the movement of large clouds of hydrogen and carbon monoxide gas within it (the velocity of distant stars being too difficult to pin down).

The new evidence that Dame and Thaddeus have accumulated shows the existence of a new arm on the other side of the Milky Way, but further from the centre than we are. The new arm is 18 kpc long and so stretches some 50 degrees across the sky.

Dame and Thaddeus conclude that this arm is an extension of the Scutum-Centaurus Arm, the rest of which is obscured behind the galactic middle.

That makes sense. The Perseus arm, which we can see more clearly, wraps 300 degrees around the galactic centre. If Dame and Thaddeus are correct, the Scutum-Centaurus Arm must be exactly symmetrical with this. That makes the Milky Way similar to the Great Barred Spiral, a barred, twin spiral some 56 million light years from here.

But why has it taken astronomers so long to find the end of this arm? The reason, says Dame and Thaddeus, is that this arm is bent. Sot it’s not in the galactic plane but slightly above it.

Which means the Milky Way is warped, like the cap from a freshly-opened beer bottle.

Ref: arxiv.org/abs/1105.2523: A Molecular Spiral Arm in the Far Outer Galaxy

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