The Double Helix Nebula, an emission nebula in Sagittarius

Image Credit: NASA/JPL-Caltech/UCLA

The Double Helix Nebula is an elongated emission nebula of about 80 light-years long, located some 25,000 light-years away from Earth in the constellation of Sagittarius (the Archer) and about 300 light-years from the supermassive black hole at the center of our galaxy. It should not be confused with the Helix Nebula, a planetary nebula only 650 light-years from Earth..

The gaseous nebula – discovered by the infrared Spitzer Space Telescope – looks like two intertwining strands wrapped around each other like a double helix as in a DNA molecule. The magnetic field at our galaxy’s center is probably responsible for the shape of the nebula.

Our galactic center has a very strong magnetic field that is highly ordered; its magnetic field lines are oriented perpendicular to the plane of the galaxy. If you take these magnetic field lines and twist them at their base, that sends what is called a torsional wave up the magnetic field lines. (The wave is like what you see if you take a long loose rope attached at its far end, throw a loop, and watch the loop travel down the rope.)

This twisting torsional wave is propagating out. We don’t see it move because it takes 100,000 years to move from where it was probably launched to where we now see it. But it’s moving fast (about 1,000 kilometers per second) because the magnetic field is so strong at the galactic center (the energy equivalent of 1,000 supernovae and about 1,000 times stronger than where we are in the galaxy’s suburbs). The fast moving magnetic wave can carry small dust particles up from the disk and trap them, providing something that can absorb and emit infrared radiation, what can be seen by Spitzer.

What launches the wave, twisting the magnetic field lines near the center of the Milky Way? The answer is probably not the supermassive black hole at the galactic center, at least not directly.

Orbiting the black hole is a massive disk of gas called the circumnuclear disk; the magnetic field lines are anchored in this disk. The disk orbits the black hole approximately once every 10,000 years. Once every 10,000 years is exactly what is needed to explain the twisting of the magnetic field lines that we see in the Double Helix Nebula.

The image – taken by the Multiband Imaging Photometer on NASA’s Spitzer Space Telescope – uses false colors because the eye is not sensitive to infrared light. The spots are infrared-luminous stars, mostly red giants and red supergiants. Many other stars are present in this region, but are too dim to appear even in this sensitive infrared image.

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