A map of Milky Way galaxy showing the Sun's position on the left. The inset shows a schematic of the Sun’s neighborhood including the Local Bubble.
A map of Milky Way galaxy showing the Sun's position. The inset shows a schematic of the Sun’s neighborhood. It includes the Local Bubble. Photo: Credit: NASA/JPL-Caltech/R. Hurt (SSC/Caltech), annotations by J. West
A team of radioastronomers has provided convincing evidence that the Sun sits in a tunnel-like structure in the galaxy. It is surrounded by enormous, elongated magnet filaments that are so large and close to us that they can be seen spread across the sky.
It can be difficult to tell how far away something is when we see it in the sky. What is it? Is it close and small, or far and distant? We have many ways to measure distances for most objects, but it can be difficult for others.
It is because of this that some structures in the sky are difficult to see. The galaxy around us glows when we use radio telescopes to map the sky. These telescopes "see" light with a lower energy and longer wavelengths than our eyes. We see the Milky Way as a flat disc and are within that disk. If we use the correct kind of projection (called a Mollweide projection) we can see the galaxy as a thick line that runs right through the galactic Equator.
This was done in the 1950s. Immediately, a long, curving filament was visible stretching upwards and away from the galactic disc near the center. The North Polar Spur is the name of this filament, as it extends up to the galactic pole. It can also be seen below.
The entire sky as radio waves (408 Mhz), shows the Milky Way galaxy in the middle. There are also several features that could be part of a magnetic "tunnel", in which the Sun is located. The North Polar Spur is visible in the large loops at the top. More diffuse emission can be seen on the extreme left or right. Credit: NASA/LAMBDA Archive Team
Soon after, diffuse emission was again visible, but not as clearly as the North Polar Spur (or NPS). It also became fainter. The Fan Region extends up from the galactic plane (from about half way to the center of the map and along the extreme right edge).
Astronomers debate the nature of these two structures. Many argue that the NPS is located in the center of the galaxy, at 26,000 light-years away. It is also aligned loosely with large X-ray bubbles that are centered in central part of the galaxy, suggesting they are connected. However, most methods of inferring its distance cannot distinguish between it being more than ten thousand light years away or a thousand light-years away. It could be located near the galactic center but it may also be a local feature in the spiral arm that the Sun is currently in.
The Fan Region is the same. In this instance, the argument that it is closer to us is stronger. Polarized light is a condition where the electromagnetic waves from incoming light align in a particular way. Some radio observations can be sensitive to it. Both the radio waves from the NPS as well as the Fan are polarized. Stars that face the Fan Region's direction also have a polarization in the light they receive. These stars are very close to the Sun, which means that the Fan Region is also close to us and polarizing our starlight.
The new work by astronomers suggests that these two features and others are part of a larger structure. It is a partially hollowed out, gently curving tunnel in galactic gas within our local spiral arm. Parallel filaments of magnetized materials run along the walls. The Sun is located near the one-side of the tunnel. Their perspective and physical form determine their shape in the sky.
The entire sky as radio waves (408 Mhz), shows the Milky Way galaxy in the middle. There are also several features that could be part of a magnetic "tunnel", in which the Sun is located. The North Polar Spur is visible in the large loops at the top. More diffuse emission can be seen on the extreme left or right. Credit: NASA/LAMBDA Archive Team
For example, imagine a tunnel that runs through a mountain. It is a long, curving cylinder. Imagine someone painting stripes along the entire length of the tunnel, from the middle to the top. One line will be close to your vehicle, the others on the left. The other lines will be visible from the driver's side window. Because of perspective, you'll see them appear converge ahead and behind.
This is almost what we see when we look at these radio filaments. The tunnel analogy fits the data quite well.
This could be caused by what? The Local Bubble is a volume of space that surrounds the Sun. It is approximately a thousand light-years across and elongated. The density of interstellar gases in it is also lower than the surrounding areas. It was probably carved by supernovae (exploding star) over the past 15-20m years. The expanding debris of these supernovae pushed against gas around them like snowplows, causing the bubble to burst.
We also know filamentary structures can develop in areas where bubbles are formed; they are often seen in supernova remnants. These structures are due to the interaction of the magnetic field of expanding stellar debris with the gas magnetic fields. This could also explain the tunnel's shape and alignment. We know that magnetic field lines run parallel to the spiral galaxies' curved arms. The direction that the filaments seem to follow is roughly the same as our local arm.
A map of Milky Way galaxy showing the Sun's position. The inset shows a schematic of the Sun’s neighborhood. It includes the Local Bubble. Photo: Credit: NASA/JPL-Caltech/R. Hurt (SSC/Caltech), annotations by J. West
This is supported by the fact that these filaments can be seen in polarized lighting. Space dust grains tend to align along magnetic field lines. Light bouncing off them becomes polarized. This is similar to how light reflected from a metal car's hood gets polarized. Polarized sunglasses can be used to reduce the glare. These magnetic field lines are also magnetized and electrons can spin around them like beads on a wire, emitting polarized light (called synchrotron emissions).
The tunnel model is stable. It is possible, and even probable, that there are other partially-hollowed tunnels all along the spiral arms.
Although the model is not perfect, it does provide some information about the shape, polarization structure and brightness of the filaments. However, certain parts of the filaments do not fit the model. It's possible that there are magnetic field lines which cause distortions to the tunnel and filaments. It's possible that there are local disturbances in the magnetic field lines that distort the tunnel and filaments. The galaxy is messy. There are stars that explode and massive nebulae that blast out black holes. These jets can cause local distortions in the magnetic field.
What impresses me most about this system is the way it unifies all these features across the sky. It is not obvious that anything seen in the sky to the north would be connected to something 100 degrees distant. It can be difficult to see the whole picture. We know of large structures in the sky that occupy large areas of space. However, they are not local and so are spread over vast areas above our heads.
It is something I would rather this model prove to be accurate. It's a wonderful idea to look up at the night sky and see features so large and close you would have to rotate your whole body to see them...assuming you have radio-sensitive ears. Although you don't have one, your imagination is useful. Knowing that I am within a series of huge structures, ranging from billions to tens or thousands of trillions, each nested inside another, all the way to the entire Milky Way Galaxy, spurs my imagination.
We are surrounded by a number of cosmic matryoshkas dolls. When we stand on Earth, we can see beyond them to the Universe beyond. It's amazing and beautiful to think of that.
