In the Milky Way Project we look for bright sturctures, like Infrared-bubbles or bowshocks. Infrared Dark Clouds are something the volunteers are adviced to ignore:
“If you see dark splotches in an image, they might be part of a ‘dark cloud’ of gas and dust. You need not classify these objects!”
Infrared Dark Clouds (IRDCs) were discovered 1996. They block the light at mid-infrared and are a dark spot in these images. At first the astronomers thought that all dark spots are cold and dense clouds. A place where massive stars form.
But not all dark spots are IRDCs. Some are “holes” in the sky. If something is a dense and cold cloud, you see bright sturctures at far-infrared images, such as images from the Herschel Space Observatory.
In this blog-post you can see combined Spitzer+Herschel images.
A IRDC compared with Spitzer and Herschel data:
You can see the red stars. Those are new born stars, called Young Stellar Objects. IRDC are also often connected to yellowballs and small bubbles.
After a lot of new stars were born, the energy from those stars heat the surrounding gas and dust and the IRDC now glows in the mid-infrared. A yellowball is born that can evolve into a bubble.
How does this works?
First we imagine a cold cloud like the one above as a ball. We (observer) are in front of this ball and a green screen is the background. From above it should look like this:
The light from the background has to pass the cloud. At the edge the path is short. In the middle the path is long. The more we look into the middle of the cloud, the less light arrives in our eyes.
Now we can predict how this cloud should look like:
Many of the IRDC are not perfect balls, but the pattern is similar.
If we now imagine a ring, it should look like this:
The light moves free. Only in the ring it is a bit absorbed. Depending on how dense the ring is and how strong the background radiation, the ring is darker or brighter. This is how it should look like:
But a perfect IRDC ring is something we should not expect in space. Everything, also gas and dust in space, prefers disorder (entropy). A not perfect ring is possible (example on the right). Keep in mid you can find any roation of the ring in space. To find an ellipse is more likely than to find a circle.
The last example is a sphere (not filled) with a thin layer. It would look like this:
It is the opposite of the ball: The edges have the longest path for the light through the ring, but the centre dose have a short path.
A thin and dark ring should be visible. Inside the ring the path is slightly shorter in the centre, so we should see something like in the image above about the ball, but inverted.
Something is important about this shpere: No matter how you rotate the sphere, it will always look like a circle in an image.
The example to the right is the coffee ring at 8.0 μm wavelength. It looks like some alien left its coffee cup on the sky and the coffee was left from the shape of the cup.
It was first disovered in the Milky Way Project by the volunteer @ZUCCO66. This is how it looks like in the Milky Way Project:
Now there is a big problem. Remember how I did talk about entropy? Everything wants to be in disorder. If something should be in order, you need energy. If you add energy to the object, a lot of energy is lost as heat, but the coffee ring is not heated. In fact it is colder than 40 Kelvin. This is -233.15°C.
To learn more about the coffee ring, some researchers, part of the Milky Way Project, requested telescope time for the Green Bank Telescope in West Virginia. This is the largest steerable radio telescope of the world.
Is it possible to solve the mystery?