Above you can see the first ever all-sky observation of polarized light emitted by interstellar dust in the Milky Way. It represents the galaxy’s magnetic fingerprint.
The data was acquired by the ESA’s Planck satellite. The European Space Agency explains why polarization—the phenomenon whereby electromagnetic fields vibrate preferentially in certain directions—is so useful:
In space, the light emitted by stars, gas and dust can also be polarised in various ways. By measuring the amount of polarisation in this light, astronomers can study the physical processes that caused the polarisation. In particular, polarisation may reveal the existence and properties of magnetic fields in the medium light has travelled through…
Even though the tiny dust grains are very cold, they do emit light but at very long wavelengths – from the infrared to the microwave domain. If the grains are not symmetrical, more of that light comes out vibrating parallel to the longest axis of the grain, making the light polarised.