Astronomy
The Interstellar Medium's Effect on Starlight Polarization
Quick fact
Starlight can become polarized by as much as a few percent when it passes through dense clouds of interstellar dust, and this polarization is observable even though dust grains are about the size of smoke particles.
Why this is interesting
When you look at a star, its light might have a secret twist—polarization—that tells you about invisible magnetic fields and tiny dust grains thousands of light-years away. How can a simple property of light reveal so much?
Read the full explanation
Understanding The Interstellar Medium's Effect on Starlight Polarization
Imagine you are looking at a star through a dusty, cloudy region of space. The light from the star travels through the interstellar medium, which is full of tiny dust grains. These grains are not random spheres; they are elongated, like tiny needles or grains of rice. As light passes, the dust grains absorb and scatter light. Normally, light from a star is unpolarized, meaning its electric field vibrates in all directions randomly. But when light interacts with these asymmetric dust grains, one polarization direction is absorbed more than the others. This process is called dichroic extinction. The result is that the transmitted light has a slight preference for a specific direction of vibration—it becomes partially polarized. The direction of this polarization is perpendicular to the alignment of the dust grains. So, by measuring the polarization angle of a star's light, we can infer the orientation of the dust grains along the line of sight. But why are the dust grains aligned? The leading theory is that they are aligned by magnetic fields in the interstellar medium. The dust grains have a slight magnetic property, and they interact with the magnetic field, causing them to align with their long axis perpendicular to the magnetic field lines. (Actually, the alignment mechanism is more complex, but the net effect is that grains align perpendicular to the magnetic field.) Therefore, the observed polarization direction tells us the direction of the magnetic field projected on the sky.
A deeper explanation
The underlying mechanism is the interaction of light with asymmetric dust grains, a form of dichroism. As starlight passes through a region containing aligned nonspherical dust grains, the component of light whose electric field oscillates in the plane containing the grain's long axis is preferentially absorbed (or scattered), leaving the perpendicular component relatively stronger. This selective extinction produces linear polarization in the transmitted light. The degree of polarization depends on the alignment efficiency of the grains, the column density of dust, and the magnetic field geometry. The importance of this effect is profound: it provides a way to map magnetic fields in the interstellar medium. Magnetic fields are crucial for regulating star formation, as they support molecular clouds against gravitational collapse and channel the flow of gas. By observing the polarization of hundreds of background stars, astronomers can create maps of magnetic field directions across the sky. These maps reveal the structure of magnetic fields in galaxies, helping to understand how gas accretes onto clouds and how stars are born. Furthermore, the polarization of starlight is also seen in the cosmic microwave background, where it provides information about the early universe. The same physical principle—polarization by scattering off particles—has analogies in other contexts, such as the polarization of light reflecting off surfaces on Earth. However, in interstellar space, the alignment of dust grains by magnetic fields is a unique phenomenon that makes starlight polarization a powerful probe of the unseen magnetic universe.