An Astronomy Ph.D. thesis in plain English - Part 2

Last Time: In the first post in this series I discussed the way in which stars get redder as dust and gas pass between us and the star, which is very similar to the effect that we see as we admire a red sunset. When the stars get redder, they usually get more faint as well, as you would expect if there is dust passing in front of them.

The next important property of the young stars that I was investigating is their polarization -- that is, the preferred direction of oscillation of the light waves being emitted from the star. A beam of light can be thought of as a combination of many individual light waves, and each wave can be thought of as having a "direction of oscillation". Think about all the different ways that you can shake a skipping rope held between two people, or a string on a guitar, or a clothesline fixed at both ends. You can shake it up and down, or side to side, or on any number of diagonals.

A light wave is a lot like these ropes and strings. The light travels from one end of the string to the other, and while it travels it is allowed to oscillate in any of the directions that you can think of to shake the string (the thing that is oscillating is actually two things - the electric and magnetic fields that are associated with the light). The normal, unpolarized light that we see all around us is composed of many different light waves, and all of them can be oscillating in their own direction: there is no preferred direction of oscillation (the first image of the yellow arrows). However, some processes in nature can produce a preferred direction of oscillation, or a polarization of the light (like the second yellow arrow, below).

Some of the most important mechanisms that cause a polarization of light are (1) the presence of a magnetic field where the light is being generated and (2) the reflection of light. In my thesis, I was mostly concerned with polarization that was caused by the reflection of light. Once again, this is something that happens all the time here on Earth. Sunlight that is reflected from a surface such as water on a lake or an ocean or snow on a ski slope tends to be polarized. Sunglass companies have capitalized on this fact by marketing sunglasses with a polarizing filter in them that cuts down on glare from the water or from the snow. The starlight that I was observing is not reflected by water or by snow, but it is reflected by the dusty rings surrounding the star, and the light that is reflected by the rings is more polarized than the light coming to us directly.


By watching the star and observing the times when the most polarized light is reaching us (and hence, the most light reflected off the dust), we can try to figure out the shape and size of the dust distribution, as well as the composition of the dust, and the density of the dust. All of these things will eventually help us to understand what the conditions for planet formation are, and how often these conditions arise in star systems outside of our own solar system.

Next Time: A curious "blueing" effect sometimes seen when the stars become very faint...