University of Wisconsin

Space Astronomy Laboratory




#4207 Eta Car
#4208 M1-92
#4211 NGC 7023
UW Astronomer: Joni Johnson

The space between the stars is not completely empty: it contains an extremely rarified mixture of gases and very fine solid particles. The gas (mostly hydrogen, with a little helium and heavier elements) is less dense than the best vacuum we can make on the ground, and the particles are smaller than smoke particles, but nevertheless they both have a profound effect on the appearance of our universe and on how it changes with time. For instance, the solid particles, through their interaction with light, have a controlling influence on the formation of new stars and planets.

The small particles (called "interstellar grains"), are of a variety of sizes and compositions, and are found in clouds around forming and exploding stars, in diffuse clouds in interstellar space, and (probably) spread evenly between. They are most easily seen through two effects: absorption, in which light from a star is lessened in its long journey from its source to our telescope, and reflection, in which its route is altered in flight, and happens to fall into the telescope, apparently arriving from black space. The absorption makes the starlight appear dimmer, redder (since more blue light is absorbed than red light), and, it turns out, polarizes it, since some of the grains act like little polaroids. The reflection makes the space around the illuminating star glow blue. This is called a "reflection nebula". Our atmosphere does some of the same things to sunlight: at sunset the sun appears to be dimmer and redder due to absorption along its lengthened path through the atmosphere, while even after it is set the sky is still blue, due to sunlight which is reflected in the atmosphere. In a sense, the whole sky is a "reflection nebula" illuminated by the sun. It turns out that the process of reflection from very small particles always heavily polarizes the light, so that both the sky and the light from astronomical reflection nebulae is very highly polarized. (This is very easy to see with a pair of polaroid sunglasses on a clear day.) The amount of polarization depends on the angle that the light has been deflected by (called the scattering angle), on the size of the particle, what it is made of, and on the wavelength of the reflected light. This makes study of the polarization of reflection nebulae a wonderful way of studying the nature of the interstellar grains.

WUPPE's study of the ultraviolet polarization from reflection nebulae (the first ever attempted) is aimed at understanding the nature of the very smallest of the interstellar grains, which are of great importance in the formation of massive hot stars, with their large output of ultraviolet light. We will be looking at how the polarization changes from the visible polarization (obtained from ground-based observation) as we go into the ultraviolet. We will be looking at several different nebulae with different geometries (scattering angle) and different histories (composition) to try to separate those effects. The observations are actually fairly tricky, since the reflected light is quite faint, and the observation "aperture" must be carefully placed by the Payload Specialist to miss the illuminating star and to hit the part of the nebula which corresponds to the most favorable scattering angle.