University of Wisconsin

Space Astronomy Laboratory



ID # 4207

UW Astronomer: Joni Johnson

Eta Carina is a very peculiar, luminous, active star, which has a record of outbursts over the last three hundred years. Before 1800, it was a star of about third magnitude (visible to the naked eye in the Southern Hemisphere). In the mid-1800's, it suffered an eruption, which caused it to be among the brightest stars in the sky for a few decades. It then faded to an object visible only in telescopes, apparently enshrouded in the matter it ejected during the eruption. The nature of these eruptions is unknown - it does not fit into the pattern of other active stars: It is not a "nova," which is an outburst caused by transfer of matter onto a compact star in a binary star system, and it is not a "supernova," in which a massive star near the end of its lifetime is completely disrupted in a massive cataclysm. Most astronomers agree that the star is near the end of its lifetime, and some think it could soon become a supernova. Whatever the current status of the star, it has resulted in a unique opportunity. The visible object now consists of a compact, quite bright nebula known as the "Homunculus" because of its resemblance to a small man. The compact nebula is lit from the inside by light from the erupting star reflecting off the very large amount of dust that has condensed out of gas as the nebula has cooled.

WUPPE's observations will focus not on the star itself, but on the reflected light in the nebula, which has among the largest known visible-wavelength polarizations of any object in the sky (about 35%). This is probably because the structure of the nebula forces the light from the star to turn a near right angle before it gets out, a very favorable angle for polarization. The polarization for a right angle reflection turns out to be a very useful measurement indicating the size of the dust particles that are doing the reflecting. The aim of the WUPPE measurement is therefore to determine the size of the dust particles that are responsible for scattering of ultraviolet light, a very important step towards understanding the formation of massive stars. This measurement is also very interesting because the dust that is doing the reflecting has apparently been ejected recently from a hot star - material that has been processed by thermonuclear fusion into the heavier elements that form the next generation of stars.