Astronomy's House of Cards
Kye Ewing - June 3, 1999
In school we all learned about the scientific method of proving a theory. Gather the facts, form a hypothesis and then test a model to find if you are correct. In physics the last part (of testing the hypothesis) can get pretty abstract, not to mention expensive! New ideas tend to be tested "in theory", based on assumptions that themselves have only been proven in theory. Add to this some truly astronomical margins of error and you have models that might make for real pretty pictures, but how much is correct and how much is only waiting to be disproved?
Star distances are one of the assumptions that countless other assumptions in astrophysics are based on. Recent data gathered by the Hipparcos Mission has greatly solidified the facts of this ground truth, verifying some ideas and trashing others. From 1989 to 1993 the Hipparcos Satellite held two astrometry instruments in Earth orbit while our planet made its way around the Sun. Star distances were measured with unprecedented accuracy, using the only known DIRECT method - parallax motion. It extended the range of accurate parallax distances by about ten times. Before Hipparcos less than 1,000 stars had measured parallaxes. Now 22,396 stars have had their parallaxes measured to a high accuracy (10%) and another 30,000 stars have been recorded at a somewhat lower accuracy - still with vastly more certainty than ever before. Repeated observations have added other dimensions to this new bank of information. Our knowledge of star motions, as well as very precise magnitude measurements have been greatly enhanced.
As it typically goes with science - answering the old questions has brought up many new ones. 8,237 new variable stars were discovered, but more than 4,000 of them are of unknown types! Their periods and luminosities will have to be measured, mostly by amateurs working with the AAVSO. Many of these stars are brighter than 9th magnitude - well within the reach of amateur scopes. Theories about the life cycles of stars use their spectral characteristics to make assumptions about what is happening to them. Being able to compare the spectral lines of stars at a confidently known distance makes further inferences about energy output more reliable. Among the stars that Hipparcos observed with high accuracy was the Hyades Cluster. Star clusters of this type are especially interesting to astrophysics because they give a sampling of stars born around the same time, with similar chemical compositions, with only their masses being different. How this group changes with time provides insights that help us understand how stars evolve.
Why do we care - other than to make those snappy looking little models? Well, the "House of Cards" that the model of our universe is supported by is now much stronger. Some of the "next tier up" assumptions are a little less shaky! One such line of theory upon assumption is the cosmic distance scale. It determines our estimations of the true size, brightness and therefore energy output of nearly everything in the universe. Of course, energy output is a very important factor in many other theories regarding the workings of our universe. Cepheid Variables pulsate in brightness at a very regular rate and at very predictable brightness ranges, easily matched to the cycle's length. This makes them an ideal universe yardstick; all galaxies have them in their hovering globular clusters. Although none are real close, they are supergiants, which means they can be observed at greater distances than most stars. This only allows measurement by some of those secondary methods mentioned earlier, with more accuracy needed in our technology to measure smaller parallaxes. The "next generation" mission that is now being planned should take astrometry to the next level. With one hundred times the precision of Hipparcos, the GAIA project will make use of greatly refined technology in both CCD imaging and data transfer. The onboard software will compile its own target list on an ongoing basis. It will be expected to send one megabyte of data per second for the entire five year mission! Talk about needing a bigger hard drive!!!