From thornbur@black-hole.physics.ubc.ca Mon Jul 14 16:55:19 1997 Path: newsfeed.cv.nrao.edu!newsgate.duke.edu!nntprelay.mathworks.com!europa.clark.net!news.msfc.nasa.gov!pecos.msfc.nasa.gov!not-for-mail From: Jonathan Thornburg Newsgroups: sci.astro.research Subject: Re: Hipparcos Parallax query Date: 14 Jul 1997 16:31:59 GMT Organization: U of British Columbia / Physics Dept / Relativity Lines: 138 Sender: astres@pecos.msfc.nasa.gov Approved: astres@pecos.msfc.nasa.gov Distribution: world Message-ID: <5qdk9t$2ki@pecos.msfc.nasa.gov> References: <5q0asr$1j1@pecos.msfc.nasa.gov> Reply-To: Jonathan Thornburg NNTP-Posting-Host: pecos.msfc.nasa.gov Keywords: Hipparcos star catalog parallax X-Posting-Tool: modtool v2.0 Xref: newsfeed.cv.nrao.edu sci.astro.research:48 In article <5q0asr$1j1@pecos.msfc.nasa.gov>, Harvey Taylor asks some questions about the Hipparcos parallax catalog: | This leaves only the question of the negative angles. | Does anybody happen to know exactly how these arose? These are due to observational errors (statistical fluctuations >from Hipparcos only observing a finite number of photons from the star, unmodelled thermal distortions of the Hipparcos optical bench, noise in the detectors, etc etc). That is, for any star we can write observed parallax = true parallax + observational error True parallaxes are always positive, but observational errors may be either positive or negative. If a particular star happens to have a negative observational error larger in magnitude than its true parallax, then its observed parallax (= the number in the Hipparcos catalog) will be negative. In the distant past (pre-1900), people often replaced negative observed parallaxes by zero, or omitted negative-observed-parallax stars from catalogs. However, early in this century it was realised that this is a very bad thing to do -- the negative observed parallaxes still carry statistical information about both the observational errors and about the statistical distribution of true parallaxes. So the modern approach is to simply leave them in the catalog. | I have been comparing the Gliese3 and the Hipparcos | preliminary data files. | | [...] | | I am trying to get a handle on the relative completeness | of these catalogues. If Hipparcos shows N stars within | M lightyears, what proportion of the actual number of | stars is this likely to be? Only a tiny proportion of the actual stars near our Sun are in the Hipparcos catalog. Due to the design of the Hipparcos satellite, the stars to be observed (i.e. the stars in the catalog) all had to be chosen _ahead_of_time_, *before* the satellite was launched. The main Hipparcos catalog was limited to around 100,000 stars, and the experimental design also placed various limits on how the catalog stars could be distributed over the sky. For example, the satellite couldn't accurately measure star positions if more than 5 or 6 sufficiently bright stars (whether in the catalog or not) stars were present within an approximately 30 arcsecond circle. And it couldn't accurate measure star positions for too-faint stars. And these restrictions also depended on position in the sky. The Hipparcos consortium obviously wanted to maximize the scientific return from the mission. Since some stars are of more scientific interest than others, the consortium spent a lot of time and effort trying to maximize the "total interest" of catalog stars. (They put out a number of public calls to astronomers as a whole for suggestions of "interesting" stars to be included in the catalog, then winnowed their preliminary catalog down based on the experimental constraints and the various stars' degree of scientific interest.) The net result is that the Hipparcos stars are a highly selective and non-random sample of all the stars within (say) 1000 light years of us. You can't easily make any inferences at all about "all nearby stars" >from the Hipparcos data by itself. (Of course, combining the Hipparcos data with other astronomical data can tell you a lot more.) In practice, the Hipparcos catalog is probably complete only out to at most 10 light years or so, and maybe less than that. Indeed, it's not unlikely that there are some stars (say) 15-20 light years from here, which haven't even been discovered yet! | What I wonder is if Tycho includes higher magnitude stars, | what percentage will be high magnitude because of distance | and what percentage will be high magnitude because they | are intrinsically dim? Mostly the former. | Related to this is another question. I have read in a | pre-Hipparcos text that trigonometric parallax measurements | are useful only up to about 50 million AUs (ie. ~800 LYs). | Is this rule of thumb out the window with the presumably | more accurate measurements of Hipparcos? It's a matter of degree. Hipparcos's original design goal was parallaxes with a standard deviation of 2 to 5 milliarcseconds, depending on brightness and position on the sky. If you want the relative error of the observed parallax (i.e. the ratio observational error / observed parallax ) to be (say) 10%, then that means (taking the 2 milliarcsecond figure for purposes of discussion) Hipparcos parallaxes are good down to 20 milliarcseconds, i.e. out to 50 parsecs (= 165 light years) distance. On the other hand, if you can live with 20% relative errors, then the parallaxes are good down to 10 milliarcseconds, i.e. 100 parsecs (= 325 light years) distance. Or if you need 1% relative errors, then they're only good down to 200 milliarcseconds, i.e. out to 5 parsecs (= 16 light years) distance. Etc etc. So the only good answer to your question is, "it depends". | BTW Hipparcos shows ~80,000 stars within 1000 lightyears. | Any guesses how many Tycho shows? The original design goal was for Tycho to cover 400,000 stars around magnitude 9-10, with the possibility of considerably- -poorer-quality data for another 800,000 stars of magnitude 10-12. Unfortunately, due to a rocket failure, Hipparcos didn't make it into its planned orbit, but was stuck in a scientifically-inferior "parking" orbit. This meant that a considerable part of the data had to be discarded due to noise from the Earth's Van Allen radiation belts. ESA (= European Space Agency = the people who ran Hipparcos) was able to partially work the problem, but I don't know how the final performance compared to the original design plans. Reference (technical discussion of the original mission design): M A C Perryman & T. D. Guyenne "The Scientific Aspects of the Hipparcos Space Astrometry Mission" ESA SP-177, May 1982 -- -- Jonathan Thornburg (personal E-mail) U of British Columbia / Physics Dept / "The 1980s were the go-go junk-bond days of early-universe cosmology" - Michael S. Turner