How to use Virtual Observatory Proper Motion of Barnards Star Florian Freistetter, ZAH, Heidelberg florian@ari.uni-heidelberg.de
Stars do move! Even if stars are called fixed stars sometime, y are not really fixed. This word was chosen in ancient times, when one did not know much about real nature of celestial bodies. To distinguish m from moving stars, that change ir position every night. Today we know, that those are planets and that also fixed stars do move althought ir motion is very small and it took some time for astronomers to measure it. better. Let's choose two images from POSSII-Catalogue (13' x 13'). The column date shows when pictures were made. We take images from 1991 and 1988. There are different reasons why a star changes its position on sky. There are apparent changes, due to motion of Earth around Sun (parallax) and due to finite velocity of light (aberration). And re is also a real change of position, due to proper motion of star. A star, moving on sky, is changing its right ascencion and declination. The following formula gives change during a certain time: = cos = sin cos The total proper motion per time unit is called ; is angle, in which star is moving (North = 0 ). Bild 1: Chosing pictures of Barnards Star Klick on Submit to load pictures in Aladin. We now can combine two images to a movie and see, if star has moved. Therefore we use blink -button from toolbar on right. We specify images we want to use and hit create : How fast is Barnards Star The star with fastest proper motion measured so far is Barnards Star. How fast it really is, can be found out with Aladin: We start Aladin and n with File -> Open... server selector. Enter Barnard Star in target -field and hit submit. The available pictures of Barnards Star are listed. To investigate proper motion, we chose two picture, that were made at different times. The longer in between, Bild 2: Making a movie
The movie now starts playing and we can see, that star is moving To measure how far star has moved, we use rgb -button from toolbar. This function is meant to use for combination of images in different wavelengths to obtain a color picture. But we can use also for our case. result will be approx. 32 arc seconds: that is apparent distance that star has moved. But during what time? With a right-click on images in Aladin-stack, we can view properties of image. There we find exact time, when pictures were made: In rgb -window, we chose one of images for red channel and one for green. Klicking create gives a new image.: Bild 5: When were pictures made? Bild 3: Combining to images The two images are now superomposed. Where stars have not moved, y appear white. But Barnards Star has moved and thus we see two images: one in green and one in red: The relevant information can be found at label epoch. In our case, pictures were made at May, 12th 1988, 09:54:00 and June, 16th 1991, 07:47:59. Or, writen in decimals: 1988,36115674196 and 1991.45468856947. We can now easily calculate time that has passed during two exposures: 3.09353182751 years. Thus, proper motion per year for Barnards star is 10.35 arcseconds/year! Furr Analysis Bild 4: Two images of Barnards Star We now magnify part of image around Barnard Star ( zoom ) and use dist -tool to measure distance between red and green image. The If star moves 10.35 arcseconds per year along celestial sphere, what is its real velocity through space? To calculate that cvalue, we have to know distance of Barnards Star. To obtain this information, we load a catalogue: File -> Load catalague -> Simbad Database The catalogues symbol is now displayed in
stack on right side. With mark -tool we select objects of catalogue in image and database entries are shown in measurement window: We now known, that Barnards Star is 1.82 parsecs away and shows an apparent motion of 10.35 arcseconds per year. Simple trigonometry gives real distance, that Barnards Star covers in a year: Bild 8: Trigonometry Bild 6: Data from catalogue Barnards Star is here listed under its or name: V* V2500 Oph ( V means variable since Barnards Star is a variable star). Klicking on its name opens Simbad-Database in webbrowser where one can fand all relevant data: The distance X that star moves during a year is 0.0000912 parsec or. 2813000000 km. That corresponds to a tangential velocity of 90 km/s or 321000 km/h. Motion on celestial sphere The visible motion of Barnards Star on sky is also influenced by or factors: motion of earth around sun; influence of moon on motion of earth, etc. The APFS-toll of german virtual observatory (GAVO) allows a visualistaion of real motion of a star on celestial sphere It can be accessed at http://dc.zah.uniheidelberg.de/apfs/res/apfs_new/hipquery/f orm. Bild 7: Barnards Star at Simbad Parallaxes mas indicates parallax of star in milliarcseconds (mas). It is given as 0.549 arcseconds We now can calculate easily distance r to Barnards Star: r = 1 / 0.549 = 1.82 pc Enter Barnard Star in Object field and specify timescale. Lets look at motion between June 1st 2009 and June 1st 2014. The output interval ( interval of generation ) should be 24 hours. As output format we choose a graphical represantation and select VOPlot.
Bild 10: Motion of Barnards Star Bild 9: Motion of Barnards Star with GAVO Clicking Go starts calculation and graphical user-interface. There we have to adjust correct columns for x and y. We want to have right ascension ( racio ) at x and declination ( dec ) at y. A click on plot draws new image. We can now see how position of star is changing during time. There are five loops that correspond to five years from 2009 to 2014 and are due to motion of earth around sun. The superimposed linear motion from lower left to upper right is real proper motion of star: Bild 11: Motion of Barnards That gets more clear, if we change output interval from 24 hours to 8766 hours (one year). The motion of earth is now filtered out and we can see linear proper motion of Barnards Star: