HR Diagram negative parallax help?!?

Question

I'm supposed to be doing this assignment to create a HR diagram, but when I've been collecting the information for the stars off of this website http://www.rssd.esa.int/SA-general/Projects/Hipparcos/apps/SelectArea.html as the assignment asked me too, some of the stars I have found to have negative trigonometric parallaxes, which then makes the distance a negative and then you can't work out the Abs magnitude/luminosity.

Does anyone know what I am supposed to do? I emailed the website for help but they just emailed back "what do YOU think it means" and then closed the question.

Any help any one can give me would be GREATLY appreciated.

Answer 1

Wouldn't you just use the absolute value of the distances?  I mean, if you travel negative one mile south, your distance from your starting point is still one mile away, right?  

However, when dealing with two distances, say, -1 mile north, and 5 miles north, then you have two distaces that are 6 miles apart.  On is 5 miles from your starting point, and one is one mile from your starting point, but they are in opposite directions from it.

<edit> Are you trying to figure out the actual luminosity of a star by calculating it's distance through parallax, and computing it's apparent magnitude?  If that's the case, it's the difference in distance you need to calculate the total distance between the negative distance and the positive one (see my -1 and +5 explaination).  Sorry if I'm way off, I'm having a hard time understanding the question.

Answer 2

I would just ignore the negative and take the absolute value.  It doesn't really matter which direction the star shifts, just by how much.

Answer 3

If the answer is negative then the parallax angle is smaller than the errors in the astrometry - so the number is meaningless.

Try looking here:

http://www.realityreviewed.com/Negative%...

Apparently they simply list all the parallax "measurements" even the ones that are zero or negative (which means the angle was really really small)

What this means is that those stars for which you get negative parallax angles, you cannot work out the distance, so you have to reject them from you sample. You may want to say why you threw out any stars in a paragraph of explanation for your teacher/professor