No stars beyond 16308.35 ly's from Sol

[jrobert]

I travelled to the edge of the celestial star universe in an attempt to find the farthest star from Sol (our home star) and found something interesting. The farthest stars are exactly 16308.35 ly's from Sol. I counted about 200 or so stars with this distance. Now that can't be right, can it? Or is this the limitation of the Hippocros catalog?

PS: I'm running a P4 1.6Ghz, 512MB, Geforce2 GTS w/32MB DDR VRAM.

[Rigel]

I think that this is not a bug in Celestia

When the stars database of Celestia is generated from the Hipparcos catalog, and when a parallax value is dubious in this catalog, then Celestia put this star at such a distance from the sun

Tell me if I'm wrong...

[jrobert]

I think I should rephrase my question... What I'd like to really know is: Why are there no stars beyond 16308.35 ly's from Sol?

[Rigel]

:o

I hope there will be an astronomer around there to give us a correct answer, but from what I know about astronomy, that's because stellar distances in Celestia are based on the parallax value of the Hipparcos Catalog, and this method - ie : measuring the parallax angle of a star" - can only be done on quite near stars. When astronomers want to know distances of more distant stars, they use other methods than parallax...

[NecroBones]

Actually, I think the more important question is why are three so many with the SAME distance. That shouldn't happen, unless the distances are being truncated, or rounded, or otherwise hard-set based on unavailable data. So is it a limitation of Celestia? Or the Hipparcos catalog? Or both?

[julesstoop]

Might this have to do with Hipparchos' maximum measuring resolution? In other words, when a parallax comes close to the smallest measureable value, it gets truncated to that value; if it's yet even smaller, it gets rounded of to zero. Hence, many stars end up at the specific distance associated with this smallest possible parallax, and no stars beyond.

By the way, I have noticed it as well in quite a funny way. If you fly up to about a distance of 100.000 ly +, and look down towards the sun, with the star brightness on maximum, you can actually see brighter stars sitting on a circle a circle at this 16000 lightyear distance, and (besides the galaxy clouds) darkness beyond.

[billybob884]

Maybe its because people dont know the exactdistances of the stars and they dont know of anyother stars beyond these. Just a guess


Mike M.

[ou8poop2]

well, i updated the star database to the newest version, and found several stars with this as the farthest distance:
put this into your favorites.cel file

"fathest" {
isFolder false
parentFolder ""
base [ 7089.238800749067 -1580.958021665661 24015.40832756709 ]
offset [ 1.275541319145646e-005 -4.664833141987401e-008 7.505682233954758e-006 ]
axis [ 0.103199 -0.0962932 0.989989 ]
angle 2.4815
time 2447763.83118801
selection "#0"
coordsys "ecliptical"
}

now, i realize that this is most likely eroneous data from tthe catalogs, but i thought there was a limit placed by chris when coding that prevented stars from showing farther than 16000 ly. So why can we see this star? if you move too far in any direction, it disappears, so are these the effects of those limits? It seems that as long as you face home to Sol, you can see it, so can you place objects farther out for now, and get the same results? And Chris, when will you ( or how can we now ) change those limits so as to see farther obects?

[granthutchison]

16308.35 ly is exactly 5000 parsecs, corresponding to a pretty tiny parallax of 0.2 milliarcseconds. I don't know what the parallax limit for Hipparcos was, but it seems to me that this sphere of stars represents a whole bunch of stars with marginal parallaxes that were all rounded to 0.2 milliarcseconds. I wouldn't be surprised if this represented the "edge" of the Hipparcos dataset, beyond which parallaxes were deemed unobtainable to any usable precision - a star with a parallax of 0.1 milliarcseconds would be twice as far away as one with 0.2 milliarcseconds, so rounding errors would swamp the data.

Grant

[Rigel]

You're right.

Those stars correspond generally to Hipparcos entries with negative parallaxes, that could not be corrected by the mean of their B-V color indexes, which is the procedure I used to generate the new stardatabases for all Tycho stars with unknown parallax values.

Therefore, they are put at that limit of 5000 pc (a few hundreds of stars on more than 2 millions). There is no mean to have a better estimation of their distances. They could be eliminated, but it seems better to keep them in the database.

Also, don't forget that we have only a good estimation of the distance of about 100000 stars today and no more, as a result of the Hipparcos mission. And as Alexis said in another post, the detectors of the Hipparcos satellite were not distance limited, but magnitude limited.

Pascal

[granthutchison]

And as Alexis said in another post, the detectors of the Hipparcos satellite were not distance limited, but magnitude limited.

This is interesting. So the "edge" to the data (apart, that is, from the artefactual 5000 pc sphere) is determined by the fact that there are vanishingly few stars bright enough to allow Hipparcos to detect their parallax at such extreme distances? So could the Hipparcos detectors have detected less than 0.2 milliarcseconds of parallax, if the star had been bright enough?

Grant

[Rigel]

It's a bit more complicated than that. If you are interested, you should visit the ESA website

http://astro.estec.esa.nl/Hipparcos/hipparcos.html

to learn more about the Hipparcos mission...

[granthutchison]

It's a bit more complicated than that. If you are interested, you should visit the ESA website
http://astro.estec.esa.nl/Hipparcos/hipparcos.html
to learn more about the Hipparcos mission...

I do find that site a bit user-beligerent - I've spent a while there in the past and still haven't quite got to grips with the observational limits of the Hipparcos survey. (As is evident from my question, no doubt! )

Grant

[alexis]

This is interesting. So the "edge" to the data (apart, that is, from the artefactual 5000 pc sphere) is determined by the fact that there are vanishingly few stars bright enough to allow Hipparcos to detect their parallax at such extreme distances?

Well, yes and no. The average parallax error of the Hipparcos catalogue is about 1 mas, slightly dependent on the apparent magnitude (brighter stars usually have somewhat more accurate parallaxes than fainter ones) but generally independent of the distance to the star.

Thus a measured parallax of (2 +- 1) mas would place the star anywhere between 300 pc and 1000 pc with 1-sigma confidence (68% likelihood), and a parallax of (0.2 +- 1) mas would correspond to a distance between 800 pc and infinity with the same confidence.

The Hipparcos sample is complete down to about V magnitude 8, though some stars in it are as faint as V = 12 mag.

To take this into perspective, the nearest known stellar neigbour of our Sun is Proxima Centauri, at a parallax of 772 mas (distance 1.3 pc) and an apparent V magnitude of 11.0. Thus even out to the very nearest stars the Hipparcos catalogue is not complete down to the luminosity of a Proxima Centauri class star!

At the other extreme we have the red super giants that at a distance of 2000 pc still could be as bright as V = 8 mag.

So could the Hipparcos detectors have detected less than 0.2 milliarcseconds of parallax, if the star had been bright enough?

Hipparcos cannot distinguish parallaxes smaller than 1 mas from zero, even though it could (and did) measure such small parallaxes. That's why one has to take care when using such measurements. The relative error of the parallaxes just inflates for small parallaxes, as you pointed out yourself.

It is therefore very sensible to clamp distances derived from Hipparcos parallaxes, as done in Celestia. One can be quite certain that all stars seen by Hipparcos must be closer than 5000 pc (except possibly some rare cases). And it really doesn't make much sense using parallaxes that yield infinite relative distance errors anyway. A practical absolute lower limit on sensible parallax measurements would be about 1 mas, corresponding to 1000 pc.

That's all for now, have a very Merry Christmas, all of you!

/Alexis

[granthutchison]

Alexis - thanks for taking the trouble to reply.
I am grateful; and better informed!

Grant